LinuxLists.cc - [PATCH RFC] mm/userfaultfd: enable writenotify while userfaultfd-wp is enabled for a VMA

2022-12-02 13:15:00

Subject: [PATCH RFC] mm/userfaultfd: enable writenotify while userfaultfd-wp is enabled for a VMA

Currently, we don't enable writenotify when enabling userfaultfd-wp on
a shared writable mapping (for now we only support SHMEM). The consequence
is that vma->vm_page_prot will still include write permissions, to be set
as default for all PTEs that get remapped (e.g., mprotect(), NUMA hinting,
page migration, ...).

This is problematic for uffd-wp: we'd have to manually check for
a uffd-wp PTE and manually write-protect that PTE, which is error prone
and the logic is the wrong way around. Prone to such issues is any code
that uses vma->vm_page_prot to set PTE permissions: primarily pte_modify()
and mk_pte(), but there might be more (move_pte() looked suspicious at
first but the protection parameter is essentially unused).

Instead, let's enable writenotify -- just like we do for softdirty
tracking -- such that PTEs will be mapped write-protected as default
and we will only allow selected PTEs that are defintly safe to be
mapped without write-protection (see can_change_pte_writable()) to be
writable. This reverses the logic and implicitly fixes and prevents any
such uffd-wp issues.

Note that when enabling userfaultfd-wp, there is no need to walk page
tables to enforce the new default protection for the PTEs: we know that
they cannot be uffd-wp'ed yet, because that can only happen afterwards.

For example, this fixes page migration and mprotect() to not map a
uffd-wp'ed PTE writable. In theory, this should also fix when NUMA-hinting
remaps pages in such (shmem) mappings -- if NUMA-hinting is applicable to
shmem with uffd as well.

Running the mprotect() reproducer [1] without this commit:
$ ./uffd-wp-mprotect
FAIL: uffd-wp did not fire
Running the mprotect() reproducer with this commit:
$ ./uffd-wp-mprotect
PASS: uffd-wp fired

[1] https://lore.kernel.org/all/[email protected]/T/#u

Reported-by: Ives van Hoorne <[email protected]>
Debugged-by: Peter Xu <[email protected]>
Fixes: b1f9e876862d ("mm/uffd: enable write protection for shmem & hugetlbfs")
Cc: [email protected]
Cc: Andrew Morton <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Alistair Popple <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Nadav Amit <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Signed-off-by: David Hildenbrand <[email protected]>
---

Based on latest upstream. userfaultfd selftests seem to pass.

---
fs/userfaultfd.c | 28 ++++++++++++++++++++++------
mm/mmap.c | 4 ++++
2 files changed, 26 insertions(+), 6 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 98ac37e34e3d..fb0733f2e623 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -108,6 +108,21 @@ static bool userfaultfd_is_initialized(struct userfaultfd_ctx *ctx)
return ctx->features & UFFD_FEATURE_INITIALIZED;
}

+static void userfaultfd_set_vm_flags(struct vm_area_struct *vma,
+ vm_flags_t flags)
+{
+ const bool uffd_wp = !!((vma->vm_flags | flags) & VM_UFFD_WP);
+
+ vma->vm_flags = flags;
+ /*
+ * For shared mappings, we want to enable writenotify while
+ * userfaultfd-wp is enabled (see vma_wants_writenotify()). We'll simply
+ * recalculate vma->vm_page_prot whenever userfaultfd-wp is involved.
+ */
+ if ((vma->vm_flags & VM_SHARED) && uffd_wp)
+ vma_set_page_prot(vma);
+}
+
static int userfaultfd_wake_function(wait_queue_entry_t *wq, unsigned mode,
int wake_flags, void *key)
{
@@ -618,7 +633,8 @@ static void userfaultfd_event_wait_completion(struct userfaultfd_ctx *ctx,
for_each_vma(vmi, vma) {
if (vma->vm_userfaultfd_ctx.ctx == release_new_ctx) {
vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
- vma->vm_flags &= ~__VM_UFFD_FLAGS;
+ userfaultfd_set_vm_flags(vma,
+ vma->vm_flags & ~__VM_UFFD_FLAGS);
}
}
mmap_write_unlock(mm);
@@ -652,7 +668,7 @@ int dup_userfaultfd(struct vm_area_struct *vma, struct list_head *fcs)
octx = vma->vm_userfaultfd_ctx.ctx;
if (!octx || !(octx->features & UFFD_FEATURE_EVENT_FORK)) {
vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
- vma->vm_flags &= ~__VM_UFFD_FLAGS;
+ userfaultfd_set_vm_flags(vma, vma->vm_flags & ~__VM_UFFD_FLAGS);
return 0;
}

@@ -733,7 +749,7 @@ void mremap_userfaultfd_prep(struct vm_area_struct *vma,
} else {
/* Drop uffd context if remap feature not enabled */
vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
- vma->vm_flags &= ~__VM_UFFD_FLAGS;
+ userfaultfd_set_vm_flags(vma, vma->vm_flags & ~__VM_UFFD_FLAGS);
}
}

@@ -895,7 +911,7 @@ static int userfaultfd_release(struct inode *inode, struct file *file)
prev = vma;
}

- vma->vm_flags = new_flags;
+ userfaultfd_set_vm_flags(vma, new_flags);
vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
}
mmap_write_unlock(mm);
@@ -1463,7 +1479,7 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
* the next vma was merged into the current one and
* the current one has not been updated yet.
*/
- vma->vm_flags = new_flags;
+ userfaultfd_set_vm_flags(vma, new_flags);
vma->vm_userfaultfd_ctx.ctx = ctx;

if (is_vm_hugetlb_page(vma) && uffd_disable_huge_pmd_share(vma))
@@ -1651,7 +1667,7 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
* the next vma was merged into the current one and
* the current one has not been updated yet.
*/
- vma->vm_flags = new_flags;
+ userfaultfd_set_vm_flags(vma, new_flags);
vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;

skip:
diff --git a/mm/mmap.c b/mm/mmap.c
index 74a84eb33b90..ce7526aa5d61 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1525,6 +1525,10 @@ int vma_wants_writenotify(struct vm_area_struct *vma, pgprot_t vm_page_prot)
if (vma_soft_dirty_enabled(vma) && !is_vm_hugetlb_page(vma))
return 1;

+ /* Do we need write faults for uffd-wp tracking? */
+ if (userfaultfd_wp(vma))
+ return 1;
+
/* Specialty mapping? */
if (vm_flags & VM_PFNMAP)
return 0;

base-commit: a4412fdd49dc011bcc2c0d81ac4cab7457092650
--
2.38.1

2022-12-02 17:24:29

by Peter Xu

[permalink] [raw]

Subject: Re: [PATCH RFC] mm/userfaultfd: enable writenotify while userfaultfd-wp is enabled for a VMA

On Fri, Dec 02, 2022 at 01:27:48PM +0100, David Hildenbrand wrote:
> Currently, we don't enable writenotify when enabling userfaultfd-wp on
> a shared writable mapping (for now we only support SHMEM). The consequence

and hugetlbfs

> is that vma->vm_page_prot will still include write permissions, to be set
> as default for all PTEs that get remapped (e.g., mprotect(), NUMA hinting,
> page migration, ...).

The thing is by default I think we want the write bit..

The simple example is (1) register UFFD_WP on shmem writable, (2) write a
page. Here we didn't wr-protect anything, so we want the write bit there.

Or the other example is when UFFDIO_COPY with flags==0 even if with
VM_UFFD_WP. We definitely wants the write bit.

We only doesn't want the write bit when uffd-wp is explicitly set.

I think fundamentally the core is uffd-wp is pte-based, so the information
resides in pte not vma. I'm not strongly objecting this patch, especially
you mentioned auto-numa so I need to have a closer look later there.
However I do think uffd-wp is slightly special because we always need to
consider pte information anyway, so a per-vma information doesn't hugely
help, IMHO.

>
> This is problematic for uffd-wp: we'd have to manually check for
> a uffd-wp PTE and manually write-protect that PTE, which is error prone
> and the logic is the wrong way around. Prone to such issues is any code
> that uses vma->vm_page_prot to set PTE permissions: primarily pte_modify()
> and mk_pte(), but there might be more (move_pte() looked suspicious at
> first but the protection parameter is essentially unused).
>
> Instead, let's enable writenotify -- just like we do for softdirty
> tracking -- such that PTEs will be mapped write-protected as default
> and we will only allow selected PTEs that are defintly safe to be
> mapped without write-protection (see can_change_pte_writable()) to be
> writable. This reverses the logic and implicitly fixes and prevents any
> such uffd-wp issues.
>
> Note that when enabling userfaultfd-wp, there is no need to walk page
> tables to enforce the new default protection for the PTEs: we know that
> they cannot be uffd-wp'ed yet, because that can only happen afterwards.
>
> For example, this fixes page migration and mprotect() to not map a
> uffd-wp'ed PTE writable. In theory, this should also fix when NUMA-hinting
> remaps pages in such (shmem) mappings -- if NUMA-hinting is applicable to
> shmem with uffd as well.
>
> Running the mprotect() reproducer [1] without this commit:
> $ ./uffd-wp-mprotect
> FAIL: uffd-wp did not fire
> Running the mprotect() reproducer with this commit:
> $ ./uffd-wp-mprotect
> PASS: uffd-wp fired
>
> [1] https://lore.kernel.org/all/[email protected]/T/#u

I still hope for a formal patch (non-rfc) we can have a reproducer outside
mprotect(). IMHO mprotect() is really ambiguously here being used with
uffd-wp, so not a good example IMO as I explained in the other thread [1].

I'll need to off-work most of the rest of today, but maybe I can also have
a look in the weekend or Monday more on the numa paths. Before that, can
we first reach a consensus that we have the mm/migrate patch there to be
merged first? These are two issues, IMHO.

I know you're against me for some reason, but until now I sincerely don't
know why. That patch sololy recovers write bit status (by removing it for
read-only) for a migration entry and that definitely makes sense to me. As
I also mentioned in the old version of that thread, we can rework migration
entries and merge READ|WRITE entries into a GENERIC entry one day if you
think proper, but that's for later.

Let me provide another example why I think recovering write bit may matter
outside uffd too so it's common and it's always good to explicit check it.

If you still remember for sparc64 (I think you're also in the loop)
pte_mkdirty will also apply write bit there; even though I don't know why
but it works like that for years. Consider below sequence:

- map a page writable, write to page (fault in, set dirty)
- mprotect(RO) on the page (keep dirty bit, vma/pte becomes RO)
- migrate the page
- mk_pte returns with WRITE bit cleared (which is correct)
- pte_mkdirty set dirty and write bit (because dirty used to set)

If without the previous mm/migrate patch [1] IIUC it'll allow the pte
writable even without VM_WRITE here after migration.

I'm not sure whether I missed something, nor can I write a reproducer
because I don't have sparc64 systems on hand, not to mention time. But
hopefully I explained why I think it's safer to just always double check on
the write bit to be the same as when before migration, irrelevant of
uffd-wp, vma pgprot or mk_pte().

For this patch itself, I'll rethink again when I read more on numa.

Thanks,

>
> Reported-by: Ives van Hoorne <[email protected]>
> Debugged-by: Peter Xu <[email protected]>
> Fixes: b1f9e876862d ("mm/uffd: enable write protection for shmem & hugetlbfs")
> Cc: [email protected]
> Cc: Andrew Morton <[email protected]>
> Cc: Hugh Dickins <[email protected]>
> Cc: Alistair Popple <[email protected]>
> Cc: Mike Rapoport <[email protected]>
> Cc: Nadav Amit <[email protected]>
> Cc: Andrea Arcangeli <[email protected]>
> Signed-off-by: David Hildenbrand <[email protected]>
> ---
>
> Based on latest upstream. userfaultfd selftests seem to pass.
>
> ---
> fs/userfaultfd.c | 28 ++++++++++++++++++++++------
> mm/mmap.c | 4 ++++
> 2 files changed, 26 insertions(+), 6 deletions(-)
>
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index 98ac37e34e3d..fb0733f2e623 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -108,6 +108,21 @@ static bool userfaultfd_is_initialized(struct userfaultfd_ctx *ctx)
> return ctx->features & UFFD_FEATURE_INITIALIZED;
> }
>
> +static void userfaultfd_set_vm_flags(struct vm_area_struct *vma,
> + vm_flags_t flags)
> +{
> + const bool uffd_wp = !!((vma->vm_flags | flags) & VM_UFFD_WP);
> +
> + vma->vm_flags = flags;
> + /*
> + * For shared mappings, we want to enable writenotify while
> + * userfaultfd-wp is enabled (see vma_wants_writenotify()). We'll simply
> + * recalculate vma->vm_page_prot whenever userfaultfd-wp is involved.
> + */
> + if ((vma->vm_flags & VM_SHARED) && uffd_wp)
> + vma_set_page_prot(vma);
> +}
> +
> static int userfaultfd_wake_function(wait_queue_entry_t *wq, unsigned mode,
> int wake_flags, void *key)
> {
> @@ -618,7 +633,8 @@ static void userfaultfd_event_wait_completion(struct userfaultfd_ctx *ctx,
> for_each_vma(vmi, vma) {
> if (vma->vm_userfaultfd_ctx.ctx == release_new_ctx) {
> vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
> - vma->vm_flags &= ~__VM_UFFD_FLAGS;
> + userfaultfd_set_vm_flags(vma,
> + vma->vm_flags & ~__VM_UFFD_FLAGS);
> }
> }
> mmap_write_unlock(mm);
> @@ -652,7 +668,7 @@ int dup_userfaultfd(struct vm_area_struct *vma, struct list_head *fcs)
> octx = vma->vm_userfaultfd_ctx.ctx;
> if (!octx || !(octx->features & UFFD_FEATURE_EVENT_FORK)) {
> vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
> - vma->vm_flags &= ~__VM_UFFD_FLAGS;
> + userfaultfd_set_vm_flags(vma, vma->vm_flags & ~__VM_UFFD_FLAGS);
> return 0;
> }
>
> @@ -733,7 +749,7 @@ void mremap_userfaultfd_prep(struct vm_area_struct *vma,
> } else {
> /* Drop uffd context if remap feature not enabled */
> vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
> - vma->vm_flags &= ~__VM_UFFD_FLAGS;
> + userfaultfd_set_vm_flags(vma, vma->vm_flags & ~__VM_UFFD_FLAGS);
> }
> }
>
> @@ -895,7 +911,7 @@ static int userfaultfd_release(struct inode *inode, struct file *file)
> prev = vma;
> }
>
> - vma->vm_flags = new_flags;
> + userfaultfd_set_vm_flags(vma, new_flags);
> vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
> }
> mmap_write_unlock(mm);
> @@ -1463,7 +1479,7 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
> * the next vma was merged into the current one and
> * the current one has not been updated yet.
> */
> - vma->vm_flags = new_flags;
> + userfaultfd_set_vm_flags(vma, new_flags);
> vma->vm_userfaultfd_ctx.ctx = ctx;
>
> if (is_vm_hugetlb_page(vma) && uffd_disable_huge_pmd_share(vma))
> @@ -1651,7 +1667,7 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
> * the next vma was merged into the current one and
> * the current one has not been updated yet.
> */
> - vma->vm_flags = new_flags;
> + userfaultfd_set_vm_flags(vma, new_flags);
> vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
>
> skip:
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 74a84eb33b90..ce7526aa5d61 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1525,6 +1525,10 @@ int vma_wants_writenotify(struct vm_area_struct *vma, pgprot_t vm_page_prot)
> if (vma_soft_dirty_enabled(vma) && !is_vm_hugetlb_page(vma))
> return 1;
>
> + /* Do we need write faults for uffd-wp tracking? */
> + if (userfaultfd_wp(vma))
> + return 1;
> +
> /* Specialty mapping? */
> if (vm_flags & VM_PFNMAP)
> return 0;
>
> base-commit: a4412fdd49dc011bcc2c0d81ac4cab7457092650
> --
> 2.38.1
>

--
Peter Xu

2022-12-02 17:40:09

by David Hildenbrand

[permalink] [raw]

Subject: Re: [PATCH RFC] mm/userfaultfd: enable writenotify while userfaultfd-wp is enabled for a VMA

On 02.12.22 17:56, David Hildenbrand wrote:
> On 02.12.22 17:33, Peter Xu wrote:
>> On Fri, Dec 02, 2022 at 01:27:48PM +0100, David Hildenbrand wrote:
>>> Currently, we don't enable writenotify when enabling userfaultfd-wp on
>>> a shared writable mapping (for now we only support SHMEM). The consequence
>>
>> and hugetlbfs
>>
>>> is that vma->vm_page_prot will still include write permissions, to be set
>>> as default for all PTEs that get remapped (e.g., mprotect(), NUMA hinting,
>>> page migration, ...).
>>
>> The thing is by default I think we want the write bit..
>>
>> The simple example is (1) register UFFD_WP on shmem writable, (2) write a
>> page. Here we didn't wr-protect anything, so we want the write bit there.
>>
>> Or the other example is when UFFDIO_COPY with flags==0 even if with
>> VM_UFFD_WP. We definitely wants the write bit.
>>
>> We only doesn't want the write bit when uffd-wp is explicitly set.
>>
>> I think fundamentally the core is uffd-wp is pte-based, so the information
>> resides in pte not vma. I'm not strongly objecting this patch, especially
>> you mentioned auto-numa so I need to have a closer look later there.
>> However I do think uffd-wp is slightly special because we always need to
>> consider pte information anyway, so a per-vma information doesn't hugely
>> help, IMHO.
>
> That's the same as softdirty tracking, IMHO.
>
> [...]
>
>>> Running the mprotect() reproducer [1] without this commit:
>>> $ ./uffd-wp-mprotect
>>> FAIL: uffd-wp did not fire
>>> Running the mprotect() reproducer with this commit:
>>> $ ./uffd-wp-mprotect
>>> PASS: uffd-wp fired
>>>
>>> [1] https://lore.kernel.org/all/[email protected]/T/#u
>>
>> I still hope for a formal patch (non-rfc) we can have a reproducer outside
>> mprotect(). IMHO mprotect() is really ambiguously here being used with
>> uffd-wp, so not a good example IMO as I explained in the other thread [1].
>
> I took the low hanging fruit to showcase that this is a more generic problem.
> The reproducer is IMHO nice because it's simple and race-free.
>
>>
>> I'll need to off-work most of the rest of today, but maybe I can also have
>> a look in the weekend or Monday more on the numa paths. Before that, can
>> we first reach a consensus that we have the mm/migrate patch there to be
>> merged first? These are two issues, IMHO.
>>
>> I know you're against me for some reason, but until now I sincerely don't
>> know why. That patch sololy recovers write bit status (by removing it for
>> read-only) for a migration entry and that definitely makes sense to me. As
>> I also mentioned in the old version of that thread, we can rework migration
>> entries and merge READ|WRITE entries into a GENERIC entry one day if you
>> think proper, but that's for later.
>
> I'm not against you. I'm against changing well-working, common code
> when it doesn't make any sense to me to change it. And now we have proof that
> mprotect() just behaves exactly the same way, using the basic rules of vma->vm_page_prot.
>
> Yes, there is broken sparc64 (below), but that shouldn't dictate our implementation.
>
>
> What *would* make sense to me, as I raised, is:
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index dff333593a8a..9fc181fd3c5a 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -213,8 +213,10 @@ static bool remove_migration_pte(struct folio *folio,
> pte = pte_mkdirty(pte);
> if (is_writable_migration_entry(entry))
> pte = maybe_mkwrite(pte, vma);
> - else if (pte_swp_uffd_wp(*pvmw.pte))
> + else if (pte_swp_uffd_wp(*pvmw.pte)) {
> pte = pte_mkuffd_wp(pte);
> + pt = pte_wrprotect(pte);
> + }
>
> if (folio_test_anon(folio) && !is_readable_migration_entry(entry))
> rmap_flags |= RMAP_EXCLUSIVE;
>
>
> It still requires patch each and every possible code location, which I dislike as
> described in the patch description. The fact that there are still uffd-wp bugs
> with your patch makes that hopefully clear. I'd be interested if they can be
> reproduced witht his patch.
>

And if NUMA hinting is indeed the problem, without this patch what would
be required would most probably be:

diff --git a/mm/memory.c b/mm/memory.c
index 8a6d5c823f91..869d35ef0e24 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4808,6 +4808,8 @@ static vm_fault_t do_numa_page(struct vm_fault *vmf)
pte = pte_mkyoung(pte);
if (was_writable)
pte = pte_mkwrite(pte);
+ if (pte_uffd_wp(pte))
+ pte = pte_wrprotect(pte);
ptep_modify_prot_commit(vma, vmf->address, vmf->pte, old_pte, pte);
update_mmu_cache(vma, vmf->address, vmf->pte);
pte_unmap_unlock(vmf->pte, vmf->ptl);

And just to make my point about the migration path clearer: doing it your way
would be:

diff --git a/mm/memory.c b/mm/memory.c
index 8a6d5c823f91..a7c4c1a57f6a 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4808,6 +4808,8 @@ static vm_fault_t do_numa_page(struct vm_fault *vmf)
pte = pte_mkyoung(pte);
if (was_writable)
pte = pte_mkwrite(pte);
+ else
+ pte = pte_wrprotect(pte);
ptep_modify_prot_commit(vma, vmf->address, vmf->pte, old_pte, pte);
update_mmu_cache(vma, vmf->address, vmf->pte);
pte_unmap_unlock(vmf->pte, vmf->ptl);

And I don't think that's the right approach.

--
Thanks,

David / dhildenb

2022-12-02 17:48:38

by David Hildenbrand

[permalink] [raw]

Subject: Re: [PATCH RFC] mm/userfaultfd: enable writenotify while userfaultfd-wp is enabled for a VMA

On 02.12.22 17:33, Peter Xu wrote:
> On Fri, Dec 02, 2022 at 01:27:48PM +0100, David Hildenbrand wrote:
>> Currently, we don't enable writenotify when enabling userfaultfd-wp on
>> a shared writable mapping (for now we only support SHMEM). The consequence
>
> and hugetlbfs
>
>> is that vma->vm_page_prot will still include write permissions, to be set
>> as default for all PTEs that get remapped (e.g., mprotect(), NUMA hinting,
>> page migration, ...).
>
> The thing is by default I think we want the write bit..
>
> The simple example is (1) register UFFD_WP on shmem writable, (2) write a
> page. Here we didn't wr-protect anything, so we want the write bit there.
>
> Or the other example is when UFFDIO_COPY with flags==0 even if with
> VM_UFFD_WP. We definitely wants the write bit.
>
> We only doesn't want the write bit when uffd-wp is explicitly set.
>
> I think fundamentally the core is uffd-wp is pte-based, so the information
> resides in pte not vma. I'm not strongly objecting this patch, especially
> you mentioned auto-numa so I need to have a closer look later there.
> However I do think uffd-wp is slightly special because we always need to
> consider pte information anyway, so a per-vma information doesn't hugely
> help, IMHO.

That's the same as softdirty tracking, IMHO.

[...]

>> Running the mprotect() reproducer [1] without this commit:
>> $ ./uffd-wp-mprotect
>> FAIL: uffd-wp did not fire
>> Running the mprotect() reproducer with this commit:
>> $ ./uffd-wp-mprotect
>> PASS: uffd-wp fired
>>
>> [1] https://lore.kernel.org/all/[email protected]/T/#u
>
> I still hope for a formal patch (non-rfc) we can have a reproducer outside
> mprotect(). IMHO mprotect() is really ambiguously here being used with
> uffd-wp, so not a good example IMO as I explained in the other thread [1].

I took the low hanging fruit to showcase that this is a more generic problem.
The reproducer is IMHO nice because it's simple and race-free.

>
> I'll need to off-work most of the rest of today, but maybe I can also have
> a look in the weekend or Monday more on the numa paths. Before that, can
> we first reach a consensus that we have the mm/migrate patch there to be
> merged first? These are two issues, IMHO.
>
> I know you're against me for some reason, but until now I sincerely don't
> know why. That patch sololy recovers write bit status (by removing it for
> read-only) for a migration entry and that definitely makes sense to me. As
> I also mentioned in the old version of that thread, we can rework migration
> entries and merge READ|WRITE entries into a GENERIC entry one day if you
> think proper, but that's for later.

I'm not against you. I'm against changing well-working, common code
when it doesn't make any sense to me to change it. And now we have proof that
mprotect() just behaves exactly the same way, using the basic rules of vma->vm_page_prot.

Yes, there is broken sparc64 (below), but that shouldn't dictate our implementation.

What *would* make sense to me, as I raised, is:

diff --git a/mm/migrate.c b/mm/migrate.c
index dff333593a8a..9fc181fd3c5a 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -213,8 +213,10 @@ static bool remove_migration_pte(struct folio *folio,
pte = pte_mkdirty(pte);
if (is_writable_migration_entry(entry))
pte = maybe_mkwrite(pte, vma);
- else if (pte_swp_uffd_wp(*pvmw.pte))
+ else if (pte_swp_uffd_wp(*pvmw.pte)) {
pte = pte_mkuffd_wp(pte);
+ pt = pte_wrprotect(pte);
+ }

if (folio_test_anon(folio) && !is_readable_migration_entry(entry))
rmap_flags |= RMAP_EXCLUSIVE;

It still requires patch each and every possible code location, which I dislike as
described in the patch description. The fact that there are still uffd-wp bugs
with your patch makes that hopefully clear. I'd be interested if they can be
reproduced witht his patch.

>
> Let me provide another example why I think recovering write bit may matter
> outside uffd too so it's common and it's always good to explicit check it.
>
> If you still remember for sparc64 (I think you're also in the loop)
> pte_mkdirty will also apply write bit there; even though I don't know why
> but it works like that for years. Consider below sequence:

Yes, and I consider that having to be fixed properly on in sparc64 code.
pte_mkdirty() must not allow write access. That's why we have pte_mkwrite()
after all. It's just plain wrong and requires custom hacks all over the place.

As raised, the whole maybe_mkwrite() logic is completely broken on sparc64.

>
> - map a page writable, write to page (fault in, set dirty)
> - mprotect(RO) on the page (keep dirty bit, vma/pte becomes RO)
> - migrate the page
> - mk_pte returns with WRITE bit cleared (which is correct)
> - pte_mkdirty set dirty and write bit (because dirty used to set)
>
> If without the previous mm/migrate patch [1] IIUC it'll allow the pte
> writable even without VM_WRITE here after migration.

That would be my reaction:

diff --git a/mm/migrate.c b/mm/migrate.c
index dff333593a8a..05183cd22f0f 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -209,12 +209,20 @@ static bool remove_migration_pte(struct folio *folio,
entry = pte_to_swp_entry(*pvmw.pte);
if (!is_migration_entry_young(entry))
pte = pte_mkold(pte);
+ /*
+ * HACK alert. sparc64 really has to fix it's pte_mkwrite()
+ * implementation to not allow random write access.
+ */
+#ifndef CONFIG_SPARC64
if (folio_test_dirty(folio) && is_migration_entry_dirty(entry))
pte = pte_mkdirty(pte);
+#endif

>
> I'm not sure whether I missed something, nor can I write a reproducer
> because I don't have sparc64 systems on hand, not to mention time. But
> hopefully I explained why I think it's safer to just always double check on
> the write bit to be the same as when before migration, irrelevant of
> uffd-wp, vma pgprot or mk_pte().
>
> For this patch itself, I'll rethink again when I read more on numa.

Most probably we'll have to agree to disagree.

--
Thanks,

David / dhildenb

2022-12-05 21:41:34

by Peter Xu

[permalink] [raw]

Subject: Re: [PATCH RFC] mm/userfaultfd: enable writenotify while userfaultfd-wp is enabled for a VMA

On Fri, Dec 02, 2022 at 06:11:17PM +0100, David Hildenbrand wrote:
> On 02.12.22 17:56, David Hildenbrand wrote:
> > On 02.12.22 17:33, Peter Xu wrote:
> > > On Fri, Dec 02, 2022 at 01:27:48PM +0100, David Hildenbrand wrote:
> > > > Currently, we don't enable writenotify when enabling userfaultfd-wp on
> > > > a shared writable mapping (for now we only support SHMEM). The consequence
> > >
> > > and hugetlbfs
> > >
> > > > is that vma->vm_page_prot will still include write permissions, to be set
> > > > as default for all PTEs that get remapped (e.g., mprotect(), NUMA hinting,
> > > > page migration, ...).
> > >
> > > The thing is by default I think we want the write bit..
> > >
> > > The simple example is (1) register UFFD_WP on shmem writable, (2) write a
> > > page. Here we didn't wr-protect anything, so we want the write bit there.
> > >
> > > Or the other example is when UFFDIO_COPY with flags==0 even if with
> > > VM_UFFD_WP. We definitely wants the write bit.
> > >
> > > We only doesn't want the write bit when uffd-wp is explicitly set.
> > >
> > > I think fundamentally the core is uffd-wp is pte-based, so the information
> > > resides in pte not vma. I'm not strongly objecting this patch, especially
> > > you mentioned auto-numa so I need to have a closer look later there.
> > > However I do think uffd-wp is slightly special because we always need to
> > > consider pte information anyway, so a per-vma information doesn't hugely
> > > help, IMHO.
> >
> > That's the same as softdirty tracking, IMHO.

Soft-dirty doesn't have a bit in the pte showing whether the page is
protected.

One wr-protects in soft-dirty with either ALL or NONE. That's per-vma.

One wr-protects in uffd-wp by wr-protect specific page or range of pages.
That's per-page.

> >
> > [...]
> >
> > > > Running the mprotect() reproducer [1] without this commit:
> > > > $ ./uffd-wp-mprotect
> > > > FAIL: uffd-wp did not fire
> > > > Running the mprotect() reproducer with this commit:
> > > > $ ./uffd-wp-mprotect
> > > > PASS: uffd-wp fired
> > > >
> > > > [1] https://lore.kernel.org/all/[email protected]/T/#u
> > >
> > > I still hope for a formal patch (non-rfc) we can have a reproducer outside
> > > mprotect(). IMHO mprotect() is really ambiguously here being used with
> > > uffd-wp, so not a good example IMO as I explained in the other thread [1].
> >
> > I took the low hanging fruit to showcase that this is a more generic problem.
> > The reproducer is IMHO nice because it's simple and race-free.

If no one is using mprotect() with uffd-wp like that, then the reproducer
may not be valid - the reproducer is defining how it should work, but does
that really stand? That's why I said it's ambiguous, because the
definition in this case is unclear.

I think numa has the problem too which I agree with you. If you attach a
numa reproducer it'll be nicer. But again I'm not convinced uffd-wp is a
per-vma thing, which seems to be what this patch is based upon.

Now I really wonder whether I should just simply wr-protect pte for
pte_mkuffd_wp() always, attached. I didn't do that from the start because
I wanted to keep the helpers operate on one bit only. But I found that
it's actually common technique to use in pgtable arch code, and it really
doesn't make sense to not wr-protect a pte if uffd-wp is set on a present
entry. It's much safer.

> >
> > >
> > > I'll need to off-work most of the rest of today, but maybe I can also have
> > > a look in the weekend or Monday more on the numa paths. Before that, can
> > > we first reach a consensus that we have the mm/migrate patch there to be
> > > merged first? These are two issues, IMHO.
> > >
> > > I know you're against me for some reason, but until now I sincerely don't
> > > know why. That patch sololy recovers write bit status (by removing it for
> > > read-only) for a migration entry and that definitely makes sense to me. As
> > > I also mentioned in the old version of that thread, we can rework migration
> > > entries and merge READ|WRITE entries into a GENERIC entry one day if you
> > > think proper, but that's for later.
> >
> > I'm not against you. I'm against changing well-working, common code
> > when it doesn't make any sense to me to change it.

This goes back to the original question of whether we should remove the
write bit for read migration entry. Well, let's just focus on others;
we're all tired of this one.

> > And now we have proof that
> > mprotect() just behaves exactly the same way, using the basic rules of vma->vm_page_prot.
> >
> > Yes, there is broken sparc64 (below), but that shouldn't dictate our implementation.

I doubt whether sparc64 is broken if it has been like that anyway, because
I know little on sparc64 so I guess I'd not speak on that.

> >
> >
> > What *would* make sense to me, as I raised, is:
> >
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index dff333593a8a..9fc181fd3c5a 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -213,8 +213,10 @@ static bool remove_migration_pte(struct folio *folio,
> > pte = pte_mkdirty(pte);
> > if (is_writable_migration_entry(entry))
> > pte = maybe_mkwrite(pte, vma);
> > - else if (pte_swp_uffd_wp(*pvmw.pte))
> > + else if (pte_swp_uffd_wp(*pvmw.pte)) {
> > pte = pte_mkuffd_wp(pte);
> > + pt = pte_wrprotect(pte);
> > + }
> > if (folio_test_anon(folio) && !is_readable_migration_entry(entry))
> > rmap_flags |= RMAP_EXCLUSIVE;
> >
> >
> > It still requires patch each and every possible code location, which I dislike as
> > described in the patch description. The fact that there are still uffd-wp bugs
> > with your patch makes that hopefully clear. I'd be interested if they can be
> > reproduced witht his patch.
> >
>
> And if NUMA hinting is indeed the problem, without this patch what would
> be required would most probably be:
>
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 8a6d5c823f91..869d35ef0e24 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4808,6 +4808,8 @@ static vm_fault_t do_numa_page(struct vm_fault *vmf)
> pte = pte_mkyoung(pte);
> if (was_writable)
> pte = pte_mkwrite(pte);
> + if (pte_uffd_wp(pte))
> + pte = pte_wrprotect(pte);
> ptep_modify_prot_commit(vma, vmf->address, vmf->pte, old_pte, pte);
> update_mmu_cache(vma, vmf->address, vmf->pte);
> pte_unmap_unlock(vmf->pte, vmf->ptl);
>
>
> And just to make my point about the migration path clearer: doing it your way
> would be:
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 8a6d5c823f91..a7c4c1a57f6a 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4808,6 +4808,8 @@ static vm_fault_t do_numa_page(struct vm_fault *vmf)
> pte = pte_mkyoung(pte);
> if (was_writable)
> pte = pte_mkwrite(pte);
> + else
> + pte = pte_wrprotect(pte);
> ptep_modify_prot_commit(vma, vmf->address, vmf->pte, old_pte, pte);
> update_mmu_cache(vma, vmf->address, vmf->pte);
> pte_unmap_unlock(vmf->pte, vmf->ptl);
>
>
> And I don't think that's the right approach.

Yes, but now I'm prone to the patch I attached which should just cover all
pte_mkuffd_wp().

Side note: since looking at the numa code, I found that after the recent
rework of removing savedwrite for numa, cdb205f9e220 ("mm/autonuma: use
can_change_(pte|pmd)_writable() to replace savedwrite"), I think it can
happen that after numa balancing one pte under uffd-wp vma (but not
wr-protected) can have its write bit lost if the migration failed during
recovering, because vma_wants_manual_pte_write_upgrade() will return false
for such case. Is it true?

--
Peter Xu

Attachments:

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2022-12-06 01:23:14

by kernel test robot

[permalink] [raw]

Subject: Re: [PATCH] mm/uffd: Always wr-protect pte in pte_mkuffd_wp()

Attachments:

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2022-12-06 11:58:52

by kernel test robot

[permalink] [raw]

Subject: Re: [PATCH] mm/uffd: Always wr-protect pte in pte_mkuffd_wp()

Hi Peter,

Thank you for the patch! Yet something to improve:

[auto build test ERROR on akpm-mm/mm-everything]

url: https://github.com/intel-lab-lkp/linux/commits/Peter-Xu/mm-uffd-Always-wr-protect-pte-in-pte_mkuffd_wp/20221206-050912
base: https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything
patch link: https://lore.kernel.org/r/Y45duzmGGUT0%2Bu8t%40x1n
patch subject: [PATCH] mm/uffd: Always wr-protect pte in pte_mkuffd_wp()
config: x86_64-randconfig-a004-20221205
compiler: clang version 14.0.6 (https://github.com/llvm/llvm-project f28c006a5895fc0e329fe15fead81e37457cb1d1)
reproduce (this is a W=1 build):
wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
chmod +x ~/bin/make.cross
# https://github.com/intel-lab-lkp/linux/commit/ddd9626e3f22a7c2c263c0db4d062a7cc60dfa15
git remote add linux-review https://github.com/intel-lab-lkp/linux
git fetch --no-tags linux-review Peter-Xu/mm-uffd-Always-wr-protect-pte-in-pte_mkuffd_wp/20221206-050912
git checkout ddd9626e3f22a7c2c263c0db4d062a7cc60dfa15
# save the config file
mkdir build_dir && cp config build_dir/.config
COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross W=1 O=build_dir ARCH=x86_64 prepare

If you fix the issue, kindly add following tag where applicable
| Reported-by: kernel test robot <[email protected]>

All errors (new ones prefixed by >>):

make[5]: *** No rule to make target 'tools/include/uapi/linux/stddef.h', needed by 'tools/bpf/resolve_btfids/libbpf/staticobjs/libbpf.o'. Stop.
make[4]: *** [Makefile:157: tools/bpf/resolve_btfids/libbpf/staticobjs/libbpf-in.o] Error 2
make[4]: *** Waiting for unfinished jobs....
make[3]: *** [Makefile:55: tools/bpf/resolve_btfids//libbpf/libbpf.a] Error 2
make[2]: *** [Makefile:76: bpf/resolve_btfids] Error 2
make[1]: *** [Makefile:1423: tools/bpf/resolve_btfids] Error 2
In file included from arch/x86/kernel/asm-offsets.c:13:
In file included from include/linux/suspend.h:5:
In file included from include/linux/swap.h:9:
In file included from include/linux/memcontrol.h:20:
In file included from include/linux/mm.h:29:
In file included from include/linux/pgtable.h:6:
>> arch/x86/include/asm/pgtable.h:316:9: error: implicit declaration of function 'pte_wrprotect' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
return pte_wrprotect(pte_set_flags(pte, _PAGE_UFFD_WP));
^
>> arch/x86/include/asm/pgtable.h:316:9: error: returning 'int' from a function with incompatible result type 'pte_t'
return pte_wrprotect(pte_set_flags(pte, _PAGE_UFFD_WP));
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>> arch/x86/include/asm/pgtable.h:335:21: error: static declaration of 'pte_wrprotect' follows non-static declaration
static inline pte_t pte_wrprotect(pte_t pte)
^
arch/x86/include/asm/pgtable.h:316:9: note: previous implicit declaration is here
return pte_wrprotect(pte_set_flags(pte, _PAGE_UFFD_WP));
^
3 errors generated.
make[2]: *** [scripts/Makefile.build:118: arch/x86/kernel/asm-offsets.s] Error 1
make[2]: Target 'prepare' not remade because of errors.
make[1]: *** [Makefile:1270: prepare0] Error 2
make[1]: Target 'prepare' not remade because of errors.
make: *** [Makefile:231: __sub-make] Error 2
make: Target 'prepare' not remade because of errors.

vim +/pte_wrprotect +316 arch/x86/include/asm/pgtable.h

313
314 static inline pte_t pte_mkuffd_wp(pte_t pte)
315 {
> 316 return pte_wrprotect(pte_set_flags(pte, _PAGE_UFFD_WP));
317 }
318
319 static inline pte_t pte_clear_uffd_wp(pte_t pte)
320 {
321 return pte_clear_flags(pte, _PAGE_UFFD_WP);
322 }
323 #endif /* CONFIG_HAVE_ARCH_USERFAULTFD_WP */
324
325 static inline pte_t pte_mkclean(pte_t pte)
326 {
327 return pte_clear_flags(pte, _PAGE_DIRTY);
328 }
329
330 static inline pte_t pte_mkold(pte_t pte)
331 {
332 return pte_clear_flags(pte, _PAGE_ACCESSED);
333 }
334
> 335 static inline pte_t pte_wrprotect(pte_t pte)
336 {
337 return pte_clear_flags(pte, _PAGE_RW);
338 }
339

--
0-DAY CI Kernel Test Service
https://01.org/lkp

Attachments:

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config (107.26 kB)
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2022-12-06 16:38:21

by Peter Xu

[permalink] [raw]

Subject: Re: [PATCH] mm/uffd: Always wr-protect pte in pte_mkuffd_wp()

On Tue, Dec 06, 2022 at 08:46:17AM +0800, kernel test robot wrote:
> vim +316 arch/x86/include/asm/pgtable.h
>
> 313
> 314 static inline pte_t pte_mkuffd_wp(pte_t pte)
> 315 {
> > 316 return pte_wrprotect(pte_set_flags(pte, _PAGE_UFFD_WP));
> 317 }

It's interesting to know the bot will test any patch I attach in an mail
reply, which is very nice...

I'll send a formal patch for this one soon.

--
Peter Xu

2022-12-06 17:06:18

by David Hildenbrand

[permalink] [raw]

Subject: Re: [PATCH RFC] mm/userfaultfd: enable writenotify while userfaultfd-wp is enabled for a VMA

>>>> I think fundamentally the core is uffd-wp is pte-based, so the information
>>>> resides in pte not vma. I'm not strongly objecting this patch, especially
>>>> you mentioned auto-numa so I need to have a closer look later there.
>>>> However I do think uffd-wp is slightly special because we always need to
>>>> consider pte information anyway, so a per-vma information doesn't hugely
>>>> help, IMHO.
>>>
>>> That's the same as softdirty tracking, IMHO.
>
> Soft-dirty doesn't have a bit in the pte showing whether the page is
> protected.

If the page is already softdirty (PTE bit), we don't need write faults
and consequently can map the page writable. If the page is not
softdirty, we need !write as default. But let's talk in detail about
vma->vm_page_prot and interaction with other wrprotect features below.

>
> One wr-protects in soft-dirty with either ALL or NONE. That's per-vma.
>
> One wr-protects in uffd-wp by wr-protect specific page or range of pages.
> That's per-page.
>

Let me try to explain clearer what the issue with uffd-wp on shmem is
right now and how to proceed from here. maybe that makes it clearer what
the vma->vm_page_prot semantics are.

vma->vm_page_prot defines the default PTE/PMD/... protection. This
protection is always *safe*, meaning, if you blindly use that protection
when (re)mapping a page, there won't be correctness issues. No need to
manually wrprotect afterwards.

That's why migration, mprotect(), NUMA faults that all rely on
vma->vm_page_prot work as expected for now.

When calculating vma->vm_page_prot, we considers three things:

(1) VMA protection. For example, no VM_WRITE? then it won't include
write permissions.
(2) Private vs. shared: If the mapping is private, we will never have
write permissions in vma->vm_page_prot, because we might have to
COW individual PTEs. We really want write faults as default.
(3) Write-notify: Some mechanism (softdirty tracking, file system, uffd-
wp) *might* require default write-protection, such that we always
properly trigger a write fault instead where we can handle these.
This only applies to shared mappings, because private mappings
always default to !write (2).

As vma->vm_page_prot is safe, it is always valid to apply them when
mapping a PTE/PMD/ ... *in addition* we can use other information, to
detect if we still can map the PTE writable (if they were removed due to
(2) or (3)).

In migration code, we use the migration type (writable migration
entries). In NUMA-hinting code we used the stored savedwrite value.
Absence of these bits does not imply that we have to map the page
wrprotected.

We never had to remove write permissions so far from the
vma->vm_page_prot default. We always only added permissions.

Now, uffd-wp on shmem as of now violates these semantics.
vma->vm_page_prot cannot always be used as a safe default, because we
might have to wrprotect individual PTEs. Note that for uffd-wp on
anonymous memory this was not an issue, because we default to !write in
vma->vm_page_prot.

The two possible ways to approach this for uffd-wp on shmem are:

(1) Obey existing vma->vm_page_prot semantics. Default to !write and
optimize the relevant cases to *add* the write bit. This is
essentially what this patch does, minus
can_change_pte_writable() optimizations on relevant code paths where
we'd want to maintain the write bit. For example, when removing
uffd-wp protection we might want to restore the write bit directly.

(2) Default to write permissions and check each and every code location
where we remap for uffd-wp ptes, to manuall wrprotect -- *remove*
the write bit. Alternatively, as you said, always wrprotect when
setting the PTE bit, which might work as well.

My claim is that (1) is less error prone, because in the worst case we
forget to optimize one code location -- instead to resulting in a BUG if
we forget to wrprotect (what we have now). But I am not going to fight
for it, because I can see that (2) can be made to work as well, as you
outline in your patch.

You seem to have decided on (2). Fair enough, you know uffd-wp best. We
just have to fix it properly and make the logic consistent whenever we
remap a page.

Is anything I said fundamentally wrong?

>>>
>>> [...]
>>>
>>>>> Running the mprotect() reproducer [1] without this commit:
>>>>> $ ./uffd-wp-mprotect
>>>>> FAIL: uffd-wp did not fire
>>>>> Running the mprotect() reproducer with this commit:
>>>>> $ ./uffd-wp-mprotect
>>>>> PASS: uffd-wp fired
>>>>>
>>>>> [1] https://lore.kernel.org/all/[email protected]/T/#u
>>>>
>>>> I still hope for a formal patch (non-rfc) we can have a reproducer outside
>>>> mprotect(). IMHO mprotect() is really ambiguously here being used with
>>>> uffd-wp, so not a good example IMO as I explained in the other thread [1].
>>>
>>> I took the low hanging fruit to showcase that this is a more generic problem.
>>> The reproducer is IMHO nice because it's simple and race-free.
>
> If no one is using mprotect() with uffd-wp like that, then the reproducer
> may not be valid - the reproducer is defining how it should work, but does
> that really stand? That's why I said it's ambiguous, because the
> definition in this case is unclear.

There are interesting variations like:

mmap(PROT_READ, MAP_POPULATE|MAP_SHARED)
uffd_wp()
mprotect(PROT_READ|PROT_WRITE)

Where we start out with all-write permissions before we enable selective
write permissions.

But I'm not going to argue about whats valid and whats not as long as
it's documented ;). I primarily wanted to showcase that the same logic
based on vma->vm_page_prot is used elsewhere, and that migration PTE
restoration is not particularly special.

>
> I think numa has the problem too which I agree with you. If you attach a
> numa reproducer it'll be nicer. But again I'm not convinced uffd-wp is a
> per-vma thing, which seems to be what this patch is based upon.

I might be able to give NUMA hinting a shot later this week, but I have
other stuff to focus on.

>
> Now I really wonder whether I should just simply wr-protect pte for
> pte_mkuffd_wp() always, attached. I didn't do that from the start because
> I wanted to keep the helpers operate on one bit only. But I found that
> it's actually common technique to use in pgtable arch code, and it really
> doesn't make sense to not wr-protect a pte if uffd-wp is set on a present
> entry. It's much safer.

Indeed, that would be much safer.

>
>>>
>>>>
>>>> I'll need to off-work most of the rest of today, but maybe I can also have
>>>> a look in the weekend or Monday more on the numa paths. Before that, can
>>>> we first reach a consensus that we have the mm/migrate patch there to be
>>>> merged first? These are two issues, IMHO.
>>>>
>>>> I know you're against me for some reason, but until now I sincerely don't
>>>> know why. That patch sololy recovers write bit status (by removing it for
>>>> read-only) for a migration entry and that definitely makes sense to me. As
>>>> I also mentioned in the old version of that thread, we can rework migration
>>>> entries and merge READ|WRITE entries into a GENERIC entry one day if you
>>>> think proper, but that's for later.
>>>
>>> I'm not against you. I'm against changing well-working, common code
>>> when it doesn't make any sense to me to change it.
>
> This goes back to the original question of whether we should remove the
> write bit for read migration entry. Well, let's just focus on others;
> we're all tired of this one.

I hope my lengthy explanation above was helpful and correct.

>
>>> And now we have proof that
>>> mprotect() just behaves exactly the same way, using the basic rules of vma->vm_page_prot.
>>>
>>> Yes, there is broken sparc64 (below), but that shouldn't dictate our implementation.
>
> I doubt whether sparc64 is broken if it has been like that anyway, because
> I know little on sparc64 so I guess I'd not speak on that.
>

I'd wish some of the sparc64 maintainers would speak up finally. Anyhow,
most probably I'll write a reproducer for some broken case and send it
to the sparc64 list. Yeah, I need to get Linux for sparc64 running
inside a VM ... :/

[...]

>
> Yes, but now I'm prone to the patch I attached which should just cover all
> pte_mkuffd_wp().

We should probably do the same for the pmd variants, and clean up the
existing wrprotect like:
pmde = pmd_wrprotect(pmd_mkuffd_wp(pmde));

But that's of course, not a fix.

>
> Side note: since looking at the numa code, I found that after the recent
> rework of removing savedwrite for numa, cdb205f9e220 ("mm/autonuma: use
> can_change_(pte|pmd)_writable() to replace savedwrite"), I think it can
> happen that after numa balancing one pte under uffd-wp vma (but not
> wr-protected) can have its write bit lost if the migration failed during
> recovering, because vma_wants_manual_pte_write_upgrade() will return false
> for such case. Is it true?

Yes, you are correct. I added that to the patch description:

"
Note that we don't optimize for the actual migration case:
(1) When migration succeeds the new PTE will not be writable because
the source PTE was not writable (protnone); in the future we
might just optimize that case similarly by reusing
can_change_pte_writable()/can_change_pmd_writable() when
removing migration PTEs.
(2) When migration fails, we'd have to recalculate the "writable"
flag because we temporarily dropped the PT lock; for now keep it
simple and set "writable=false".
"

Case (1) would, with your current patch, always lose the write bit
during migration, even if vma->vm_page_prot included it. We most might
want to optimize that in the future.

Case (2) is rather a corner case, and unless people complain about it
being a real performance issue, it felt cleaner (less code) to not
optimize for that now.

Again Peter, I am not against you, not at all. Sorry if I gave you the
impression. I highly appreciate your work and this discussion.

--
Thanks,

David / dhildenb

2022-12-06 18:45:02

by kernel test robot

[permalink] [raw]

Subject: Re: [PATCH] mm/uffd: Always wr-protect pte in pte_mkuffd_wp()

Hi Peter,

Thank you for the patch! Yet something to improve:

[auto build test ERROR on akpm-mm/mm-everything]

url: https://github.com/intel-lab-lkp/linux/commits/Peter-Xu/mm-uffd-Always-wr-protect-pte-in-pte_mkuffd_wp/20221206-050912
base: https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything
patch link: https://lore.kernel.org/r/Y45duzmGGUT0%2Bu8t%40x1n
patch subject: [PATCH] mm/uffd: Always wr-protect pte in pte_mkuffd_wp()
config: i386-randconfig-a003-20221205
compiler: clang version 14.0.6 (https://github.com/llvm/llvm-project f28c006a5895fc0e329fe15fead81e37457cb1d1)
reproduce (this is a W=1 build):
wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
chmod +x ~/bin/make.cross
# https://github.com/intel-lab-lkp/linux/commit/ddd9626e3f22a7c2c263c0db4d062a7cc60dfa15
git remote add linux-review https://github.com/intel-lab-lkp/linux
git fetch --no-tags linux-review Peter-Xu/mm-uffd-Always-wr-protect-pte-in-pte_mkuffd_wp/20221206-050912
git checkout ddd9626e3f22a7c2c263c0db4d062a7cc60dfa15
# save the config file
mkdir build_dir && cp config build_dir/.config
COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross W=1 O=build_dir ARCH=i386 SHELL=/bin/bash

If you fix the issue, kindly add following tag where applicable
| Reported-by: kernel test robot <[email protected]>

All errors (new ones prefixed by >>):

In file included from fs/btrfs/disk-io.c:13:
In file included from include/linux/migrate.h:8:
In file included from include/linux/hugetlb.h:830:
In file included from arch/x86/include/asm/hugetlb.h:6:
>> include/asm-generic/hugetlb.h:40:9: error: implicit declaration of function 'huge_pte_wrprotect' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
return huge_pte_wrprotect(pte_mkuffd_wp(pte));
^
>> include/asm-generic/hugetlb.h:40:9: error: returning 'int' from a function with incompatible result type 'pte_t'
return huge_pte_wrprotect(pte_mkuffd_wp(pte));
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>> include/asm-generic/hugetlb.h:108:21: error: static declaration of 'huge_pte_wrprotect' follows non-static declaration
static inline pte_t huge_pte_wrprotect(pte_t pte)
^
include/asm-generic/hugetlb.h:40:9: note: previous implicit declaration is here
return huge_pte_wrprotect(pte_mkuffd_wp(pte));
^
3 errors generated.
--
In file included from mm/mprotect.c:13:
In file included from include/linux/hugetlb.h:830:
In file included from arch/x86/include/asm/hugetlb.h:6:
>> include/asm-generic/hugetlb.h:40:9: error: implicit declaration of function 'huge_pte_wrprotect' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
return huge_pte_wrprotect(pte_mkuffd_wp(pte));
^
>> include/asm-generic/hugetlb.h:40:9: error: returning 'int' from a function with incompatible result type 'pte_t'
return huge_pte_wrprotect(pte_mkuffd_wp(pte));
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>> include/asm-generic/hugetlb.h:108:21: error: static declaration of 'huge_pte_wrprotect' follows non-static declaration
static inline pte_t huge_pte_wrprotect(pte_t pte)
^
include/asm-generic/hugetlb.h:40:9: note: previous implicit declaration is here
return huge_pte_wrprotect(pte_mkuffd_wp(pte));
^
In file included from mm/mprotect.c:15:
include/linux/mman.h:154:9: warning: division by zero is undefined [-Wdivision-by-zero]
_calc_vm_trans(flags, MAP_SYNC, VM_SYNC ) |
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/linux/mman.h:132:21: note: expanded from macro '_calc_vm_trans'
: ((x) & (bit1)) / ((bit1) / (bit2))))
^ ~~~~~~~~~~~~~~~~~
1 warning and 3 errors generated.

vim +/huge_pte_wrprotect +40 include/asm-generic/hugetlb.h

37
38 static inline pte_t huge_pte_mkuffd_wp(pte_t pte)
39 {
> 40 return huge_pte_wrprotect(pte_mkuffd_wp(pte));
41 }
42
43 static inline pte_t huge_pte_clear_uffd_wp(pte_t pte)
44 {
45 return pte_clear_uffd_wp(pte);
46 }
47
48 static inline int huge_pte_uffd_wp(pte_t pte)
49 {
50 return pte_uffd_wp(pte);
51 }
52
53 #ifndef __HAVE_ARCH_HUGE_PTE_CLEAR
54 static inline void huge_pte_clear(struct mm_struct *mm, unsigned long addr,
55 pte_t *ptep, unsigned long sz)
56 {
57 pte_clear(mm, addr, ptep);
58 }
59 #endif
60
61 #ifndef __HAVE_ARCH_HUGETLB_FREE_PGD_RANGE
62 static inline void hugetlb_free_pgd_range(struct mmu_gather *tlb,
63 unsigned long addr, unsigned long end,
64 unsigned long floor, unsigned long ceiling)
65 {
66 free_pgd_range(tlb, addr, end, floor, ceiling);
67 }
68 #endif
69
70 #ifndef __HAVE_ARCH_HUGE_SET_HUGE_PTE_AT
71 static inline void set_huge_pte_at(struct mm_struct *mm, unsigned long addr,
72 pte_t *ptep, pte_t pte)
73 {
74 set_pte_at(mm, addr, ptep, pte);
75 }
76 #endif
77
78 #ifndef __HAVE_ARCH_HUGE_PTEP_GET_AND_CLEAR
79 static inline pte_t huge_ptep_get_and_clear(struct mm_struct *mm,
80 unsigned long addr, pte_t *ptep)
81 {
82 return ptep_get_and_clear(mm, addr, ptep);
83 }
84 #endif
85
86 #ifndef __HAVE_ARCH_HUGE_PTEP_CLEAR_FLUSH
87 static inline pte_t huge_ptep_clear_flush(struct vm_area_struct *vma,
88 unsigned long addr, pte_t *ptep)
89 {
90 return ptep_clear_flush(vma, addr, ptep);
91 }
92 #endif
93
94 #ifndef __HAVE_ARCH_HUGE_PTE_NONE
95 static inline int huge_pte_none(pte_t pte)
96 {
97 return pte_none(pte);
98 }
99 #endif
100
101 /* Please refer to comments above pte_none_mostly() for the usage */
102 static inline int huge_pte_none_mostly(pte_t pte)
103 {
104 return huge_pte_none(pte) || is_pte_marker(pte);
105 }
106
107 #ifndef __HAVE_ARCH_HUGE_PTE_WRPROTECT
> 108 static inline pte_t huge_pte_wrprotect(pte_t pte)
109 {
110 return pte_wrprotect(pte);
111 }
112 #endif
113

--
0-DAY CI Kernel Test Service
https://01.org/lkp

Attachments:

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config (152.63 kB)
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2022-12-06 19:18:20

by Hugh Dickins

[permalink] [raw]

Subject: Re: [PATCH RFC] mm/userfaultfd: enable writenotify while userfaultfd-wp is enabled for a VMA

On Tue, 6 Dec 2022, David Hildenbrand wrote:
...
>
> We never had to remove write permissions so far from the vma->vm_page_prot
> default. We always only added permissions.
>
>
> Now, uffd-wp on shmem as of now violates these semantics. vma->vm_page_prot
> cannot always be used as a safe default, because we might have to wrprotect
> individual PTEs. Note that for uffd-wp on anonymous memory this was not an
> issue, because we default to !write in vma->vm_page_prot.
>
>
> The two possible ways to approach this for uffd-wp on shmem are:
>
> (1) Obey existing vma->vm_page_prot semantics. Default to !write and
> optimize the relevant cases to *add* the write bit. This is
> essentially what this patch does, minus
> can_change_pte_writable() optimizations on relevant code paths where
> we'd want to maintain the write bit. For example, when removing
> uffd-wp protection we might want to restore the write bit directly.
>
> (2) Default to write permissions and check each and every code location
> where we remap for uffd-wp ptes, to manuall wrprotect -- *remove*
> the write bit. Alternatively, as you said, always wrprotect when
> setting the PTE bit, which might work as well.
>
>
> My claim is that (1) is less error prone, because in the worst case we forget
> to optimize one code location -- instead to resulting in a BUG if we forget to
> wrprotect (what we have now). But I am not going to fight for it, because I
> can see that (2) can be made to work as well, as you outline in your patch.
>
> You seem to have decided on (2). Fair enough, you know uffd-wp best. We just
> have to fix it properly and make the logic consistent whenever we remap a
> page.
>
...
>
> But I'm not going to argue about whats valid and whats not as long as it's
> documented ;). I primarily wanted to showcase that the same logic based on
> vma->vm_page_prot is used elsewhere, and that migration PTE restoration is not
> particularly special.

I have not been following the uffd-wp work, but I believe that David's
painstaking and excellent account of vm_page_prot is correct. Peter,
please I beg you to follow his advice and go for (1) for uffd-wp.

I do not share David's faith in "documented": documented or not,
depart from safe convention and you will be adding (at least the
opportunity for) serious bugs.

Hugh

2022-12-06 22:17:14

by Peter Xu

[permalink] [raw]

Subject: Re: [PATCH RFC] mm/userfaultfd: enable writenotify while userfaultfd-wp is enabled for a VMA

Hi, Hugh,

On Tue, Dec 06, 2022 at 11:09:40AM -0800, Hugh Dickins wrote:
> I have not been following the uffd-wp work, but I believe that David's
> painstaking and excellent account of vm_page_prot is correct. Peter,
> please I beg you to follow his advice and go for (1) for uffd-wp.

Thanks for jumping in, I will. Your input definitely matters.

I think the fundamental moot point was whether vm_page_prot is the "safest"
or "default" behavior of vma.

I thought it was the "default" and I don't want to have VM_UFFD_WP regions
to have default no-write attribute comparing to generic ones. I wanted it
to behave exactly like a normal shmem vma if no one explicitly does
something else.

If it's "safest" and you also agree then I think indeed VM_UFFD_WP falls
into that category; frankly I don't have a solid clue before. Maybe that
also matches better with the recent vma_wants_manual_pte_write_upgrade(),
or we start to lose write bit in do_numa_page() at least for uffd-wp
protected ranges when recovering the ptes, and it'll be ugly to have:

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 3a3d23b3dbe2..718d540b9eb4 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2086,7 +2086,7 @@ static inline bool vma_wants_manual_pte_write_upgrade(struct vm_area_struct *vma
* private mappings, that's always the case when we have write
* permissions as we properly have to handle COW.
*/
- if (vma->vm_flags & VM_SHARED)
+ if ((vma->vm_flags & VM_SHARED) && !userfaultfd_wp(vma))
return vma_wants_writenotify(vma, vma->vm_page_prot);
return !!(vma->vm_flags & VM_WRITE);

To recover the write bit.

I tried to move forward with the 47ba8bcc33b2 ("mm/migrate: fix read-only
page got writable when recover pte", 2022-11-25) in mm-unstable upstream
because that seems pretty obvious to me and that's verified in the test
beds, but I obviously failed anyway. Let's adjust the patches with the
best we can have.

> I do not share David's faith in "documented": documented or not,
> depart from safe convention and you will be adding (at least the
> opportunity for) serious bugs.

Yes I agree not having the write bit is safer. That's also why I wanted to
move the wrprotect into pte_mkuffd_wp. I assume from "resolving problem"
pov they're the same, but I'll follow your advise.

David, would you please repost a new patch for this one and copy Ives to
make sure it'll work for him in his systems?

I'd suggest to drop the mprotect example, I'll reply later on that to the
other email but I still don't know whether it's a good example for a reader
to understand the problem.

No reproducer needed for numa I think - I guess Ives's test case would be
far enough to verify it if possible. I also hope what Ives saw was the
numa balancing issue you reported, so maybe it'll resolve all problem he
has. Then with that verified and queued we can drop the mm/migrate patch.

Thanks,

--
Peter Xu

2022-12-06 22:46:48

by Peter Xu

[permalink] [raw]

Subject: Re: [PATCH RFC] mm/userfaultfd: enable writenotify while userfaultfd-wp is enabled for a VMA

On Tue, Dec 06, 2022 at 05:28:07PM +0100, David Hildenbrand wrote:
> > If no one is using mprotect() with uffd-wp like that, then the reproducer
> > may not be valid - the reproducer is defining how it should work, but does
> > that really stand? That's why I said it's ambiguous, because the
> > definition in this case is unclear.
>
> There are interesting variations like:
>
> mmap(PROT_READ, MAP_POPULATE|MAP_SHARED)
> uffd_wp()
> mprotect(PROT_READ|PROT_WRITE)
>
> Where we start out with all-write permissions before we enable selective
> write permissions.

Could you elaborate what's the difference of above comparing to:

mmap(PROT_READ|PROT_WRITE, MAP_POPULATE|MAP_SHARED)
uffd_wp()

?

[...]

> Yes, you are correct. I added that to the patch description:
>
> "
> Note that we don't optimize for the actual migration case:
> (1) When migration succeeds the new PTE will not be writable because
> the source PTE was not writable (protnone); in the future we
> might just optimize that case similarly by reusing
> can_change_pte_writable()/can_change_pmd_writable() when
> removing migration PTEs.
> (2) When migration fails, we'd have to recalculate the "writable"
> flag because we temporarily dropped the PT lock; for now keep it
> simple and set "writable=false".
> "
>
> Case (1) would, with your current patch, always lose the write bit during
> migration, even if vma->vm_page_prot included it. We most might want to
> optimize that in the future.
>
> Case (2) is rather a corner case, and unless people complain about it being
> a real performance issue, it felt cleaner (less code) to not optimize for
> that now.

As I didn't have a closer look on the savedwrite removal patchset so I may
not speak anything sensible here.. What I hope is that we don't lose write
bits easily, after all we tried to even safe the dirty and young bits to
avoid the machine cycles in the MMUs.

>
> Again Peter, I am not against you, not at all. Sorry if I gave you the
> impression. I highly appreciate your work and this discussion.

No worry on that part. You're doing great in this email explaining things
and write things up, especially I'm happy Hugh confirmed it so it's good to
have those. Let's start with something like this when you NAK something
next time. :)

Thanks,

--
Peter Xu

2022-12-07 14:35:02

by David Hildenbrand

[permalink] [raw]

Subject: Re: [PATCH RFC] mm/userfaultfd: enable writenotify while userfaultfd-wp is enabled for a VMA

On 06.12.22 22:27, Peter Xu wrote:
> On Tue, Dec 06, 2022 at 05:28:07PM +0100, David Hildenbrand wrote:
>>> If no one is using mprotect() with uffd-wp like that, then the reproducer
>>> may not be valid - the reproducer is defining how it should work, but does
>>> that really stand? That's why I said it's ambiguous, because the
>>> definition in this case is unclear.
>>
>> There are interesting variations like:
>>
>> mmap(PROT_READ, MAP_POPULATE|MAP_SHARED)
>> uffd_wp()
>> mprotect(PROT_READ|PROT_WRITE)
>>
>> Where we start out with all-write permissions before we enable selective
>> write permissions.
>
> Could you elaborate what's the difference of above comparing to:
>
> mmap(PROT_READ|PROT_WRITE, MAP_POPULATE|MAP_SHARED)
> uffd_wp()
>
> ?

That mapping would temporarily allow write access. I'd imagine that
something like that might be useful when atomically replacing an
existing mapping (MAP_FIXED), and the VMA might already be in use by
other threads. or when you really want to catch any possible write access.

For example, libvhost-user.c in QEMU uses for ordinary postcopy:

/*
* In postcopy we're using PROT_NONE here to catch anyone
* accessing it before we userfault.
*/
mmap_addr = mmap(0, dev_region->size + dev_region->mmap_offset,
PROT_NONE, MAP_SHARED | MAP_NORESERVE,
vmsg->fds[0], 0);

I'd imagine, when using uffd-wp (VM snapshotting with shmem?) one might
use PROT_READ instead before the write-protection is properly set.
Because read access would be fine in the meantime.

But I'm just pulling use cases out of my magic hat ;) Nothing stops user
space from doing things that are not clearly forbidden (well, even then
users might complain, but that's a different story).

[...]

>> Case (2) is rather a corner case, and unless people complain about it being
>> a real performance issue, it felt cleaner (less code) to not optimize for
>> that now.
>
> As I didn't have a closer look on the savedwrite removal patchset so I may
> not speak anything sensible here.. What I hope is that we don't lose write
> bits easily, after all we tried to even safe the dirty and young bits to
> avoid the machine cycles in the MMUs.

Hopefully, someone will complain loudly if that corner case is relevant.

>
>>
>> Again Peter, I am not against you, not at all. Sorry if I gave you the
>> impression. I highly appreciate your work and this discussion.
>
> No worry on that part. You're doing great in this email explaining things
> and write things up, especially I'm happy Hugh confirmed it so it's good to
> have those. Let's start with something like this when you NAK something
> next time. :)

:)

--
Thanks,

David / dhildenb

2022-12-07 16:07:26

by David Hildenbrand

[permalink] [raw]

Subject: Re: [PATCH RFC] mm/userfaultfd: enable writenotify while userfaultfd-wp is enabled for a VMA

>
> David, would you please repost a new patch for this one and copy Ives to
> make sure it'll work for him in his systems?

Yes, will do. I think has was already cc'ed.

>
> I'd suggest to drop the mprotect example, I'll reply later on that to the
> other email but I still don't know whether it's a good example for a reader
> to understand the problem.

Yes, can do.

>
> No reproducer needed for numa I think - I guess Ives's test case would be
> far enough to verify it if possible. I also hope what Ives saw was the
> numa balancing issue you reported, so maybe it'll resolve all problem he
> has. Then with that verified and queued we can drop the mm/migrate patch.

I tried writing a numa hinting reproducer, but so far I assume that it's
with current code not (easily) possible for shmem.

We'd have to call change_prot_numa() in order to protnone these PTEs.
That can either happen via

a) mbind() with MPOL_MF_LAZY. However, user space is currently not able
to set that flag (dead code ...).
b) task_numa_work(). However, vma_policy_mof() seems to always fail on
shmem and prevent us from reaching that code. Reason is that shmem
implements vm_ops->get_policy, and it seems to be impossible to get
MPOL_F_MOF set. At least I haven't found an easy way or I am missing
something.

So numa hinting might not immediately explain the lost write faults.

... but are there other ways to reach do_numa_page(), even without
active NUMA hinting? I found at least one:

map = mmap(NULL, size, PROT_WRITE, MAP_SHARED|MAP_ANON, -1, 0);
memset(map, 0, size);
uffd_wp_range(map, size);

On upstream during the next write fault, we'll end up in do_numa_page()
and simply remap the page writable due to vm_page_prot, not triggering a
write fault. I can see the "numa_hint_faults" counter in /proc/vmstat
increasing accordingly, so we're really in do_numa_page().

PROT_WRITE on shmem with uffd-wp is completely non-functional as it
seems. It seems to work with anon memory. And with my patch, it also
works with shmem.

Attaching a simple reproducer (uffd-wp-prot-write.c).

Independent of uffd-wp on shmem, we seem to be missing propagating the
uffd-wp bit when splitting the huge zeropage. So uffd-wp'ing the huge
zeropage and then splitting it loses the uffd-wp markers. :/

Fix seems easy and I just tested my possible fix. Attaching a simple
reproducer (uffd-wp-huge-zero.c).

--
Thanks,

David / dhildenb

Attachments:

uffd-wp-prot-write.c (3.63 kB)
uffd-wp-huge-zero.c (4.05 kB)
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2022-12-07 16:25:26

by Peter Xu

[permalink] [raw]

Subject: Re: [PATCH RFC] mm/userfaultfd: enable writenotify while userfaultfd-wp is enabled for a VMA

On Wed, Dec 07, 2022 at 02:33:58PM +0100, David Hildenbrand wrote:
> On 06.12.22 22:27, Peter Xu wrote:
> > On Tue, Dec 06, 2022 at 05:28:07PM +0100, David Hildenbrand wrote:
> > > > If no one is using mprotect() with uffd-wp like that, then the reproducer
> > > > may not be valid - the reproducer is defining how it should work, but does
> > > > that really stand? That's why I said it's ambiguous, because the
> > > > definition in this case is unclear.
> > >
> > > There are interesting variations like:
> > >
> > > mmap(PROT_READ, MAP_POPULATE|MAP_SHARED)
> > > uffd_wp()
> > > mprotect(PROT_READ|PROT_WRITE)
> > >
> > > Where we start out with all-write permissions before we enable selective
> > > write permissions.
> >
> > Could you elaborate what's the difference of above comparing to:
> >
> > mmap(PROT_READ|PROT_WRITE, MAP_POPULATE|MAP_SHARED)
> > uffd_wp()
> >
> > ?
>
> That mapping would temporarily allow write access. I'd imagine that
> something like that might be useful when atomically replacing an existing
> mapping (MAP_FIXED), and the VMA might already be in use by other threads.
> or when you really want to catch any possible write access.
>
> For example, libvhost-user.c in QEMU uses for ordinary postcopy:
>
> /*
> * In postcopy we're using PROT_NONE here to catch anyone
> * accessing it before we userfault.
> */
> mmap_addr = mmap(0, dev_region->size + dev_region->mmap_offset,
> PROT_NONE, MAP_SHARED | MAP_NORESERVE,
> vmsg->fds[0], 0);

I assume this is for missing mode only. More on wr-protect mode below.

Personally I don't see immediately on whether this is needed. If the
process itself is trusted then it should be under control of anyone who
will be accessing the pages.. If the other threads are not trusted, then
there's no way to stop anyone from mprotect(RW) after mprotect(NONE)
anyway..

So I may not really get the gut of it.

Another way to make sure no one access it is right after receiving the
memory range from QEMU (VhostUserMemoryRegion), if VuDev.postcopy_listening
is set, then we register the range with UFFD missing immediately. After
all if postcopy_listening is set it means we passed the advise phase
already (VHOST_USER_POSTCOPY_ADVISE). Any potential access will be blocked
until QEMU starts to read on that uffd.

>
> I'd imagine, when using uffd-wp (VM snapshotting with shmem?) one might use
> PROT_READ instead before the write-protection is properly set. Because read
> access would be fine in the meantime.

It'll be different for wr-protect IIUC, because unlike missing protections,
we don't worry about writes happening before UFFDIO_WRITEPROTECT.

IMHO the solo thing the vhost-user proc needs to do is one
UFFDIO_WRITEPROTECT for each of the range when QEMU tells it to, then it'll
be fine. Pre-writes are fine.

Sorry I probably went a bit off-topic. I just want to make sure I don't
miss any real use case of having mprotect being useful for uffd-wp being
there, because that used to be a grey area for me.

>
> But I'm just pulling use cases out of my magic hat ;) Nothing stops user
> space from doing things that are not clearly forbidden (well, even then
> users might complain, but that's a different story).

Yes, I think those are always fine but the user just cannot assume it'll
work as they assumed how it will work.

If "doing things that are not clearly forbidden" triggers a host warning or
crash that's a bug, OTOH if the outcome is limited to the process itself
then from kernel pov I think we're good. I used to even thought about
forbid mprotect() on uffd-wp but I'm not sure whether it's good idea either.

Let's see whether I missed something above, if so I'll rethink.

Thanks,

--
Peter Xu

2022-12-07 17:56:42

by Peter Xu

[permalink] [raw]

Subject: Re: [PATCH RFC] mm/userfaultfd: enable writenotify while userfaultfd-wp is enabled for a VMA

On Wed, Dec 07, 2022 at 04:32:17PM +0100, David Hildenbrand wrote:
> >
> > David, would you please repost a new patch for this one and copy Ives to
> > make sure it'll work for him in his systems?
>
> Yes, will do. I think has was already cc'ed.
>
> >
> > I'd suggest to drop the mprotect example, I'll reply later on that to the
> > other email but I still don't know whether it's a good example for a reader
> > to understand the problem.
>
> Yes, can do.
>
> >
> > No reproducer needed for numa I think - I guess Ives's test case would be
> > far enough to verify it if possible. I also hope what Ives saw was the
> > numa balancing issue you reported, so maybe it'll resolve all problem he
> > has. Then with that verified and queued we can drop the mm/migrate patch.
>
> I tried writing a numa hinting reproducer, but so far I assume that it's
> with current code not (easily) possible for shmem.
>
> We'd have to call change_prot_numa() in order to protnone these PTEs. That
> can either happen via
>
> a) mbind() with MPOL_MF_LAZY. However, user space is currently not able
> to set that flag (dead code ...).
> b) task_numa_work(). However, vma_policy_mof() seems to always fail on
> shmem and prevent us from reaching that code. Reason is that shmem
> implements vm_ops->get_policy, and it seems to be impossible to get
> MPOL_F_MOF set. At least I haven't found an easy way or I am missing
> something.
>
> So numa hinting might not immediately explain the lost write faults.
>
> ... but are there other ways to reach do_numa_page(), even without active
> NUMA hinting? I found at least one:
>
>
> map = mmap(NULL, size, PROT_WRITE, MAP_SHARED|MAP_ANON, -1, 0);
> memset(map, 0, size);
> uffd_wp_range(map, size);
>
> On upstream during the next write fault, we'll end up in do_numa_page() and
> simply remap the page writable due to vm_page_prot, not triggering a write
> fault. I can see the "numa_hint_faults" counter in /proc/vmstat increasing
> accordingly, so we're really in do_numa_page().

Seems true. I think fundamentally it's because numa hint rely on PROT_NONE
as the hint, and it explicitly checks against mprotect(PROT_NONE) using the
accessible check:

if (pte_protnone(vmf->orig_pte) && vma_is_accessible(vmf->vma))
return do_numa_page(vmf);

I'm not sure whether we should also add a pte_uffd_wp(vmf->orig_pte) here
to mask out the uffd-wp cases.

So far it seems the outcome is not extremely bad - PROT_WRITE only mappings
are rare in real life, and also with the protnone recovery code (and along
with the vm_page_prot patch coming) we'll be able to still recover the pte
into a uffd-wp-ed pte without PROTNONE bit set. But I don't have a solid
clue yet on what's the best.

>
>
> PROT_WRITE on shmem with uffd-wp is completely non-functional as it seems.
> It seems to work with anon memory. And with my patch, it also works with
> shmem.

There are definitely two problems: (1) unexpected hints to numa even if
it's not, and (2) correct recover of uffd-wp bit when numa hint wants to
recover the old pte. Yes I think it somehow verified that this patch will
also fix the numa recovery path.

>
> Attaching a simple reproducer (uffd-wp-prot-write.c).
>
>
>
> Independent of uffd-wp on shmem, we seem to be missing propagating the
> uffd-wp bit when splitting the huge zeropage. So uffd-wp'ing the huge
> zeropage and then splitting it loses the uffd-wp markers. :/

For this one, thanks for the reproducer. I'm not extremely sure whether
it's a bug.

Firstly, I think your reproducer should just work well with shmem, afaiu,
because shmem is based on pte markers and it should only work on pte level
(not pmd). The huge zero pmd should got split right after wr-protected.
So the reproducer shouldn't go wrong on shmem at all with/without any
recent fix. Let me know otherwise.

For anon, I'm not sure it's a bug, because there's a semantic difference on
anon/shmem. The thing is losing wr-protect on the zero page is the same as
losing wr-protect on a page that is not mapped. For anon currently we
can't track a page that is not mapped and we skip those ranges (being zero
when read). So fundamentally I am not sure whether it'll be an issue for
existing anon uffd-wp users because if it is then it's more than zero
pages.

I know that such a semantic difference between anon and shmem is not good
at all, but just to say maybe that worth another discussion.

Thanks,

>
> Fix seems easy and I just tested my possible fix. Attaching a simple
> reproducer (uffd-wp-huge-zero.c).
>
> --
> Thanks,
>
> David / dhildenb

> #define _GNU_SOURCE
> #include <stdio.h>
> #include <stdlib.h>
> #include <stdint.h>
> #include <stdbool.h>
> #include <inttypes.h>
> #include <string.h>
> #include <fcntl.h>
> #include <unistd.h>
> #include <errno.h>
> #include <poll.h>
> #include <pthread.h>
> #include <sys/mman.h>
> #include <sys/syscall.h>
> #include <sys/ioctl.h>
> #include <linux/memfd.h>
> #include <linux/userfaultfd.h>
>
> static size_t pagesize;
> static int uffd;
> static int pagemap_fd;
>
> #define SIZE (1024*1024*1024ull)
> #define barrier() __asm__ __volatile__("": : :"memory")
>
> volatile bool uffd_triggered;
>
> static void uffd_wp_range(char *start, size_t size, bool wp)
> {
> struct uffdio_writeprotect uffd_writeprotect;
>
> uffd_writeprotect.range.start = (unsigned long) start;
> uffd_writeprotect.range.len = size;
> if (wp) {
> uffd_writeprotect.mode = UFFDIO_WRITEPROTECT_MODE_WP;
> } else {
> uffd_writeprotect.mode = 0;
> }
> if (ioctl(uffd, UFFDIO_WRITEPROTECT, &uffd_writeprotect)) {
> fprintf(stderr, "UFFDIO_WRITEPROTECT failed: %d\n", errno);
> exit(1);
> }
> }
>
> static void *uffd_thread_fn(void *arg)
> {
> static struct uffd_msg msg;
> ssize_t nread;
>
> while (1) {
> struct pollfd pollfd;
> int nready;
>
> pollfd.fd = uffd;
> pollfd.events = POLLIN;
> nready = poll(&pollfd, 1, -1);
> if (nready == -1) {
> fprintf(stderr, "poll() failed: %d\n", errno);
> exit(1);
> }
>
> nread = read(uffd, &msg, sizeof(msg));
> if (nread <= 0)
> continue;
>
> if (msg.event != UFFD_EVENT_PAGEFAULT ||
> !(msg.arg.pagefault.flags & UFFD_PAGEFAULT_FLAG_WP)) {
> printf("FAIL: wrong uffd-wp event fired\n");
> exit(1);
> }
>
> /* un-protect */
> uffd_triggered = true;
> uffd_wp_range((char *)(uintptr_t)msg.arg.pagefault.address, pagesize, false);
> }
> return arg;
> }
>
> static int setup_uffd(char *map, size_t size)
> {
> struct uffdio_api uffdio_api;
> struct uffdio_register uffdio_register;
> pthread_t thread;
>
> uffd = syscall(__NR_userfaultfd,
> O_CLOEXEC | O_NONBLOCK | UFFD_USER_MODE_ONLY);
> if (uffd < 0) {
> fprintf(stderr, "syscall() failed: %d\n", errno);
> return -errno;
> }
>
> uffdio_api.api = UFFD_API;
> uffdio_api.features = UFFD_FEATURE_PAGEFAULT_FLAG_WP;
> if (ioctl(uffd, UFFDIO_API, &uffdio_api) < 0) {
> fprintf(stderr, "UFFDIO_API failed: %d\n", errno);
> return -errno;
> }
>
> if (!(uffdio_api.features & UFFD_FEATURE_PAGEFAULT_FLAG_WP)) {
> fprintf(stderr, "UFFD_FEATURE_WRITEPROTECT missing\n");
> return -ENOSYS;
> }
>
> uffdio_register.range.start = (unsigned long) map;
> uffdio_register.range.len = size;
> uffdio_register.mode = UFFDIO_REGISTER_MODE_WP;
> if (ioctl(uffd, UFFDIO_REGISTER, &uffdio_register) < 0) {
> fprintf(stderr, "UFFDIO_REGISTER failed: %d\n", errno);
> return -errno;
> }
>
> pthread_create(&thread, NULL, uffd_thread_fn, NULL);
>
> return 0;
> }
>
> int main(void)
> {
> const size_t size = SIZE;
> char *map, *cur;
>
> pagesize = getpagesize();
> pagemap_fd = open("/proc/self/pagemap", O_RDONLY);
> if (pagemap_fd < 0) {
> fprintf(stderr, "Opening /proc/self/pagemap failed\n");
> return 1;
> }
>
> map = mmap(NULL, size, PROT_WRITE, MAP_PRIVATE|MAP_ANON, -1, 0);
> if (map == MAP_FAILED) {
> fprintf(stderr, "mmap() failed\n");
> return -errno;
> }
>
> if (madvise(map, size, MADV_NOHUGEPAGE)) {
> fprintf(stderr, "MADV_HUGEPAGE failed\n");
> return -errno;
> }
>
> /* Populate all pages ... */
> memset(map, 0, size);
>
> if (setup_uffd(map, size))
> return 1;
>
> /* ... and write-protect them using uffd-wp. */
> uffd_wp_range(map, size, true);
>
> /* Test if all faults trigger. */
> for (cur = map; cur < map + size; cur += pagesize) {
> uffd_triggered = false;
> barrier();
>
> /* Trigger a write fault. */
> *cur = 1;
>
> barrier();
> if (!uffd_triggered) {
> printf("FAIL: uffd-wp did not trigger\n");
> return 1;
> }
> }
>
> printf("PASS: uffd-wp triggered\n");
> return 0;
> }

> #define _GNU_SOURCE
> #include <stdio.h>
> #include <stdlib.h>
> #include <stdint.h>
> #include <stdbool.h>
> #include <inttypes.h>
> #include <string.h>
> #include <fcntl.h>
> #include <unistd.h>
> #include <errno.h>
> #include <poll.h>
> #include <pthread.h>
> #include <sys/mman.h>
> #include <sys/syscall.h>
> #include <sys/ioctl.h>
> #include <linux/memfd.h>
> #include <linux/userfaultfd.h>
>
> static size_t pagesize;
> static int uffd;
> static int pagemap_fd;
>
> #define SIZE (128*1024*1024ull)
> #define barrier() __asm__ __volatile__("": : :"memory")
>
> volatile bool uffd_triggered;
>
> static void uffd_wp_range(char *start, size_t size, bool wp)
> {
> struct uffdio_writeprotect uffd_writeprotect;
>
> uffd_writeprotect.range.start = (unsigned long) start;
> uffd_writeprotect.range.len = size;
> if (wp) {
> uffd_writeprotect.mode = UFFDIO_WRITEPROTECT_MODE_WP;
> } else {
> uffd_writeprotect.mode = 0;
> }
> if (ioctl(uffd, UFFDIO_WRITEPROTECT, &uffd_writeprotect)) {
> fprintf(stderr, "UFFDIO_WRITEPROTECT failed: %d\n", errno);
> exit(1);
> }
> }
>
> static void *uffd_thread_fn(void *arg)
> {
> static struct uffd_msg msg;
> ssize_t nread;
>
> while (1) {
> struct pollfd pollfd;
> int nready;
>
> pollfd.fd = uffd;
> pollfd.events = POLLIN;
> nready = poll(&pollfd, 1, -1);
> if (nready == -1) {
> fprintf(stderr, "poll() failed: %d\n", errno);
> exit(1);
> }
>
> nread = read(uffd, &msg, sizeof(msg));
> if (nread <= 0)
> continue;
>
> if (msg.event != UFFD_EVENT_PAGEFAULT ||
> !(msg.arg.pagefault.flags & UFFD_PAGEFAULT_FLAG_WP)) {
> printf("FAIL: wrong uffd-wp event fired\n");
> exit(1);
> }
>
> /* un-protect */
> uffd_triggered = true;
> uffd_wp_range((char *)(uintptr_t)msg.arg.pagefault.address, pagesize, false);
> }
> return arg;
> }
>
> static int setup_uffd(char *map, size_t size)
> {
> struct uffdio_api uffdio_api;
> struct uffdio_register uffdio_register;
> pthread_t thread;
>
> uffd = syscall(__NR_userfaultfd,
> O_CLOEXEC | O_NONBLOCK | UFFD_USER_MODE_ONLY);
> if (uffd < 0) {
> fprintf(stderr, "syscall() failed: %d\n", errno);
> return -errno;
> }
>
> uffdio_api.api = UFFD_API;
> uffdio_api.features = UFFD_FEATURE_PAGEFAULT_FLAG_WP;
> if (ioctl(uffd, UFFDIO_API, &uffdio_api) < 0) {
> fprintf(stderr, "UFFDIO_API failed: %d\n", errno);
> return -errno;
> }
>
> if (!(uffdio_api.features & UFFD_FEATURE_PAGEFAULT_FLAG_WP)) {
> fprintf(stderr, "UFFD_FEATURE_WRITEPROTECT missing\n");
> return -ENOSYS;
> }
>
> uffdio_register.range.start = (unsigned long) map;
> uffdio_register.range.len = size;
> uffdio_register.mode = UFFDIO_REGISTER_MODE_WP;
> if (ioctl(uffd, UFFDIO_REGISTER, &uffdio_register) < 0) {
> fprintf(stderr, "UFFDIO_REGISTER failed: %d\n", errno);
> return -errno;
> }
>
> pthread_create(&thread, NULL, uffd_thread_fn, NULL);
>
> return 0;
> }
>
> int main(void)
> {
> const size_t size = SIZE;
> char *map, *cur;
>
> pagesize = getpagesize();
> pagemap_fd = open("/proc/self/pagemap", O_RDONLY);
> if (pagemap_fd < 0) {
> fprintf(stderr, "Opening /proc/self/pagemap failed\n");
> return 1;
> }
>
> map = mmap(NULL, size, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANON, -1, 0);
> if (map == MAP_FAILED) {
> fprintf(stderr, "mmap() failed\n");
> return -errno;
> }
>
> if (madvise(map, size, MADV_HUGEPAGE)) {
> fprintf(stderr, "MADV_HUGEPAGE failed\n");
> return -errno;
> }
>
> if (setup_uffd(map, size))
> return 1;
>
> /* Populate the shared zeropage, hopefullt also the huge one.*/
> madvise(map, size, MADV_POPULATE_READ);
>
> /* ... and write-protect all pages using uffd-wp. */
> uffd_wp_range(map, size, true);
>
> /*
> * Discard every second odd page, this will split any huge zero
> * THP and will hopefully keep the PTE protected using uffd-wp.
> *
> * Any mechanism to PTE-map the THP would do.
> */
> for (cur = map + pagesize; cur < map + size; cur += 2 * pagesize)
> madvise(cur, pagesize, MADV_DONTNEED);
>
> /*
> * Test every second even page (-> all remaining ones) if we get our
> * uffd-wp events.
> */
> for (cur = map; cur < map + size; cur += 2 * pagesize) {
> uffd_triggered = false;
>
> barrier();
> /* Trigger a write fault. */
> *cur = 1;
> barrier();
>
> if (!uffd_triggered) {
> printf("FAIL: uffd-wp did not trigger\n");
> return 1;
> }
> }
>
> printf("PASS: uffd-wp triggered\n");
> return 0;
> }

--
Peter Xu

2022-12-07 20:24:37

by David Hildenbrand

[permalink] [raw]

Subject: Re: [PATCH RFC] mm/userfaultfd: enable writenotify while userfaultfd-wp is enabled for a VMA

>>
>> On upstream during the next write fault, we'll end up in do_numa_page() and
>> simply remap the page writable due to vm_page_prot, not triggering a write
>> fault. I can see the "numa_hint_faults" counter in /proc/vmstat increasing
>> accordingly, so we're really in do_numa_page().
>
> Seems true. I think fundamentally it's because numa hint rely on PROT_NONE
> as the hint, and it explicitly checks against mprotect(PROT_NONE) using the
> accessible check:
>
> if (pte_protnone(vmf->orig_pte) && vma_is_accessible(vmf->vma))
> return do_numa_page(vmf);
>
> I'm not sure whether we should also add a pte_uffd_wp(vmf->orig_pte) here
> to mask out the uffd-wp cases.

:/ more special UFFD-wp casing, I'm not sure sure about that.

Most importantly, once someone unlocks NUMA hinting for shmem (e.g.,
MPOL_MF_LAZY, MPOL_F_NUMA_BALANCING) this might be problematic. That at
least makes it sound fragile to me.

>
> So far it seems the outcome is not extremely bad - PROT_WRITE only mappings
> are rare in real life, and also with the protnone recovery code (and along
> with the vm_page_prot patch coming) we'll be able to still recover the pte
> into a uffd-wp-ed pte without PROTNONE bit set. But I don't have a solid
> clue yet on what's the best.
>

Yes, just another way to trigger surprise uffd-wp behavior (at least
surprising for me ;) ). But this time, not involving mprotect(). I
suspect there are more cases, but I might be wrong.

I was primarily trying to find out which other cases might be affected.

[..]

>>
>>
>> Independent of uffd-wp on shmem, we seem to be missing propagating the
>> uffd-wp bit when splitting the huge zeropage. So uffd-wp'ing the huge
>> zeropage and then splitting it loses the uffd-wp markers. :/
>
> For this one, thanks for the reproducer. I'm not extremely sure whether
> it's a bug.
>
> Firstly, I think your reproducer should just work well with shmem, afaiu,
> because shmem is based on pte markers and it should only work on pte level
> (not pmd). The huge zero pmd should got split right after wr-protected.
> So the reproducer shouldn't go wrong on shmem at all with/without any
> recent fix. Let me know otherwise.

shmem doesn't use the huge shared zeropage, so it should be fine.

>
> For anon, I'm not sure it's a bug, because there's a semantic difference on
> anon/shmem. The thing is losing wr-protect on the zero page is the same as
> losing wr-protect on a page that is not mapped. For anon currently we
> can't track a page that is not mapped and we skip those ranges (being zero
> when read). So fundamentally I am not sure whether it'll be an issue for
> existing anon uffd-wp users because if it is then it's more than zero
> pages.

I think it's a bug, although most probably a low priority one.

Once user space successfully placed an uffd-wp marker, and e.g.,
verified using pagemap that it is indeed placed, the system should not
silently drop it.

The behavior between an ordinary THP and a huge zeropage differs. For
THP, we handle the split correctly and don't lose the marker. Assuming
the huge zeropage woud be disabled, the behavior would be (IMHO)
correct. The test case would pass.

For example, QEMU with uffd-wp based snapshotting will make sure that
all virtual addresses are populated (e.g., mapping the shared,
eventually the huge zeropage -- populate_read_range()), before
protecting using uffd-wp. Losing a uffd-wp marker would be problematic.

The good news is that we barely will end up PTE-mapping the huge
zeropage unless there is real user-space interaction (mprotect(),
mremap(), mmap()), so this shouldn't trigger in the QEMU use-case.

Anyhow, I'll send a patch in a couple of days and we can discuss
further. It's independent of the other discussion, just wanted to report
my findings after staring at that code for way too long today.

--
Thanks,

David / dhildenb

2022-12-07 20:34:36

by Peter Xu

[permalink] [raw]

Subject: Re: [PATCH RFC] mm/userfaultfd: enable writenotify while userfaultfd-wp is enabled for a VMA

On Wed, Dec 07, 2022 at 08:53:36PM +0100, David Hildenbrand wrote:
> Once user space successfully placed an uffd-wp marker, and e.g., verified
> using pagemap that it is indeed placed, the system should not silently drop
> it.

Note that the anon path doesn't use pte markers. We won't lose a pte
marker, hopefully, if we do that's a more severe one.

>
> The behavior between an ordinary THP and a huge zeropage differs. For THP,
> we handle the split correctly and don't lose the marker. Assuming the huge
> zeropage woud be disabled, the behavior would be (IMHO) correct. The test
> case would pass.
>
> For example, QEMU with uffd-wp based snapshotting will make sure that all
> virtual addresses are populated (e.g., mapping the shared, eventually the
> huge zeropage -- populate_read_range()), before protecting using uffd-wp.
> Losing a uffd-wp marker would be problematic.
>
> The good news is that we barely will end up PTE-mapping the huge zeropage
> unless there is real user-space interaction (mprotect(), mremap(), mmap()),
> so this shouldn't trigger in the QEMU use-case.

Ah yes, I forgot that part. If it's not affected then it's better.

>
>
> Anyhow, I'll send a patch in a couple of days and we can discuss further.
> It's independent of the other discussion, just wanted to report my findings
> after staring at that code for way too long today.

Thanks, that works for me.

--
Peter Xu

2022-12-07 20:46:58

by David Hildenbrand

[permalink] [raw]

Subject: Re: [PATCH RFC] mm/userfaultfd: enable writenotify while userfaultfd-wp is enabled for a VMA

>> For example, libvhost-user.c in QEMU uses for ordinary postcopy:
>>
>> /*
>> * In postcopy we're using PROT_NONE here to catch anyone
>> * accessing it before we userfault.
>> */
>> mmap_addr = mmap(0, dev_region->size + dev_region->mmap_offset,
>> PROT_NONE, MAP_SHARED | MAP_NORESERVE,
>> vmsg->fds[0], 0);
>
> I assume this is for missing mode only. More on wr-protect mode below.
>
> Personally I don't see immediately on whether this is needed. If the
> process itself is trusted then it should be under control of anyone who
> will be accessing the pages.. If the other threads are not trusted, then
> there's no way to stop anyone from mprotect(RW) after mprotect(NONE)
> anyway..

I think there is a difference between code that can read/write memory
(e.g., rings/buffers in libvhost-user.c, where I think this was added to
detect such early access) and code that can execute arbitrary mprotect()
to voluntarily break the system. I think that's the whole reason
libvhost-user.c went that direction.

>
> So I may not really get the gut of it.
>
> Another way to make sure no one access it is right after receiving the
> memory range from QEMU (VhostUserMemoryRegion), if VuDev.postcopy_listening
> is set, then we register the range with UFFD missing immediately. After
> all if postcopy_listening is set it means we passed the advise phase
> already (VHOST_USER_POSTCOPY_ADVISE). Any potential access will be blocked
> until QEMU starts to read on that uffd.
>
>>
>> I'd imagine, when using uffd-wp (VM snapshotting with shmem?) one might use
>> PROT_READ instead before the write-protection is properly set. Because read
>> access would be fine in the meantime.
>
> It'll be different for wr-protect IIUC, because unlike missing protections,
> we don't worry about writes happening before UFFDIO_WRITEPROTECT.
>
> IMHO the solo thing the vhost-user proc needs to do is one
> UFFDIO_WRITEPROTECT for each of the range when QEMU tells it to, then it'll
> be fine. Pre-writes are fine.
>
> Sorry I probably went a bit off-topic. I just want to make sure I don't
> miss any real use case of having mprotect being useful for uffd-wp being
> there, because that used to be a grey area for me.
>
>>
>> But I'm just pulling use cases out of my magic hat ;) Nothing stops user
>> space from doing things that are not clearly forbidden (well, even then
>> users might complain, but that's a different story).
>
> Yes, I think those are always fine but the user just cannot assume it'll
> work as they assumed how it will work.
>
> If "doing things that are not clearly forbidden" triggers a host warning or
> crash that's a bug, OTOH if the outcome is limited to the process itself
> then from kernel pov I think we're good. I used to even thought about
> forbid mprotect() on uffd-wp but I'm not sure whether it's good idea either.
>
> Let's see whether I missed something above, if so I'll rethink.

Let's not get distracted too much. As a reminder, I wrote that test case
to showcase that other kernel code behaves just like the migration code
does. It was the long hanging fruit to make a point, I'm happy to
exclude it for now.

Now, my 2 cents on the whole topic regarding "supported", "not
supported" etc:

(1) If something is not supported we should bail out or at least warn
the user. I'm pretty sure there are other uffd-wp dummy users like
me. Skimming over the man userfaultfd page nothing in particular
regarding PROT_WRITE, mprotect(), ... maybe I looked at the wrong
page.
(2) If something is easy to support, support it instead of having all
these surprises for users and having to document them and warn the
user. Makes all these discussions regarding what's supported, what's
a valid use case, etc ... much easier.
(3) Inconsistency confuses users. If something used to work for anon,
in an ideal world, we make shmem behave in a similar, non-surprising
way.
(4) There are much smarter people like me with much more advanced
magical hats. I'm pretty sure they will come up with use cases that
I am not even able to anticipate right now.
(5) Users will make any imaginable mistake possible and point at the
doc, that nothing speaks against it and that the system didn't bail
out.

Again, just my 2 cents. Maybe the dos and don'ts of userfaultfd-wp are
properly documented already and we just don't bail out.

--
Thanks,

David / dhildenb

2022-12-08 15:38:01

by Peter Xu

[permalink] [raw]

Subject: Re: [PATCH RFC] mm/userfaultfd: enable writenotify while userfaultfd-wp is enabled for a VMA

On Wed, Dec 07, 2022 at 09:10:41PM +0100, David Hildenbrand wrote:
> Now, my 2 cents on the whole topic regarding "supported", "not supported"
> etc:
>
> (1) If something is not supported we should bail out or at least warn
> the user. I'm pretty sure there are other uffd-wp dummy users like
> me. Skimming over the man userfaultfd page nothing in particular
> regarding PROT_WRITE, mprotect(), ... maybe I looked at the wrong
> page.
> (2) If something is easy to support, support it instead of having all
> these surprises for users and having to document them and warn the
> user. Makes all these discussions regarding what's supported, what's
> a valid use case, etc ... much easier.
> (3) Inconsistency confuses users. If something used to work for anon,
> in an ideal world, we make shmem behave in a similar, non-surprising
> way.
> (4) There are much smarter people like me with much more advanced
> magical hats. I'm pretty sure they will come up with use cases that
> I am not even able to anticipate right now.
> (5) Users will make any imaginable mistake possible and point at the
> doc, that nothing speaks against it and that the system didn't bail
> out.
>
> Again, just my 2 cents. Maybe the dos and don'ts of userfaultfd-wp are
> properly documented already and we just don't bail out.

I just don't know how to properly document it with all the information. If
things missing we can always work on top, but again I hope we don't go too
far from what will become useful.

Note that I never said mprotect is not supported... AFAIR there is a use
case where one can (with non-cooperative mode) use uffd-wp to track a
process who does mprotect. For anon uffd-wp it should work already, now
this also reminded me maybe yes with the vm_page_prot patch for shmem from
you it'll also work with shmem and it's still good to have that. In that
case IIUC we'll just notify uffd-wp first then with SIGBUS.

Said that, it's still unclear how these things are used altogether in an
intended way. Let me give some examples.

- What if uffd-wp is registered with SIGBUS mode? How it'll work with
mprotect(RO) too which also use SIGBUS?

- What if uffd-wp tracks a process that also use uffd-wp? Now nest cannot
work, but do we need to document it explicitly, or should we just
implemented nesting of uffd-wp?

- Even if uffd-wp seems to work well with mprotect(RO), what about all the
rest modes combined? Uffd has missing and minor mode, mprotect can do
more than RO. One thing we used to discuss but a slight different case:
what happens if one registers with uffd missing and also mprotect(NONE)?
When the page accessed IIUC we will notify SIGBUS first instead of uffd
because IIRC we'll check vma flags earlier. Is this the behavior we
wanted? What's the expected behavior? This will also be a different
order we notify comparing to the case of "uffd-wp with mprotect(RO)"
because in that case we notify uffd-wp before SIGBUS. Should we revert
the order there just to align with everything?

Sorry to dump these examples. What I wanted to say is to me there're just
so many things to consider and that's just a start. I am not sure whether
it'll be even worth it to decide which should be the right order and make
everything very much defined, even if I still think 99% of the people will
not even care, when someone cares as a start from 1% then 0.99% of them
will find that they can actually do it cleaner with other approaches with
the same kernel facilities.

What about the last 0.01%? They're the driven force to enhance the kernel,
that's always my understanding. They'll either start to ask: "hey I have a
use case that want to use uffd with mprotect() in this way, and that cannot
be done by existing infras. Would it make sense to allow it to happen?".
Or they come with patches. That's how things evolve to me.

These may be seen as excuses of not having proper documentation, personally
sometimes it's not easy for me to draw the line myself on which should be
properly documented and which may not be needed. What I worry is over
engineer and we spend time on things that may not as important or more
priority than something else.

Going back - the solo request I was asking to not use a mprotect example is
mprotect is really not the one that the majority should use for using
uffd-wp. It's not easy to help people understand the problem at all.
That's all for that.

Thanks,

--
Peter Xu