When we try to visit the pagemap of a tagged userspace pointer, we find
that the start_vaddr is not correct because of the tag.
To fix it, we should untag the usespace pointers in pagemap_read().
I tested with 5.10-rc4 and the issue remains.
Explaination from Catalin in [1]:
"
Arguably, that's a user-space bug since tagged file offsets were never
supported. In this case it's not even a tag at bit 56 as per the arm64
tagged address ABI but rather down to bit 47. You could say that the
problem is caused by the C library (malloc()) or whoever created the
tagged vaddr and passed it to this function. It's not a kernel
regression as we've never supported it.
Now, pagemap is a special case where the offset is usually not generated
as a classic file offset but rather derived by shifting a user virtual
address. I guess we can make a concession for pagemap (only) and allow
such offset with the tag at bit (56 - PAGE_SHIFT + 3).
"
My test code is baed on [2]:
A userspace pointer which has been tagged by 0xb4: 0xb400007662f541c8
=== userspace program ===
uint64 OsLayer::VirtualToPhysical(void *vaddr) {
uint64 frame, paddr, pfnmask, pagemask;
int pagesize = sysconf(_SC_PAGESIZE);
off64_t off = ((uintptr_t)vaddr) / pagesize * 8; // off = 0xb400007662f541c8 / pagesize * 8 = 0x5a00003b317aa0
int fd = open(kPagemapPath, O_RDONLY);
...
if (lseek64(fd, off, SEEK_SET) != off || read(fd, &frame, 8) != 8) {
int err = errno;
string errtxt = ErrorString(err);
if (fd >= 0)
close(fd);
return 0;
}
...
}
=== kernel fs/proc/task_mmu.c ===
static ssize_t pagemap_read(struct file *file, char __user *buf,
size_t count, loff_t *ppos)
{
...
src = *ppos;
svpfn = src / PM_ENTRY_BYTES; // svpfn == 0xb400007662f54
start_vaddr = svpfn << PAGE_SHIFT; // start_vaddr == 0xb400007662f54000
end_vaddr = mm->task_size;
/* watch out for wraparound */
// svpfn == 0xb400007662f54
// (mm->task_size >> PAGE) == 0x8000000
if (svpfn > mm->task_size >> PAGE_SHIFT) // the condition is true because of the tag 0xb4
start_vaddr = end_vaddr;
ret = 0;
while (count && (start_vaddr < end_vaddr)) { // we cannot visit correct entry because start_vaddr is set to end_vaddr
int len;
unsigned long end;
...
}
...
}
[1] https://lore.kernel.org/patchwork/patch/1343258/
[2] https://github.com/stressapptest/stressapptest/blob/master/src/os.cc#L158
Cc: Andrew Morton <[email protected]>
Cc: Alexey Dobriyan <[email protected]>
Cc: Andrey Konovalov <[email protected]>
Cc: Alexander Potapenko <[email protected]>
Cc: Vincenzo Frascino <[email protected]>
Cc: Andrey Ryabinin <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Cc: Marco Elver <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Eric W. Biederman <[email protected]>
Cc: Song Bao Hua (Barry Song) <[email protected]>
Cc: [email protected] # v5.4-
Signed-off-by: Miles Chen <[email protected]>
---
Change since v1:
1. Follow Eirc's and Catalin's suggestion to avoid overflow
2. Cc to stable v5.4-
3. add explaination from Catalin to the commit message
---
fs/proc/task_mmu.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 217aa2705d5d..92b277388f05 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -1599,11 +1599,15 @@ static ssize_t pagemap_read(struct file *file, char __user *buf,
src = *ppos;
svpfn = src / PM_ENTRY_BYTES;
- start_vaddr = svpfn << PAGE_SHIFT;
end_vaddr = mm->task_size;
/* watch out for wraparound */
- if (svpfn > mm->task_size >> PAGE_SHIFT)
+ start_vaddr = end_vaddr;
+ if (svpfn < (ULONG_MAX >> PAGE_SHIFT))
+ start_vaddr = untagged_addr(svpfn << PAGE_SHIFT);
+
+ /* Ensure the address is inside the task */
+ if (start_vaddr > mm->task_size)
start_vaddr = end_vaddr;
/*
--
2.18.0
On Fri, Nov 27, 2020 at 01:07:38PM +0800, Miles Chen wrote:
> Cc: Will Deacon <[email protected]>
That should be [email protected].
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 217aa2705d5d..92b277388f05 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -1599,11 +1599,15 @@ static ssize_t pagemap_read(struct file *file, char __user *buf,
>
> src = *ppos;
> svpfn = src / PM_ENTRY_BYTES;
> - start_vaddr = svpfn << PAGE_SHIFT;
> end_vaddr = mm->task_size;
>
> /* watch out for wraparound */
> - if (svpfn > mm->task_size >> PAGE_SHIFT)
> + start_vaddr = end_vaddr;
> + if (svpfn < (ULONG_MAX >> PAGE_SHIFT))
Does this need to be strict less-than? I think a less-than or equal
would work better.
> + start_vaddr = untagged_addr(svpfn << PAGE_SHIFT);
> +
> + /* Ensure the address is inside the task */
> + if (start_vaddr > mm->task_size)
> start_vaddr = end_vaddr;
Otherwise the logic looks fine to me. With the above:
Reviewed-by: Catalin Marinas <[email protected]>
Hi Miles,
On 11/27/20 5:07 AM, Miles Chen wrote:
> When we try to visit the pagemap of a tagged userspace pointer, we find
> that the start_vaddr is not correct because of the tag.
> To fix it, we should untag the usespace pointers in pagemap_read().
>
Nit: s/usespace/userspace/ (please search and replace all occurrences :) )
> I tested with 5.10-rc4 and the issue remains.
>
> Explaination from Catalin in [1]:
>
Nit: s/Explaination/Explanation/ (please search and replace all occurrences :) )
> "
> Arguably, that's a user-space bug since tagged file offsets were never
> supported. In this case it's not even a tag at bit 56 as per the arm64
> tagged address ABI but rather down to bit 47. You could say that the
> problem is caused by the C library (malloc()) or whoever created the
> tagged vaddr and passed it to this function. It's not a kernel
> regression as we've never supported it.
>
> Now, pagemap is a special case where the offset is usually not generated
> as a classic file offset but rather derived by shifting a user virtual
> address. I guess we can make a concession for pagemap (only) and allow
> such offset with the tag at bit (56 - PAGE_SHIFT + 3).
> "
>
> My test code is baed on [2]:
Nit: s/baed/based/ (please search and replace all occurrences :) )
[...]
> ---
> fs/proc/task_mmu.c | 8 ++++++--
> 1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 217aa2705d5d..92b277388f05 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -1599,11 +1599,15 @@ static ssize_t pagemap_read(struct file *file, char __user *buf,
>
> src = *ppos;
> svpfn = src / PM_ENTRY_BYTES;
> - start_vaddr = svpfn << PAGE_SHIFT;
> end_vaddr = mm->task_size;
>
> /* watch out for wraparound */
> - if (svpfn > mm->task_size >> PAGE_SHIFT)
> + start_vaddr = end_vaddr;
> + if (svpfn < (ULONG_MAX >> PAGE_SHIFT))
It seems that 'svpfn' should be less-then-equal (<=) '(ULONG_MAX >>
PAGE_SHIFT)'. Is there any specific reason why this is not the case?
> + start_vaddr = untagged_addr(svpfn << PAGE_SHIFT);
> +
> + /* Ensure the address is inside the task */
> + if (start_vaddr > mm->task_size)
> start_vaddr = end_vaddr;
>
> /*
>
Otherwise:
Reviewed-by: Vincenzo Frascino <[email protected]>
--
Regards,
Vincenzo
On Thu, 2020-12-03 at 11:45 +0000, Vincenzo Frascino wrote:
> Hi Miles,
>
> On 11/27/20 5:07 AM, Miles Chen wrote:
> > When we try to visit the pagemap of a tagged userspace pointer, we find
> > that the start_vaddr is not correct because of the tag.
> > To fix it, we should untag the usespace pointers in pagemap_read().
> >
>
> Nit: s/usespace/userspace/ (please search and replace all occurrences :) )
>
> > I tested with 5.10-rc4 and the issue remains.
> >
> > Explaination from Catalin in [1]:
> >
>
> Nit: s/Explaination/Explanation/ (please search and replace all occurrences :) )
thanks, I will fix this and push v3
>
> > "
> > Arguably, that's a user-space bug since tagged file offsets were never
> > supported. In this case it's not even a tag at bit 56 as per the arm64
> > tagged address ABI but rather down to bit 47. You could say that the
> > problem is caused by the C library (malloc()) or whoever created the
> > tagged vaddr and passed it to this function. It's not a kernel
> > regression as we've never supported it.
> >
> > Now, pagemap is a special case where the offset is usually not generated
> > as a classic file offset but rather derived by shifting a user virtual
> > address. I guess we can make a concession for pagemap (only) and allow
> > such offset with the tag at bit (56 - PAGE_SHIFT + 3).
> > "
> >
> > My test code is baed on [2]:
>
> Nit: s/baed/based/ (please search and replace all occurrences :) )
thanks, I will fix this and push v3
Miles
>
> [...]
>
> > ---
> > fs/proc/task_mmu.c | 8 ++++++--
> > 1 file changed, 6 insertions(+), 2 deletions(-)
> >
> > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> > index 217aa2705d5d..92b277388f05 100644
> > --- a/fs/proc/task_mmu.c
> > +++ b/fs/proc/task_mmu.c
> > @@ -1599,11 +1599,15 @@ static ssize_t pagemap_read(struct file *file, char __user *buf,
> >
> > src = *ppos;
> > svpfn = src / PM_ENTRY_BYTES;
> > - start_vaddr = svpfn << PAGE_SHIFT;
> > end_vaddr = mm->task_size;
> >
> > /* watch out for wraparound */
> > - if (svpfn > mm->task_size >> PAGE_SHIFT)
> > + start_vaddr = end_vaddr;
> > + if (svpfn < (ULONG_MAX >> PAGE_SHIFT))
>
> It seems that 'svpfn' should be less-then-equal (<=) '(ULONG_MAX >>
> PAGE_SHIFT)'. Is there any specific reason why this is not the case?
>
> > + start_vaddr = untagged_addr(svpfn << PAGE_SHIFT);
> > +
> > + /* Ensure the address is inside the task */
> > + if (start_vaddr > mm->task_size)
> > start_vaddr = end_vaddr;
> >
> > /*
> >
>
> Otherwise:
>
> Reviewed-by: Vincenzo Frascino <[email protected]>
>
On Thu, 2020-12-03 at 11:30 +0000, Catalin Marinas wrote:
> On Fri, Nov 27, 2020 at 01:07:38PM +0800, Miles Chen wrote:
> > Cc: Will Deacon <[email protected]>
>
> That should be [email protected].
ok, I will fix it and submit v3
>
> > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> > index 217aa2705d5d..92b277388f05 100644
> > --- a/fs/proc/task_mmu.c
> > +++ b/fs/proc/task_mmu.c
> > @@ -1599,11 +1599,15 @@ static ssize_t pagemap_read(struct file *file, char __user *buf,
> >
> > src = *ppos;
> > svpfn = src / PM_ENTRY_BYTES;
> > - start_vaddr = svpfn << PAGE_SHIFT;
> > end_vaddr = mm->task_size;
> >
> > /* watch out for wraparound */
> > - if (svpfn > mm->task_size >> PAGE_SHIFT)
> > + start_vaddr = end_vaddr;
> > + if (svpfn < (ULONG_MAX >> PAGE_SHIFT))
>
> Does this need to be strict less-than? I think a less-than or equal
> would work better.
Thanks, I will fix it and submit v3.
>
> > + start_vaddr = untagged_addr(svpfn << PAGE_SHIFT);
> > +
> > + /* Ensure the address is inside the task */
> > + if (start_vaddr > mm->task_size)
> > start_vaddr = end_vaddr;
>
> Otherwise the logic looks fine to me. With the above:
>
> Reviewed-by: Catalin Marinas <[email protected]>