Date:   Wed, 15 Feb 2023 16:45:25 -0500
From:   Peter Xu <peterx@redhat.com>
To:     Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc:     Paul Gofman <pgofman@codeweavers.com>, david@redhat.com,
        Andrew Morton <akpm@linux-foundation.org>,
        kernel@collabora.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 1/2] mm/userfaultfd: Support WP on multiple VMAs
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On Wed, Feb 15, 2023 at 12:08:11PM +0500, Muhammad Usama Anjum wrote:
> Hi Peter,
> 
> Thank you for your review!
> 
> On 2/15/23 2:50 AM, Peter Xu wrote:
> > On Tue, Feb 14, 2023 at 01:49:50PM +0500, Muhammad Usama Anjum wrote:
> >> On 2/14/23 2:11 AM, Peter Xu wrote:
> >>> On Mon, Feb 13, 2023 at 10:50:39PM +0500, Muhammad Usama Anjum wrote:
> >>>> On 2/13/23 9:54 PM, Peter Xu wrote:
> >>>>> On Mon, Feb 13, 2023 at 09:31:23PM +0500, Muhammad Usama Anjum wrote:
> >>>>>> mwriteprotect_range() errors out if [start, end) doesn't fall in one
> >>>>>> VMA. We are facing a use case where multiple VMAs are present in one
> >>>>>> range of interest. For example, the following pseudocode reproduces the
> >>>>>> error which we are trying to fix:
> >>>>>>
> >>>>>> - Allocate memory of size 16 pages with PROT_NONE with mmap
> >>>>>> - Register userfaultfd
> >>>>>> - Change protection of the first half (1 to 8 pages) of memory to
> >>>>>>   PROT_READ | PROT_WRITE. This breaks the memory area in two VMAs.
> >>>>>> - Now UFFDIO_WRITEPROTECT_MODE_WP on the whole memory of 16 pages errors
> >>>>>>   out.
> >>>>>>
> >>>>>> This is a simple use case where user may or may not know if the memory
> >>>>>> area has been divided into multiple VMAs.
> >>>>>>
> >>>>>> Reported-by: Paul Gofman <pgofman@codeweavers.com>
> >>>>>> Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
> >>>>>> ---
> >>>>>> Changes since v1:
> >>>>>> - Correct the start and ending values passed to uffd_wp_range()
> >>>>>> ---
> >>>>>>  mm/userfaultfd.c | 38 ++++++++++++++++++++++----------------
> >>>>>>  1 file changed, 22 insertions(+), 16 deletions(-)
> >>>>>>
> >>>>>> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> >>>>>> index 65ad172add27..bccea08005a8 100644
> >>>>>> --- a/mm/userfaultfd.c
> >>>>>> +++ b/mm/userfaultfd.c
> >>>>>> @@ -738,9 +738,12 @@ int mwriteprotect_range(struct mm_struct *dst_mm, unsigned long start,
> >>>>>>  			unsigned long len, bool enable_wp,
> >>>>>>  			atomic_t *mmap_changing)
> >>>>>>  {
> >>>>>> +	unsigned long end = start + len;
> >>>>>> +	unsigned long _start, _end;
> >>>>>>  	struct vm_area_struct *dst_vma;
> >>>>>>  	unsigned long page_mask;
> >>>>>>  	int err;
> >>>>>
> >>>>> I think this needs to be initialized or it can return anything when range
> >>>>> not mapped.
> >>>> It is being initialized to -EAGAIN already. It is not visible in this patch.
> >>>
> >>> I see, though -EAGAIN doesn't look suitable at all.  The old retcode for
> >>> !vma case is -ENOENT, so I think we'd better keep using it if we want to
> >>> have this patch.
> >> I'll update in next version.
> >>
> >>>
> >>>>
> >>>>>
> >>>>>> +	VMA_ITERATOR(vmi, dst_mm, start);
> >>>>>>  
> >>>>>>  	/*
> >>>>>>  	 * Sanitize the command parameters:
> >>>>>> @@ -762,26 +765,29 @@ int mwriteprotect_range(struct mm_struct *dst_mm, unsigned long start,
> >>>>>>  	if (mmap_changing && atomic_read(mmap_changing))
> >>>>>>  		goto out_unlock;
> >>>>>>  
> >>>>>> -	err = -ENOENT;
> >>>>>> -	dst_vma = find_dst_vma(dst_mm, start, len);
> >>>>>> +	for_each_vma_range(vmi, dst_vma, end) {
> >>>>>> +		err = -ENOENT;
> >>>>>>  
> >>>>>> -	if (!dst_vma)
> >>>>>> -		goto out_unlock;
> >>>>>> -	if (!userfaultfd_wp(dst_vma))
> >>>>>> -		goto out_unlock;
> >>>>>> -	if (!vma_can_userfault(dst_vma, dst_vma->vm_flags))
> >>>>>> -		goto out_unlock;
> >>>>>> +		if (!dst_vma->vm_userfaultfd_ctx.ctx)
> >>>>>> +			break;
> >>>>>> +		if (!userfaultfd_wp(dst_vma))
> >>>>>> +			break;
> >>>>>> +		if (!vma_can_userfault(dst_vma, dst_vma->vm_flags))
> >>>>>> +			break;
> >>>>>>  
> >>>>>> -	if (is_vm_hugetlb_page(dst_vma)) {
> >>>>>> -		err = -EINVAL;
> >>>>>> -		page_mask = vma_kernel_pagesize(dst_vma) - 1;
> >>>>>> -		if ((start & page_mask) || (len & page_mask))
> >>>>>> -			goto out_unlock;
> >>>>>> -	}
> >>>>>> +		if (is_vm_hugetlb_page(dst_vma)) {
> >>>>>> +			err = -EINVAL;
> >>>>>> +			page_mask = vma_kernel_pagesize(dst_vma) - 1;
> >>>>>> +			if ((start & page_mask) || (len & page_mask))
> >>>>>> +				break;
> >>>>>> +		}
> >>>>>>  
> >>>>>> -	uffd_wp_range(dst_mm, dst_vma, start, len, enable_wp);
> >>>>>> +		_start = (dst_vma->vm_start > start) ? dst_vma->vm_start : start;
> >>>>>> +		_end = (dst_vma->vm_end < end) ? dst_vma->vm_end : end;
> >>>>>>  
> >>>>>> -	err = 0;
> >>>>>> +		uffd_wp_range(dst_mm, dst_vma, _start, _end - _start, enable_wp);
> >>>>>> +		err = 0;
> >>>>>> +	}
> >>>>>>  out_unlock:
> >>>>>>  	mmap_read_unlock(dst_mm);
> >>>>>>  	return err;
> >>>>>
> >>>>> This whole patch also changes the abi, so I'm worried whether there can be
> >>>>> app that relies on the existing behavior.
> >>>> Even if a app is dependent on it, this change would just don't return error
> >>>> if there are multiple VMAs under the hood and handle them correctly. Most
> >>>> apps wouldn't care about VMAs anyways. I don't know if there would be any
> >>>> drastic behavior change, other than the behavior becoming nicer.
> >>>
> >>> So this logic existed since the initial version of uffd-wp.  It has a good
> >>> thing that it strictly checks everything and it makes sense since uffd-wp
> >>> is per-vma attribute.  In short, the old code fails clearly.
> >>>
> >>> While the new proposal is not: if -ENOENT we really have no idea what
> >>> happened at all; some ranges can be wr-protected but we don't know where
> >>> starts to go wrong.
> >> The return error codes can be made to return in better way somewhat. The
> >> return error codes shouldn't block a correct functionality enhancement patch.
> >>
> >>>
> >>> Now I'm looking at the original problem..
> >>>
> >>>  - Allocate memory of size 16 pages with PROT_NONE with mmap
> >>>  - Register userfaultfd
> >>>  - Change protection of the first half (1 to 8 pages) of memory to
> >>>    PROT_READ | PROT_WRITE. This breaks the memory area in two VMAs.
> >>>  - Now UFFDIO_WRITEPROTECT_MODE_WP on the whole memory of 16 pages errors
> >>>    out.
> >>>
> >>> Why the user app should wr-protect 16 pages at all?
> >> Taking arguments from Paul here.
> >>
> >> The app is free to insert guard pages inside the range (with PROT_NONE) and
> >> change the protection of memory freely. Not sure why it is needed to
> >> complicate things by denying any flexibility. We should never restrict what
> >> is possible and what not. All of these different access attributes and
> >> their any combination of interaction _must_ work without question. The
> >> application should be free to change protection on any sub-range and it
> >> shouldn't break the PAGE_IS_WRITTEN + UFFD_WRITE_PROTECT promise which
> >> PAGEMAP_IOCTL (patches are in progress) and UFFD makes.
> > 
> > Because uffd-wp has a limitation on e.g. it cannot nest so far.  I'm fine
> > with allowing mprotect() happening, but just to mention so far it cannot do
> > "any combinations" yet.
> > 
> >>
> >>>
> >>> If so, uffd_wp_range() will be ran upon a PROT_NONE range which doesn't
> >>> make sense at all, no matter whether the user is aware of vma concept or
> >>> not...  because it's destined that it's a vain effort.
> >> It is not a vain effort. The user want to watch/find the dirty pages of a
> >> range while working with it: reserve and watch at once while Write
> >> protecting or un-protecting as needed. There may be several different use
> >> cases. Inserting guard pages to catch out of range access, map something
> >> only when it is needed; unmap or PROT_NONE pages when they are set free in
> >> the app etc.
> > 
> > Fair enough.
> > 
> >>
> >>>
> >>> So IMHO it's the user app needs fixing here, not the interface?  I think
> >>> it's the matter of whether the monitor is aware of mprotect() being
> >>> invoked.
> >> No. The common practice is to allocate a big memory chunk at once and have
> >> own allocator over it (whether it is some specific allocator in a game or a
> >> .net allocator with garbage collector). From the usage point of view it is
> >> very limiting to demand constant memory attributes for the whole range.
> >>
> >> That said, if we do have the way to do exactly what we want with reset
> >> through pagemap fd and it is just UFFD ioctl will be working differently,
> >> it is not a blocker of course, just weird api design.
> > 
> > Do you mean you'll disable ENGAGE_WP && !GET in your other series?  Yes, if
> > this will service your goal, it'll be perfect to remove that interface.
> No, we cannot remove it.

If this patch can land, I assume ioctl(UFFDIO_WP) can start to service the
dirty tracking purpose, then why do you still need "ENGAGE_WP && !GET"?

Note, I'm not asking to drop ENGAGE_WP entirely, only when !GET.

> 
> > 
> >>
> >>>
> >>> In short, I hope we're working on things that helps at least someone, and
> >>> we should avoid working on things that does not have clear benefit yet.
> >>> With the WP_ENGAGE new interface being proposed, I just didn't see any
> >>> benefit of changing the current interface, especially if the change can
> >>> bring uncertainties itself (e.g., should we fail upon !uffd-wp vmas, or
> >>> should we skip?).
> >> We can work on solving uncertainties in case of error conditions. Fail if
> >> !uffd-wp vma comes.
> > 
> > Let me try to double check with you here:
> > 
> > I assume you want to skip any vma that is not mapped at all, as the loop
> > already does so.  So it'll succeed if there're memory holes.
> > 
> > You also want to explicitly fail if some vma is not registered with uffd-wp
> > when walking the vma list, am I right?  IOW, the tracee _won't_ ever have a
> > chance to unregister uffd-wp itself, right?
> Yes, fail if any VMA doesn't have uffd-wp. This fail means the
> write-protection or un-protection failed on a region of memory with error
> -ENOENT. This is already happening in this current patch. The unregister
> code would remain same. The register and unregister ioctls are already
> going over all the VMAs in a range. I'm not rigid on anything. Let me
> define the interface below.
> 
> > 
> >>
> >>>
> >>>>
> >>>>>
> >>>>> Is this for the new pagemap effort?  Can this just be done in the new
> >>>>> interface rather than changing the old?
> >>>> We found this bug while working on pagemap patches. It is already being
> >>>> handled in the new interface. We just thought that this use case can happen
> >>>> pretty easily and unknowingly. So the support should be added.
> >>>
> >>> Thanks.  My understanding is that it would have been reported if it
> >>> affected any existing uffd-wp user.
> >> I would consider the UFFD WP a recent functionality and it may not being
> >> used in wide range of app scenarios.
> > 
> > Yes I think so.
> > 
> > Existing users should in most cases be applying the ioctl upon valid vmas
> > somehow.  I think the chance is low that someone relies on the errcode to
> > make other decisions, but I just cannot really tell because the user app
> > can do many weird things.
> Correct. The user can use any combination of operation
> (mmap/mprotect/uffd). They must work in harmony.

No uffd - that's exactly what I'm saying: mprotect is fine here, but uffd
is probably not, not in a nested way.  When you try to UFFDIO_REGISTER upon
some range that already been registered (by the tracee), it'll fail for you
immediately:

		/*
		 * Check that this vma isn't already owned by a
		 * different userfaultfd. We can't allow more than one
		 * userfaultfd to own a single vma simultaneously or we
		 * wouldn't know which one to deliver the userfaults to.
		 */
		ret = -EBUSY;
		if (cur->vm_userfaultfd_ctx.ctx &&
		    cur->vm_userfaultfd_ctx.ctx != ctx)
			goto out_unlock;

So if this won't work for you, then AFAICT uffd-wp won't work for you (just
like soft-dirty but in another way, sorry), at least not until someone
starts to work on the nested.

> 
> > 
> >>
> >>>
> >>>>
> >>>> Also mwriteprotect_range() gives a pretty straight forward way to WP or
> >>>> un-WP a range. Async WP can be used in coordination with pagemap file
> >>>> (PM_UFFD_WP flag in PTE) as well. There may be use cases for it. On another
> >>>> note, I don't see any use cases of WP async and PM_UFFD_WP flag as
> >>>> !PM_UFFD_WP flag doesn't give direct information if the page is written for
> >>>> !present pages.
> >>>
> >>> Currently we do maintain PM_UFFD_WP even for swap entries, so if it was
> >>> written then I think we'll know even if the page was swapped out:
> >>>
> >>> 	} else if (is_swap_pte(pte)) {
> >>> 		if (pte_swp_uffd_wp(pte))
> >>> 			flags |= PM_UFFD_WP;
> >>> 		if (pte_marker_entry_uffd_wp(entry))
> >>> 			flags |= PM_UFFD_WP;
> >>>
> >>> So it's working?
> >>>
> >>>>
> >>>>>
> >>>>> Side note: in your other pagemap series, you can optimize "WP_ENGAGE &&
> >>>>> !GET" to not do generic pgtable walk at all, but use what it does in this
> >>>>> patch for the initial round or wr-protect.
> >>>> Yeah, it is implemented with some optimizations.
> >>>
> >>> IIUC in your latest public version is not optimized, but I can check the
> >>> new version when it comes.
> >> I've some optimizations in next version keeping the code lines minimum. The
> >> best optimization would be to add a page walk dedicated for this engaging
> >> write-protect. I don't want to do that. Lets try to improve this patch in
> >> how ever way possible. So that WP from UFFD ioctl can be used.
> > 
> > If you want to do this there, I think it can be simply/mostly a copy-paste
> > of this patch over there by looping over the vmas and apply the protections.
> I wouldn't want to do this. PAGEMAP IOCTL performing an operation
> anonymously to UFFD_WP wouldn't look good when we can improve UFFD_WP itself.
> 
> > 
> > I think it's fine to do it with ioctl(UFFDIO_WP), but I want to be crystal
> > clear on what interface you're looking for before changing it, and let's
> > define it properly.
> Thank you. Let me crystal clear why we have sent this patch and what
> difference we want.
> 
> Just like UFFDIO_REGISTER and UFFDIO_UNREGISTER don't care if the requested
> range has multiple VMAs in the memory region, we want the same thing with
> UFFDIO_WRITEPROTCET. It looks more uniform and obvious to us as user
> doesn't care about VMAs. The user only cares about the memory he wants to
> write-protect. So just update the inside code of UFFDIO_WRITEPROTECT such
> that it starts to act like UFFDIO_REGISTER/UFFDIO_UNREGISTER. It shouldn't
> complain if there are multiple VMAs involved under the hood.
> 
> This patch is visiting all the VMAs in the memory range. The attached
> patches are my wip v3. If you feel like there can be better way to achieve
> this, please don't hesitate to send the v3 yourself.

As I said I think you have a point, and let's cross finger this is fine
(which I mostly agree with).

We can fail on !uffd-wp vmas, sounds reasonable to me.  But let's sync up
with above to make sure this works for you.

Since you've got a patch here, let me comment directly below.

> 
> Thank you so much!
> 
> -- 
> BR,
> Muhammad Usama Anjum

> From f69069dddda206b190706eef7b2dad3a67a6df10 Mon Sep 17 00:00:00 2001
> From: Muhammad Usama Anjum <usama.anjum@collabora.com>
> Date: Thu, 9 Feb 2023 16:13:23 +0500
> Subject: [PATCH v3 1/2] mm/userfaultfd: Support WP on multiple VMAs
> To: peterx@redhat.com, david@redhat.com
> Cc: usama.anjum@collabora.com, kernel@collabora.com
> 
> mwriteprotect_range() errors out if [start, end) doesn't fall in one
> VMA. We are facing a use case where multiple VMAs are present in one
> range of interest. For example, the following pseudocode reproduces the
> error which we are trying to fix:
> 
> - Allocate memory of size 16 pages with PROT_NONE with mmap
> - Register userfaultfd
> - Change protection of the first half (1 to 8 pages) of memory to
>   PROT_READ | PROT_WRITE. This breaks the memory area in two VMAs.
> - Now UFFDIO_WRITEPROTECT_MODE_WP on the whole memory of 16 pages errors
>   out.
> 
> This is a simple use case where user may or may not know if the memory
> area has been divided into multiple VMAs.
> 
> Reported-by: Paul Gofman <pgofman@codeweavers.com>
> Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
> ---
> Changes since v2:
> - Correct the return error code
> 
> Changes since v1:
> - Correct the start and ending values passed to uffd_wp_range()
> ---
>  mm/userfaultfd.c | 45 ++++++++++++++++++++++++++-------------------
>  1 file changed, 26 insertions(+), 19 deletions(-)
> 
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index 65ad172add27..a3b48a99b942 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -738,9 +738,12 @@ int mwriteprotect_range(struct mm_struct *dst_mm, unsigned long start,
>  			unsigned long len, bool enable_wp,
>  			atomic_t *mmap_changing)
>  {
> +	unsigned long end = start + len;
> +	unsigned long _start, _end;
>  	struct vm_area_struct *dst_vma;
>  	unsigned long page_mask;
> -	int err;
> +	int err = -ENOENT;
> +	VMA_ITERATOR(vmi, dst_mm, start);
>  
>  	/*
>  	 * Sanitize the command parameters:
> @@ -758,30 +761,34 @@ int mwriteprotect_range(struct mm_struct *dst_mm, unsigned long start,
>  	 * operation (e.g. mremap) running in parallel, bail out and
>  	 * request the user to retry later
>  	 */
> -	err = -EAGAIN;
> -	if (mmap_changing && atomic_read(mmap_changing))
> +	if (mmap_changing && atomic_read(mmap_changing)) {
> +		err = -EAGAIN;
>  		goto out_unlock;
> +	}

Let's keep the original code and simply set -ENOENT afterwards.

>  
> -	err = -ENOENT;
> -	dst_vma = find_dst_vma(dst_mm, start, len);
> +	for_each_vma_range(vmi, dst_vma, end) {
> +		err = -ENOENT;
>  
> -	if (!dst_vma)
> -		goto out_unlock;
> -	if (!userfaultfd_wp(dst_vma))
> -		goto out_unlock;
> -	if (!vma_can_userfault(dst_vma, dst_vma->vm_flags))
> -		goto out_unlock;
> +		if (!dst_vma->vm_userfaultfd_ctx.ctx)
> +			break;

What is this check for?

> +		if (!userfaultfd_wp(dst_vma))
> +			break;
> +		if (!vma_can_userfault(dst_vma, dst_vma->vm_flags))
> +			break;

I think this is not useful at all (even in the old code)?  It could have
sneaked in somehow when I took the code over from Shaohua/Andrea.  Maybe we
should clean it up since at it.

>  
> -	if (is_vm_hugetlb_page(dst_vma)) {
> -		err = -EINVAL;
> -		page_mask = vma_kernel_pagesize(dst_vma) - 1;
> -		if ((start & page_mask) || (len & page_mask))
> -			goto out_unlock;
> -	}
> +		if (is_vm_hugetlb_page(dst_vma)) {
> +			err = -EINVAL;
> +			page_mask = vma_kernel_pagesize(dst_vma) - 1;
> +			if ((start & page_mask) || (len & page_mask))
> +				break;
> +		}
>  
> -	uffd_wp_range(dst_mm, dst_vma, start, len, enable_wp);
> +		_start = (dst_vma->vm_start > start) ? dst_vma->vm_start : start;
> +		_end = (dst_vma->vm_end < end) ? dst_vma->vm_end : end;

Maybe:

                _start = MAX(dst_vma->vm_start, start);
                _end = MIN(dst_vma->vm_end, end);

?

>  
> -	err = 0;
> +		uffd_wp_range(dst_mm, dst_vma, _start, _end - _start, enable_wp);
> +		err = 0;
> +	}
>  out_unlock:
>  	mmap_read_unlock(dst_mm);
>  	return err;
> -- 
> 2.39.1
> 

> From ac119b22aa00248ed67c7ac6e285a12391943b15 Mon Sep 17 00:00:00 2001
> From: Muhammad Usama Anjum <usama.anjum@collabora.com>
> Date: Mon, 13 Feb 2023 21:15:01 +0500
> Subject: [PATCH v3 2/2] mm/userfaultfd: add VM_WARN_ONCE()
> To: peterx@redhat.com, david@redhat.com
> Cc: usama.anjum@collabora.com, kernel@collabora.com
> 
> Add VM_WARN_ONCE() to uffd_wp_range() to detect range (start, len) abuse.
> 
> Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
> ---
>  mm/userfaultfd.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index a3b48a99b942..0536e23ba5f4 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -716,6 +716,8 @@ void uffd_wp_range(struct mm_struct *dst_mm, struct vm_area_struct *dst_vma,
>  	unsigned int mm_cp_flags;
>  	struct mmu_gather tlb;
>  
> +	VM_WARN_ONCE(start < dst_vma->vm_start || start + len > dst_vma->vm_end,
> +		     "The address range exceeds VMA boundary.\n");
>  	if (enable_wp)
>  		mm_cp_flags = MM_CP_UFFD_WP;
>  	else
> -- 
> 2.39.1
> 


-- 
Peter Xu