[PATCH v2 0/2] mm/uffd: Fix vma merge/split

[PATCH v2 0/2] mm/uffd: Fix vma merge/split

[Peter Xu]

v2:
- Added r-bs for Lorenzo and Liam
- Reworded patch 1's commit message [Lorenzo]

This series contains two patches that fix vma merge/split for userfaultfd
on two separate issues.  The patchset is based on akpm/mm-hotfixes-unstable
with 2f628010799e reverted (where patch 1 should be used to replace it
which seems to be the plan we reached).

Patch 1 fixes a regression since 6.1+ due to something we overlooked when
converting to maple tree apis.  The plan is we use patch 1 to replace the
commit "2f628010799e (mm: userfaultfd: avoid passing an invalid range to
vma_merge())" in mm-hostfixes-unstable tree if possible, so as to bring
uffd vma operations back aligned with the rest code again.

Patch 2 fixes a long standing issue that vma can be left unmerged even if
we can for either uffd register or unregister.

Many thanks to Lorenzo on either noticing this issue from the assert
movement patch, looking at this problem, and also provided a reproducer on
the unmerged vma issue [1].

Please have a look, thanks.

[1] https://gist.github.com/lorenzo-stoakes/a11a10f5f479e7a977fc456331266e0e

Peter Xu (2):
  mm/uffd: Fix vma operation where start addr cuts part of vma
  mm/uffd: Allow vma to merge as much as possible

 fs/userfaultfd.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

-- 
2.39.1

[PATCH v2 1/2] mm/uffd: Fix vma operation where start addr cuts part of vma

[Peter Xu]

It seems vma merging with uffd paths is broken with either
register/unregister, where right now we can feed wrong parameters to
vma_merge() and it's found by recent patch which moved asserts upwards in
vma_merge() by Lorenzo Stoakes:

https://lore.kernel.org/all/ZFunF7DmMdK05MoF@FVFF77S0Q05N.cambridge.arm.com/

It's possible that "start" is contained within vma but not clamped to its
start.  We need to convert this into either "cannot merge" case or "can
merge" case 4 which permits subdivision of prev by assigning vma to
prev. As we loop, each subsequent VMA will be clamped to the start.

This patch will eliminate the report and make sure vma_merge() calls will
become legal again.

One thing to mention is that the "Fixes: 29417d292bd0" below is there only
to help explain where the warning can start to trigger, the real commit to
fix should be 69dbe6daf104.  Commit 29417d292bd0 helps us to identify the
issue, but unfortunately we may want to keep it in Fixes too just to ease
kernel backporters for easier tracking.

Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Reported-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Lorenzo Stoakes <lstoakes@gmail.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Fixes: 29417d292bd0 ("mm/mmap/vma_merge: always check invariants")
Fixes: 69dbe6daf104 ("userfaultfd: use maple tree iterator to iterate VMAs")
Closes: https://lore.kernel.org/all/ZFunF7DmMdK05MoF@FVFF77S0Q05N.cambridge.arm.com/
Cc: linux-stable <stable@vger.kernel.org>
Signed-off-by: Peter Xu <peterx@redhat.com>
---
 fs/userfaultfd.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 0fd96d6e39ce..17c8c345dac4 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -1459,6 +1459,8 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
 
 	vma_iter_set(&vmi, start);
 	prev = vma_prev(&vmi);
+	if (vma->vm_start < start)
+		prev = vma;
 
 	ret = 0;
 	for_each_vma_range(vmi, vma, end) {
@@ -1625,6 +1627,9 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
 
 	vma_iter_set(&vmi, start);
 	prev = vma_prev(&vmi);
+	if (vma->vm_start < start)
+		prev = vma;
+
 	ret = 0;
 	for_each_vma_range(vmi, vma, end) {
 		cond_resched();
-- 
2.39.1

[PATCH v2 2/2] mm/uffd: Allow vma to merge as much as possible

[Peter Xu]

We used to not pass in the pgoff correctly when register/unregister uffd
regions, it caused incorrect behavior on vma merging and can cause
mergeable vmas being separate after ioctls return.

For example, when we have:

  vma1(range 0-9, with uffd), vma2(range 10-19, no uffd)

Then someone unregisters uffd on range (5-9), it should logically become:

  vma1(range 0-4, with uffd), vma2(range 5-19, no uffd)

But with current code we'll have:

  vma1(range 0-4, with uffd), vma3(range 5-9, no uffd), vma2(range 10-19, no uffd)

This patch allows such merge to happen correctly before ioctl returns.

This behavior seems to have existed since the 1st day of uffd.  Since pgoff
for vma_merge() is only used to identify the possibility of vma merging,
meanwhile here what we did was always passing in a pgoff smaller than what
we should, so there should have no other side effect besides not merging
it.  Let's still tentatively copy stable for this, even though I don't see
anything will go wrong besides vma being split (which is mostly not user
visible).

Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: linux-stable <stable@vger.kernel.org>
Reported-by: Lorenzo Stoakes <lstoakes@gmail.com>
Acked-by: Lorenzo Stoakes <lstoakes@gmail.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Fixes: 86039bd3b4e6 ("userfaultfd: add new syscall to provide memory externalization")
Signed-off-by: Peter Xu <peterx@redhat.com>
---
 fs/userfaultfd.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 17c8c345dac4..4e800bb7d2ab 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -1332,6 +1332,7 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
 	bool basic_ioctls;
 	unsigned long start, end, vma_end;
 	struct vma_iterator vmi;
+	pgoff_t pgoff;
 
 	user_uffdio_register = (struct uffdio_register __user *) arg;
 
@@ -1484,8 +1485,9 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
 		vma_end = min(end, vma->vm_end);
 
 		new_flags = (vma->vm_flags & ~__VM_UFFD_FLAGS) | vm_flags;
+		pgoff = vma->vm_pgoff + ((start - vma->vm_start) >> PAGE_SHIFT);
 		prev = vma_merge(&vmi, mm, prev, start, vma_end, new_flags,
-				 vma->anon_vma, vma->vm_file, vma->vm_pgoff,
+				 vma->anon_vma, vma->vm_file, pgoff,
 				 vma_policy(vma),
 				 ((struct vm_userfaultfd_ctx){ ctx }),
 				 anon_vma_name(vma));
@@ -1565,6 +1567,7 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
 	unsigned long start, end, vma_end;
 	const void __user *buf = (void __user *)arg;
 	struct vma_iterator vmi;
+	pgoff_t pgoff;
 
 	ret = -EFAULT;
 	if (copy_from_user(&uffdio_unregister, buf, sizeof(uffdio_unregister)))
@@ -1667,8 +1670,9 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
 			uffd_wp_range(vma, start, vma_end - start, false);
 
 		new_flags = vma->vm_flags & ~__VM_UFFD_FLAGS;
+		pgoff = vma->vm_pgoff + ((start - vma->vm_start) >> PAGE_SHIFT);
 		prev = vma_merge(&vmi, mm, prev, start, vma_end, new_flags,
-				 vma->anon_vma, vma->vm_file, vma->vm_pgoff,
+				 vma->anon_vma, vma->vm_file, pgoff,
 				 vma_policy(vma),
 				 NULL_VM_UFFD_CTX, anon_vma_name(vma));
 		if (prev) {
-- 
2.39.1

Re: [PATCH v2 1/2] mm/uffd: Fix vma operation where start addr cuts part of vma

[Andrew Morton]

On Wed, 17 May 2023 15:09:15 -0400 Peter Xu <peterx@redhat.com> wrote:

> It seems vma merging with uffd paths is broken with either
> register/unregister, where right now we can feed wrong parameters to
> vma_merge() and it's found by recent patch which moved asserts upwards in
> vma_merge() by Lorenzo Stoakes:
> 
> https://lore.kernel.org/all/ZFunF7DmMdK05MoF@FVFF77S0Q05N.cambridge.arm.com/
> 
> It's possible that "start" is contained within vma but not clamped to its
> start.  We need to convert this into either "cannot merge" case or "can
> merge" case 4 which permits subdivision of prev by assigning vma to
> prev. As we loop, each subsequent VMA will be clamped to the start.
> 
> This patch will eliminate the report and make sure vma_merge() calls will
> become legal again.
> 
> One thing to mention is that the "Fixes: 29417d292bd0" below is there only
> to help explain where the warning can start to trigger, the real commit to
> fix should be 69dbe6daf104.  Commit 29417d292bd0 helps us to identify the
> issue, but unfortunately we may want to keep it in Fixes too just to ease
> kernel backporters for easier tracking.
> 
> Cc: Lorenzo Stoakes <lstoakes@gmail.com>
> Cc: Mike Rapoport (IBM) <rppt@kernel.org>
> Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
> Reported-by: Mark Rutland <mark.rutland@arm.com>
> Reviewed-by: Lorenzo Stoakes <lstoakes@gmail.com>
> Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
> Fixes: 29417d292bd0 ("mm/mmap/vma_merge: always check invariants")
> Fixes: 69dbe6daf104 ("userfaultfd: use maple tree iterator to iterate VMAs")

I don't know how -stable maintainers are to handle more than a single
Fixes: target, given that Fixes: means "kernels which have that patch
need this one".  Can we narrow this down to a single commit for this
purpose?

Re: [PATCH v2 1/2] mm/uffd: Fix vma operation where start addr cuts part of vma

[Peter Xu]

On Wed, May 17, 2023 at 01:23:21PM -0700, Andrew Morton wrote:
> On Wed, 17 May 2023 15:09:15 -0400 Peter Xu <peterx@redhat.com> wrote:
> 
> > It seems vma merging with uffd paths is broken with either
> > register/unregister, where right now we can feed wrong parameters to
> > vma_merge() and it's found by recent patch which moved asserts upwards in
> > vma_merge() by Lorenzo Stoakes:
> > 
> > https://lore.kernel.org/all/ZFunF7DmMdK05MoF@FVFF77S0Q05N.cambridge.arm.com/
> > 
> > It's possible that "start" is contained within vma but not clamped to its
> > start.  We need to convert this into either "cannot merge" case or "can
> > merge" case 4 which permits subdivision of prev by assigning vma to
> > prev. As we loop, each subsequent VMA will be clamped to the start.
> > 
> > This patch will eliminate the report and make sure vma_merge() calls will
> > become legal again.
> > 
> > One thing to mention is that the "Fixes: 29417d292bd0" below is there only
> > to help explain where the warning can start to trigger, the real commit to
> > fix should be 69dbe6daf104.  Commit 29417d292bd0 helps us to identify the
> > issue, but unfortunately we may want to keep it in Fixes too just to ease
> > kernel backporters for easier tracking.
> > 
> > Cc: Lorenzo Stoakes <lstoakes@gmail.com>
> > Cc: Mike Rapoport (IBM) <rppt@kernel.org>
> > Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
> > Reported-by: Mark Rutland <mark.rutland@arm.com>
> > Reviewed-by: Lorenzo Stoakes <lstoakes@gmail.com>
> > Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
> > Fixes: 29417d292bd0 ("mm/mmap/vma_merge: always check invariants")
> > Fixes: 69dbe6daf104 ("userfaultfd: use maple tree iterator to iterate VMAs")
> 
> I don't know how -stable maintainers are to handle more than a single
> Fixes: target, given that Fixes: means "kernels which have that patch
> need this one".  Can we narrow this down to a single commit for this
> purpose?

Please just keep:

Fixes: 69dbe6daf104 ("userfaultfd: use maple tree iterator to iterate VMAs")

I just noticed 29417d292bd0 is only in rc1 so no backport needed anyway.
We definitely need 69dbe6daf104 marked Fixes for backport till 6.1+.

Thanks,

-- 
Peter Xu