[PATCH 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@xxxxxxxxxx>
Cc: Mike Rapoport (IBM) <rppt@xxxxxxxxxx>
Cc: linux-stable <stable@xxxxxxxxxxxxxxx>
Fixes: 86039bd3b4e6 ("userfaultfd: add new syscall to provide memory externalization")
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
---
 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