mm/uffd: allow vma to merge as much as possible

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).

Link: https://lkml.kernel.org/r/20230517190916.3429499-3-peterx@redhat.com
Fixes: 86039bd3b4e6 ("userfaultfd: add new syscall to provide memory externalization")
Signed-off-by: Peter Xu <peterx@redhat.com>
Reported-by: Lorenzo Stoakes <lstoakes@gmail.com>
Acked-by: Lorenzo Stoakes <lstoakes@gmail.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 17c8c34..4e800bb 100644 (file)
--- 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) {