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

[Peter Xu <peterx@redhat.com>]

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