[PATCH 1/1] mm: lock VMAs skipped by a failed queue_pages_range()

[Suren Baghdasaryan <surenb@google.com>]

When queue_pages_range() encounters an unmovable page, it terminates
its page walk. This walk, among other things, locks the VMAs in the range.
This termination might result in some VMAs being left unlock after
queue_pages_range() completes. Since do_mbind() continues to operate on
these VMAs despite the failure from queue_pages_range(), it will encounter
an unlocked VMA.
This mbind() behavior has been modified several times before and might
need some changes to either finish the page walk even in the presence
of unmovable pages or to error out immediately after the failure to
queue_pages_range(). However that requires more discussions, so to
fix the immediate issue, explicitly lock the VMAs in the range if
queue_pages_range() failed. The added condition does not save much
but is added for documentation purposes to understand when this extra
locking is needed.

Fixes: 49b0638502da ("mm: enable page walking API to lock vmas during the walk")
Reported-by: syzbot+b591856e0f0139f83023@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/000000000000f392a60604a65085@google.com/
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
 mm/mempolicy.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index 42b5567e3773..cbc584e9b6ca 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -1342,6 +1342,9 @@ static long do_mbind(unsigned long start, unsigned long len,
 	vma_iter_init(&vmi, mm, start);
 	prev = vma_prev(&vmi);
 	for_each_vma_range(vmi, vma, end) {
+		/* If queue_pages_range failed then not all VMAs might be locked */
+		if (ret)
+			vma_start_write(vma);
 		err = mbind_range(&vmi, vma, &prev, start, end, new);
 		if (err)
 			break;
-- 
2.42.0.459.ge4e396fd5e-goog