[PATCH 23/46] mm/userfaultfd: write-lock the VMA before waking waiters when unregistering

[David Hildenbrand (Arm)]

userfaultfd_unregister() wakes pending missing faults before clearing the
VMA's userfaultfd registration. With per-VMA fault locking, holding
mmap_lock for write does not prevent another fault from queuing after
the wakeup but before userfaultfd_clear_vma() write-locks the VMA. That
waiter can then remain asleep indefinitely.

Call vma_start_write() before waking waiters. Existing fault handlers
queue themselves before dropping the VMA read lock, and new readers are
excluded until mmap_write_unlock().

Fixes: 29a22b9e08d7 ("mm: handle userfaults under VMA lock")
Cc: stable at vger.kernel.org
Reported-by: Josef Bacik <josef at toxicpanda.com>
Signed-off-by: David Hildenbrand (Arm) <david at kernel.org>
---
 mm/userfaultfd.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 669f71d485d02..7e30b5ff17be1 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -3991,6 +3991,9 @@ static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
 			start = vma->vm_start;
 		vma_end = min(end, vma->vm_end);
 
+		/* Prevent new userfaults from being queued after the wakeup. */
+		vma_start_write(vma);
+
 		if (userfaultfd_missing(vma)) {
 			/*
 			 * Wake any concurrent pending userfault while

-- 
2.43.0