[PATCH 6.6.y] mm/userfaultfd: write-lock the VMA before waking waiters when unregistering

[Sasha Levin <sashal@kernel.org>]

From: "David Hildenbrand (Arm)" <david@kernel.org>

[ Upstream commit 35f7ecd55ce7e51cb6d5ef49e6dd51f3c80f61e4 ]

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@vger.kernel.org
Reported-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Link: https://patch.msgid.link/20261031-fixes_core-v3-23-d6437171ecc3@kernel.org
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
[ sashal: adjusted the patch location because userfaultfd still resides in the filesystem subsystem ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/userfaultfd.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index bd64d6b9cf899..11b4ed7efa5f8 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -1664,6 +1664,9 @@ 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
-- 
2.53.0