[PATCH 5.15.y] mm/userfaultfd: compare exact registration modes before skipping

[Sasha Levin <sashal@kernel.org>]

From: Gregory Price <gourry@gourry.net>

[ Upstream commit 05f64f211a25f7d9771d4fde109aad97174a1cf3 ]

Changing registration from MISSING|WP to WP returns success but leaves
MISSING enabled. The no-op check sees that WP is already set and skips
the update, so later accesses to missing pages still wait for userspace.

Skip the update only when the current and requested UFFD modes match
exactly.

Fixes: 63b2d4174c4a ("userfaultfd: wp: add the writeprotect API to userfaultfd ioctl")
Cc: stable@vger.kernel.org
Signed-off-by: Gregory Price (Meta) <gourry@gourry.net>
[ david: apply fixup from Lorenzo ]
Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Link: https://patch.msgid.link/20261031-fixes_core-v3-26-d6437171ecc3@kernel.org
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
[ sashal: replaced vma_flags_t helpers with a direct vm_flags bitmask comparison ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 fs/userfaultfd.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index ea7e9db135eb4..1c4b0cb54d878 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -1435,7 +1435,7 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
 		 * userfaultfd and with the right tracking mode too.
 		 */
 		if (vma->vm_userfaultfd_ctx.ctx == ctx &&
-		    (vma->vm_flags & vm_flags) == vm_flags)
+		    (vma->vm_flags & __VM_UFFD_FLAGS) == vm_flags)
 			goto skip;
 
 		if (vma->vm_start > start)
-- 
2.53.0