[PATCH 5.4 14/28] gfs2: Fix use-after-free in gfs2_glock_shrink_scan

From: Greg Kroah-Hartman
Date: Wed Jun 16 2021 - 11:34:24 EST

From: Hillf Danton <hdanton@xxxxxxxx>

[ Upstream commit 1ab19c5de4c537ec0d9b21020395a5b5a6c059b2 ]

The GLF_LRU flag is checked under lru_lock in gfs2_glock_remove_from_lru() to
remove the glock from the lru list in __gfs2_glock_put().

On the shrink scan path, the same flag is cleared under lru_lock but because
of cond_resched_lock(&lru_lock) in gfs2_dispose_glock_lru(), progress on the
put side can be made without deleting the glock from the lru list.

Keep GLF_LRU across the race window opened by cond_resched_lock(&lru_lock) to
ensure correct behavior on both sides - clear GLF_LRU after list_del under

Reported-by: syzbot <syzbot+34ba7ddbf3021981a228@xxxxxxxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Hillf Danton <hdanton@xxxxxxxx>
Signed-off-by: Andreas Gruenbacher <agruenba@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
fs/gfs2/glock.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/gfs2/glock.c b/fs/gfs2/glock.c
index 9e1685a30bf8..01c41087f067 100644
--- a/fs/gfs2/glock.c
+++ b/fs/gfs2/glock.c
@@ -1552,6 +1552,7 @@ __acquires(&lru_lock)
while(!list_empty(list)) {
gl = list_entry(list->next, struct gfs2_glock, gl_lru);
+ clear_bit(GLF_LRU, &gl->gl_flags);
if (!spin_trylock(&gl->gl_lockref.lock)) {
list_add(&gl->gl_lru, &lru_list);
@@ -1597,7 +1598,6 @@ static long gfs2_scan_glock_lru(int nr)
if (!test_bit(GLF_LOCK, &gl->gl_flags)) {
list_move(&gl->gl_lru, &dispose);
- clear_bit(GLF_LRU, &gl->gl_flags);