gfs2: Fix use-after-free in gfs2_glock_shrink_scan

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
lru_lock.

Reported-by: syzbot <syzbot+34ba7ddbf3021981a228@syzkaller.appspotmail.com>
Signed-off-by: Hillf Danton <hdanton@sina.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

diff --git a/fs/gfs2/glock.c b/fs/gfs2/glock.c
index 797949e..d9cb261 100644 (file)
--- a/fs/gfs2/glock.c
+++ b/fs/gfs2/glock.c
@@ -1795,6 +1795,7 @@ __acquires(&lru_lock)
        while(!list_empty(list)) {
                gl = list_first_entry(list, struct gfs2_glock, gl_lru);
                list_del_init(&gl->gl_lru);
+               clear_bit(GLF_LRU, &gl->gl_flags);
                if (!spin_trylock(&gl->gl_lockref.lock)) {
 add_back_to_lru:
                        list_add(&gl->gl_lru, &lru_list);
@@ -1840,7 +1841,6 @@ static long gfs2_scan_glock_lru(int nr)
                if (!test_bit(GLF_LOCK, &gl->gl_flags)) {
                        list_move(&gl->gl_lru, &dispose);
                        atomic_dec(&lru_count);
-                       clear_bit(GLF_LRU, &gl->gl_flags);
                        freed++;
                        continue;
                }