[PATCH] gfs2: Fix slab-use-after-free in gfs2_qd_dispose

[Deepanshu Kartikey <kartikey406@gmail.com>]

In gfs2_qd_dispose(), the quota data (qd) is removed from qd_list and
qd_hlist, but not removed from the LRU list before call_rcu() is
invoked to free the object.

This can lead to a use-after-free when the shrinker (gfs2_qd_shrink_scan)
runs concurrently with filesystem unmount. The shrinker attempts to walk
the LRU list via list_lru_shrink_walk(), accessing quota data that has
already been freed by the RCU callback (gfs2_qd_dealloc).

Fix this by calling list_lru_del_obj() to remove the quota data from
the LRU list before scheduling the RCU callback.

Fixes: faada74a90563 ("gfs2: Factor out duplicate quota data disposal code")
Reported-by: syzbot+046b605f01802054bff0@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=046b605f01802054bff0
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
---
Note: Only compile-tested. No reproducer available from syzbot.
---
 fs/gfs2/quota.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/gfs2/quota.c b/fs/gfs2/quota.c
index b1692f12a602..85b0b5105b4b 100644
--- a/fs/gfs2/quota.c
+++ b/fs/gfs2/quota.c
@@ -125,6 +125,8 @@ static void gfs2_qd_dispose(struct gfs2_quota_data *qd)
 	hlist_bl_del_rcu(&qd->qd_hlist);
 	spin_unlock_bucket(qd->qd_hash);
 
+	list_lru_del_obj(&gfs2_qd_lru, &qd->qd_lru);
+
 	if (!gfs2_withdrawn(sdp)) {
 		gfs2_assert_warn(sdp, !qd->qd_change);
 		gfs2_assert_warn(sdp, !qd->qd_slot_ref);
-- 
2.43.0