[PATCH] ocfs2: fix slab-use-after-free due to dangling pointer dqi_priv

[Dennis Lam <dennis.lamerice@gmail.com>]

When mounting ocfs2 and then remounting it as read-only, a
slab-use-after-free occurs after the user uses a syscall to
quota_getnextquota. Specifically, sb_dqinfo(sb, type)->dqi_priv is the
dangling pointer.

During the remounting process, the pointer dqi_priv is freed but is
never set as null leaving it to to be accessed. Additionally, the
read-only option for remounting sets the DQUOT_SUSPENDED flag instead of
setting the DQUOT_USAGE_ENABLED flags. Moreover, later in the process of
getting the next quota, the function ocfs2_get_next_id is called and
only checks the quota usage flags and not the quota suspended flags.

To fix this, I set dqi_priv to null when it is freed after remounting
with read-only and put a check for DQUOT_SUSPENDED in ocfs2_get_next_id.

Signed-off-by: Dennis Lam <dennis.lamerice@gmail.com>
Reported-by: syzbot+d173bf8a5a7faeede34c@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/6731d26f.050a0220.1fb99c.014b.GAE@google.com/T/
---
 fs/ocfs2/quota_global.c | 3 ++-
 fs/ocfs2/quota_local.c  | 1 +
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/ocfs2/quota_global.c b/fs/ocfs2/quota_global.c
index 2b0daced98eb..c6d38340d6d4 100644
--- a/fs/ocfs2/quota_global.c
+++ b/fs/ocfs2/quota_global.c
@@ -893,7 +893,8 @@ static int ocfs2_get_next_id(struct super_block *sb, struct kqid *qid)
 	int status = 0;
 
 	trace_ocfs2_get_next_id(from_kqid(&init_user_ns, *qid), type);
-	if (!sb_has_quota_loaded(sb, type)) {
+	if (!sb_has_quota_loaded(sb, type) ||
+			sb_dqopt(sb)->flags & DQUOT_SUSPENDED) {
 		status = -ESRCH;
 		goto out;
 	}
diff --git a/fs/ocfs2/quota_local.c b/fs/ocfs2/quota_local.c
index 73d3367c533b..2956d888c131 100644
--- a/fs/ocfs2/quota_local.c
+++ b/fs/ocfs2/quota_local.c
@@ -867,6 +867,7 @@ static int ocfs2_local_free_info(struct super_block *sb, int type)
 	brelse(oinfo->dqi_libh);
 	brelse(oinfo->dqi_lqi_bh);
 	kfree(oinfo);
+	info->dqi_priv = NULL;
 	return status;
 }
 
-- 
2.47.0