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

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 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.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 8f9e8f5 ("ocfs2: Fix Q_GETNEXTQUOTA for filesystem without quotas")
Signed-off-by: Dennis Lam <[email protected]>
Reported-by: [email protected]
Tested-by: [email protected]
Closes: https://lore.kernel.org/all/[email protected]/T/
Reviewed-by: Joseph Qi <[email protected]>
Cc: Mark Fasheh <[email protected]>
Cc: Joel Becker <[email protected]>
Cc: Junxiao Bi <[email protected]>
Cc: Changwei Ge <[email protected]>
Cc: Jun Piao <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

fs/ocfs2/quota_global.c

@@ -893,7 +893,7 @@ 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_active(sb, type)){
 		status = -ESRCH;
 		goto out;
 	}

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;
 }