FS-Cache: fix dereference of NULL user_key_payload

From: Eric Biggers <[email protected]>

When the file /proc/fs/fscache/objects (available with CONFIG_FSCACHE_OBJECT_LIST=y) is opened, we request a user key with description "fscache:objlist", then access its payload. However, a revoked key has a NULL payload, and we failed to check for this. request_key() *does* skip revoked keys, but there is still a window where the key can be revoked before we access its payload.

Fix it by checking for a NULL payload, treating it like a key which was already revoked at the time it was requested.

---

Changed since v1: added Reviewed-by and resent as standalone patch. Can this please be taken through the keyrings tree?

 fs/fscache/object-list.c | 7 +++++++
 1 file changed, 7 insertions(+)

index b5ab06fabc60..0438d4cd91ef 100644
@@ -331,6 +331,13 @@ static void fscache_objlist_config(struct fscache_objlist_data *data)
 	rcu_read_lock();
 
 	confkey = user_key_payload_rcu(key);
+	if (!confkey) {
+		/* key was revoked */
+		rcu_read_unlock();
+		key_put(key);
+		goto no_config;
+	}
+
 	buf = confkey->data;
 
 	for (len = confkey->datalen - 1; len >= 0; len--) {

2.14.2.920.gcf0c67979c-goog