FS-Cache: fix dereference of NULL user_key_payload

From: Eric Biggers <[email protected]>

commit d124b2c53c7bee6569d2a2d0b18b4a1afde00134 upstream. Please apply to stable for 4.3 and earlier.

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.

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

index 51dde817e1f2..11cf71ac1cb5 100644
@@ -330,6 +330,13 @@ static void fscache_objlist_config(struct fscache_objlist_data *data)
 	rcu_read_lock();
 
 	confkey = key->payload.data;
+	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+		/* key was revoked */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--) {

2.15.0.rc0.271.g36b669edcc-goog

Applied to 3.18, thanks.

greg k-h