FS-Cache: fix dereference of NULL user_key_payload

commit d124b2c53c7bee6569d2a2d0b18b4a1afde00134 upstream.

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.

Fixes: 4fbf4291aa15 ("FS-Cache: Allow the current state of all objects to be dumped")
Reviewed-by: James Morris <james.l.morris@oracle.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/fs/fscache/object-list.c b/fs/fscache/object-list.c
index b8179ca6bf9d125aa7862012eb61e5a43b85234e..8a96aeb2391f219ecd86ad199f7c0b43b84f395d 100644 (file)
--- a/fs/fscache/object-list.c
+++ b/fs/fscache/object-list.c
@@ -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+               /* key was revoked */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--) {