FS-Cache: fix dereference of NULL user_key_payload

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>
Cc: <stable@vger.kernel.org>    [v2.6.32+]
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>

diff --git a/fs/fscache/object-list.c b/fs/fscache/object-list.c
index b5ab06fabc60a3bd0a7a308f45e4cb5b850398c1..0438d4cd91ef74ee98f8990936d07c3bf8a67b30 100644 (file)
--- a/fs/fscache/object-list.c
+++ b/fs/fscache/object-list.c
@@ -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+               /* key was revoked */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--) {