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Subject[PATCH 3.16 110/204] FS-Cache: fix dereference of NULL user_key_payload
3.16.52-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Eric Biggers <ebiggers@google.com>

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

--- a/fs/fscache/object-list.c
+++ b/fs/fscache/object-list.c
@@ -330,6 +330,13 @@ static void fscache_objlist_config(struc
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+ /* key was revoked */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--) {
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