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    Subject[PATCH 02/15] FS-Cache: fix dereference of NULL user_key_payload
    Date
    From: Eric Biggers <ebiggers@google.com>

    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>
    ---

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

    diff --git a/fs/fscache/object-list.c b/fs/fscache/object-list.c
    index b5ab06fabc60..0438d4cd91ef 100644
    --- 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    + /* key was revoked */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--) {
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