[PATCH 03/15] lib/digsig: fix dereference of NULL user_key_payload

[David Howells]

From: Eric Biggers <ebigg...@google.com>

digsig_verify() requests a user key, then accesses 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 acquire its semaphore.

Fix it by checking for a NULL payload, treating it like a key which was
already revoked at the time it was requested.

Fixes: 051dbb918c7f ("crypto: digital signature verification support")
Reviewed-by: James Morris <james.l.mor...@oracle.com>
Cc: <sta...@vger.kernel.org>    [v3.3+]
Cc: Dmitry Kasatkin <dmitry.kasat...@intel.com>
Signed-off-by: Eric Biggers <ebigg...@google.com>
Signed-off-by: David Howells <dhowe...@redhat.com>
---

 lib/digsig.c |    6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/lib/digsig.c b/lib/digsig.c
index 03d7c63837ae..6ba6fcd92dd1 100644
--- a/lib/digsig.c
+++ b/lib/digsig.c
@@ -87,6 +87,12 @@ static int digsig_verify_rsa(struct key *key,
        down_read(&key->sem);
        ukp = user_key_payload_locked(key);
 
+       if (!ukp) {
+               /* key was revoked before we acquired its semaphore */
+               err = -EKEYREVOKED;
+               goto err1;
+       }
+
        if (ukp->datalen < sizeof(*pkh))
                goto err1;