LSM: Check for NULL cred-security on free

From: Casey Schaufler <casey@schaufler-ca.com>

Check that the cred security blob has been set before trying
to clean it up. There is a case during credential initialization
that could result in this.

Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
Acked-by: John Johansen <john.johansen@canonical.com>
Signed-off-by: James Morris <james.morris@microsoft.com>
Reported-by:  <syzbot+69ca07954461f189e808@syzkaller.appspotmail.com>

security/security.c

@@ -1027,6 +1027,13 @@ int security_cred_alloc_blank(struct cred *cred, gfp_t gfp)
 
 void security_cred_free(struct cred *cred)
 {
+	/*
+	 * There is a failure case in prepare_creds() that
+	 * may result in a call here with ->security being NULL.
+	 */
+	if (unlikely(cred->security == NULL))
+		return;
+
 	call_void_hook(cred_free, cred);
 }