audit: printk USER_AVC messages when audit isn't enabled

When the audit=1 kernel parameter is absent and auditd is not running,
AUDIT_USER_AVC messages are being silently discarded.

AUDIT_USER_AVC messages should be sent to userspace using printk(), as
mentioned in the commit message of 4a4cd63 ("AUDIT: Optimise the
audit-disabled case for discarding user messages").

When audit_enabled is 0, audit_receive_msg() discards all user messages
except for AUDIT_USER_AVC messages. However, audit_log_common_recv_msg()
refuses to allocate an audit_buffer if audit_enabled is 0. The fix is to
special case AUDIT_USER_AVC messages in both functions.

It looks like commit 50397bd ("[AUDIT] clean up audit_receive_msg()")
introduced this bug.

Cc: <[email protected]> # v2.6.25+
Signed-off-by: Tyler Hicks <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Eric Paris <[email protected]>
Cc: [email protected]
Acked-by: Kees Cook <[email protected]>
Signed-off-by: Richard Guy Briggs <[email protected]>
Signed-off-by: Eric Paris <[email protected]>

kernel/audit.c

@@ -612,7 +612,7 @@ static int audit_log_common_recv_msg(struct audit_buffer **ab, u16 msg_type)
 	int rc = 0;
 	uid_t uid = from_kuid(&init_user_ns, current_uid());
 
-	if (!audit_enabled) {
+	if (!audit_enabled && msg_type != AUDIT_USER_AVC) {
 		*ab = NULL;
 		return rc;
 	}