fix bogus reporting of signals by audit

Async signals should not be reported as sent by current in audit log.  As
it is, we call audit_signal_info() too early in check_kill_permission().
Note that check_kill_permission() has that test already - it needs to know
if it should apply current-based permission checks.  So the solution is to
move the call of audit_signal_info() between those.

Bogosity in question is easily reproduced - add a rule watching for e.g.
kill(2) from specific process (so that audit_signal_info() would not
short-circuit to nothing), say load_policy, watch the bogus OBJ_PID entry
in audit logs claiming that write(2) on selinuxfs file issued by
load_policy(8) had somehow managed to send a signal to syslogd...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Acked-by: Steve Grubb <sgrubb@redhat.com>
Acked-by: Eric Paris <eparis@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/signal.c

@@ -531,18 +531,18 @@ static int check_kill_permission(int sig, struct siginfo *info,
 	if (!valid_signal(sig))
 		return error;
 
-	error = audit_signal_info(sig, t); /* Let audit system see the signal */
-	if (error)
-		return error;
-
-	error = -EPERM;
-	if ((info == SEND_SIG_NOINFO || (!is_si_special(info) && SI_FROMUSER(info)))
-	    && ((sig != SIGCONT) ||
-		(process_session(current) != process_session(t)))
-	    && (current->euid ^ t->suid) && (current->euid ^ t->uid)
-	    && (current->uid ^ t->suid) && (current->uid ^ t->uid)
-	    && !capable(CAP_KILL))
+	if (info == SEND_SIG_NOINFO || (!is_si_special(info) && SI_FROMUSER(info))) {
+		error = audit_signal_info(sig, t); /* Let audit system see the signal */
+		if (error)
+			return error;
+		error = -EPERM;
+		if (((sig != SIGCONT) ||
+			(process_session(current) != process_session(t)))
+		    && (current->euid ^ t->suid) && (current->euid ^ t->uid)
+		    && (current->uid ^ t->suid) && (current->uid ^ t->uid)
+		    && !capable(CAP_KILL))
 		return error;
+	}
 
 	return security_task_kill(t, info, sig, 0);
 }