signals: cleanup security_task_kill() usage/implementation

Every implementation of ->task_kill() does nothing when the signal comes from
the kernel.  This is correct, but means that check_kill_permission() should
call security_task_kill() only for SI_FROMUSER() case, and we can remove the
same check from ->task_kill() implementations.

(sadly, check_kill_permission() is the last user of signal->session/__session
 but we can't s/task_session_nr/task_session/ here).

NOTE: Eric W.  Biederman pointed out cap_task_kill() should die, and I think
he is very right.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Serge Hallyn <serue@us.ibm.com>
Cc: Roland McGrath <roland@redhat.com>
Cc: Casey Schaufler <casey@schaufler-ca.com>
Cc: David Quigley <dpquigl@tycho.nsa.gov>
Cc: Eric Paris <eparis@redhat.com>
Cc: Harald Welte <laforge@gnumonks.org>
Cc: Pavel Emelyanov <xemul@openvz.org>
Cc: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/signal.c

@@ -533,22 +533,23 @@ static int rm_from_queue(unsigned long mask, struct sigpending *s)
 static int check_kill_permission(int sig, struct siginfo *info,
 				 struct task_struct *t)
 {
-	int error = -EINVAL;
+	int error;
+
 	if (!valid_signal(sig))
-		return error;
+		return -EINVAL;
 
-	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) ||
-			(task_session_nr(current) != task_session_nr(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_FROMKERNEL(info)))
+		return 0;
+
+	error = audit_signal_info(sig, t); /* Let audit system see the signal */
+	if (error)
 		return error;
-	}
+
+	if (((sig != SIGCONT) || (task_session_nr(current) != task_session_nr(t)))
+	    && (current->euid ^ t->suid) && (current->euid ^ t->uid)
+	    && (current->uid ^ t->suid) && (current->uid ^ t->uid)
+	    && !capable(CAP_KILL))
+		return -EPERM;
 
 	return security_task_kill(t, info, sig, 0);
 }

security/selinux/hooks.c

@@ -3286,9 +3286,6 @@ static int selinux_task_kill(struct task_struct *p, struct siginfo *info,
 	if (rc)
 		return rc;
 
-	if (info != SEND_SIG_NOINFO && (is_si_special(info) || SI_FROMKERNEL(info)))
-		return 0;
-
 	if (!sig)
 		perm = PROCESS__SIGNULL; /* null signal; existence test */
 	else

security/smack/smack_lsm.c

@@ -1130,15 +1130,6 @@ static int smack_task_movememory(struct task_struct *p)
 static int smack_task_kill(struct task_struct *p, struct siginfo *info,
 			   int sig, u32 secid)
 {
-	/*
-	 * Special cases where signals really ought to go through
-	 * in spite of policy. Stephen Smalley suggests it may
-	 * make sense to change the caller so that it doesn't
-	 * bother with the LSM hook in these cases.
-	 */
-	if (info != SEND_SIG_NOINFO &&
-	    (is_si_special(info) || SI_FROMKERNEL(info)))
-		return 0;
 	/*
 	 * Sending a signal requires that the sender
 	 * can write the receiver.