audit: Use rcu for task lookup protection

Protect the task lookups in audit_receive_msg() with rcu_read_lock()
instead of tasklist_lock and use lock/unlock_sighand to protect
against the exit race.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Eric Paris <eparis@redhat.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

kernel/audit.c

@@ -873,17 +873,16 @@ static int audit_receive_msg(struct sk_buff *skb, struct nlmsghdr *nlh)
 	case AUDIT_TTY_GET: {
 		struct audit_tty_status s;
 		struct task_struct *tsk;
+		unsigned long flags;
 
-		read_lock(&tasklist_lock);
+		rcu_read_lock();
 		tsk = find_task_by_vpid(pid);
-		if (!tsk)
-			err = -ESRCH;
-		else {
-			spin_lock_irq(&tsk->sighand->siglock);
+		if (tsk && lock_task_sighand(tsk, &flags)) {
 			s.enabled = tsk->signal->audit_tty != 0;
-			spin_unlock_irq(&tsk->sighand->siglock);
-		}
-		read_unlock(&tasklist_lock);
+			unlock_task_sighand(tsk, &flags);
+		} else
+			err = -ESRCH;
+		rcu_read_unlock();
 
 		if (!err)
 			audit_send_reply(NETLINK_CB(skb).pid, seq,
@@ -893,22 +892,21 @@ static int audit_receive_msg(struct sk_buff *skb, struct nlmsghdr *nlh)
 	case AUDIT_TTY_SET: {
 		struct audit_tty_status *s;
 		struct task_struct *tsk;
+		unsigned long flags;
 
 		if (nlh->nlmsg_len < sizeof(struct audit_tty_status))
 			return -EINVAL;
 		s = data;
 		if (s->enabled != 0 && s->enabled != 1)
 			return -EINVAL;
-		read_lock(&tasklist_lock);
+		rcu_read_lock();
 		tsk = find_task_by_vpid(pid);
-		if (!tsk)
-			err = -ESRCH;
-		else {
-			spin_lock_irq(&tsk->sighand->siglock);
+		if (tsk && lock_task_sighand(tsk, &flags)) {
 			tsk->signal->audit_tty = s->enabled != 0;
-			spin_unlock_irq(&tsk->sighand->siglock);
-		}
-		read_unlock(&tasklist_lock);
+			unlock_task_sighand(tsk, &flags);
+		} else
+			err = -ESRCH;
+		rcu_read_unlock();
 		break;
 	}
 	default: