tty: fix leakage of -ERESTARTSYS to userland

Spotted by Satoru Takeuchi.

kill_pgrp(task_pgrp(current)) sends the signal to the current's thread
group, but can choose any sub-thread as a target for signal_wake_up().
This means that job_control() and tty_check_change() may return
-ERESTARTSYS without signal_pending().

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
Cc: Roland McGrath <roland@redhat.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/char/n_tty.c

@@ -1191,6 +1191,7 @@ static int job_control(struct tty_struct *tty, struct file *file)
 			    is_current_pgrp_orphaned())
 				return -EIO;
 			kill_pgrp(task_pgrp(current), SIGTTIN, 1);
+			set_thread_flag(TIF_SIGPENDING);
 			return -ERESTARTSYS;
 		}
 	}

drivers/char/tty_io.c

@@ -1148,7 +1148,8 @@ int tty_check_change(struct tty_struct * tty)
 		return 0;
 	if (is_current_pgrp_orphaned())
 		return -EIO;
-	(void) kill_pgrp(task_pgrp(current), SIGTTOU, 1);
+	kill_pgrp(task_pgrp(current), SIGTTOU, 1);
+	set_thread_flag(TIF_SIGPENDING);
 	return -ERESTARTSYS;
 }