tty: Don't call panic() at tty_ldisc_init()

BugLink: http://bugs.launchpad.net/bugs/1768825

commit 903f9db1 upstream.

syzbot is reporting kernel panic [1] triggered by memory allocation failure
at tty_ldisc_get() from tty_ldisc_init(). But since both tty_ldisc_get()
and caller of tty_ldisc_init() can cleanly handle errors, tty_ldisc_init()
does not need to call panic() when tty_ldisc_get() failed.

[1] https://syzkaller.appspot.com/bug?id=883431818e036ae6a9981156a64b821110f39187Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reported-by: syzbot <syzkaller@googlegroups.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

drivers/tty/tty_io.c

@@ -3169,7 +3169,10 @@ struct tty_struct *alloc_tty_struct(struct tty_driver *driver, int idx)
 
 	kref_init(&tty->kref);
 	tty->magic = TTY_MAGIC;
-	tty_ldisc_init(tty);
+	if (tty_ldisc_init(tty)) {
+		kfree(tty);
+		return NULL;
+	}
 	tty->session = NULL;
 	tty->pgrp = NULL;
 	mutex_init(&tty->legacy_mutex);

drivers/tty/tty_ldisc.c

@@ -802,12 +802,13 @@ void tty_ldisc_release(struct tty_struct *tty)
  *	the tty structure is not completely set up when this call is made.
  */
 
-void tty_ldisc_init(struct tty_struct *tty)
+int tty_ldisc_init(struct tty_struct *tty)
 {
 	struct tty_ldisc *ld = tty_ldisc_get(tty, N_TTY);
 	if (IS_ERR(ld))
-		panic("n_tty: init_tty");
+		return PTR_ERR(ld);
 	tty->ldisc = ld;
+	return 0;
 }
 
 /**

include/linux/tty.h

@@ -587,7 +587,7 @@ extern int tty_unregister_ldisc(int disc);
 extern int tty_set_ldisc(struct tty_struct *tty, int disc);
 extern int tty_ldisc_setup(struct tty_struct *tty, struct tty_struct *o_tty);
 extern void tty_ldisc_release(struct tty_struct *tty);
-extern void tty_ldisc_init(struct tty_struct *tty);
+extern int __must_check tty_ldisc_init(struct tty_struct *tty);
 extern void tty_ldisc_deinit(struct tty_struct *tty);
 extern void tty_ldisc_begin(void);