[PATCH] tty: Don't call panic() at tty_ldisc_init()

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>

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=883431818e036ae6a9981156a64b821110f39187

Signed-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>
---
 drivers/tty/tty_io.c    | 5 ++++-
 drivers/tty/tty_ldisc.c | 5 +++--
 include/linux/tty.h     | 2 +-
 3 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 63114ea..7c838b9 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -2816,7 +2816,10 @@ 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);
diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
index 050f4d6..08ddb2c 100644
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -824,12 +824,13 @@ 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;
 }
 
 /**
diff --git a/include/linux/tty.h b/include/linux/tty.h
index 47f8af2..1dd587ba 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -701,7 +701,7 @@ static inline int tty_port_users(struct tty_port *port)
 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 int tty_ldisc_receive_buf(struct tty_ldisc *ld, const unsigned char *p,
 				 char *f, int count);
-- 
1.8.3.1