From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail.kernel.org ([198.145.29.99]:34188 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755989AbeD3T1c (ORCPT ); Mon, 30 Apr 2018 15:27:32 -0400 From: Greg Kroah-Hartman To: linux-kernel@vger.kernel.org Cc: Greg Kroah-Hartman , stable@vger.kernel.org, syzbot+40b7287c2dc987c48c81@syzkaller.appspotmail.com, Tetsuo Handa , Jiri Slaby , Dmitry Vyukov , Johannes Weiner , Alan Cox , Christoph Hellwig , Michal Hocko Subject: [PATCH 4.14 32/91] tty: Avoid possible error pointer dereference at tty_ldisc_restore(). Date: Mon, 30 Apr 2018 12:24:14 -0700 Message-Id: <20180430184005.847588872@linuxfoundation.org> In-Reply-To: <20180430184004.216234025@linuxfoundation.org> References: <20180430184004.216234025@linuxfoundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Sender: stable-owner@vger.kernel.org List-ID: 4.14-stable review patch. If anyone has any objections, please let me know. ------------------ From: Tetsuo Handa commit 598c2d41ff44889dd8eced4f117403e472158d85 upstream. syzbot is reporting crashes [1] triggered by memory allocation failure at tty_ldisc_get() from tty_ldisc_restore(). While syzbot stops at WARN_ON() due to panic_on_warn == true, panic_on_warn == false will after all trigger an OOPS by dereferencing old->ops->num if IS_ERR(old) == true. We can simplify tty_ldisc_restore() as three calls (old->ops->num, N_TTY, N_NULL) to tty_ldisc_failto() in addition to avoiding possible error pointer dereference. If someone reports kernel panic triggered by forcing all memory allocations for tty_ldisc_restore() to fail, we can consider adding __GFP_NOFAIL for tty_ldisc_restore() case. [1] https://syzkaller.appspot.com/bug?id=6ac359c61e71d22e06db7f8f88243feb11d927e7 Reported-by: syzbot+40b7287c2dc987c48c81@syzkaller.appspotmail.com Signed-off-by: Tetsuo Handa Cc: Greg Kroah-Hartman Cc: Jiri Slaby Cc: Dmitry Vyukov Cc: Johannes Weiner Cc: Alan Cox Cc: Christoph Hellwig Cc: Michal Hocko Cc: stable Signed-off-by: Greg Kroah-Hartman --- drivers/tty/tty_ldisc.c | 13 +++++-------- 1 file changed, 5 insertions(+), 8 deletions(-) --- a/drivers/tty/tty_ldisc.c +++ b/drivers/tty/tty_ldisc.c @@ -526,19 +526,16 @@ static int tty_ldisc_failto(struct tty_s static void tty_ldisc_restore(struct tty_struct *tty, struct tty_ldisc *old) { /* There is an outstanding reference here so this is safe */ - old = tty_ldisc_get(tty, old->ops->num); - WARN_ON(IS_ERR(old)); - tty->ldisc = old; - tty_set_termios_ldisc(tty, old->ops->num); - if (tty_ldisc_open(tty, old) < 0) { - tty_ldisc_put(old); + if (tty_ldisc_failto(tty, old->ops->num) < 0) { + const char *name = tty_name(tty); + + pr_warn("Falling back ldisc for %s.\n", name); /* The traditional behaviour is to fall back to N_TTY, we want to avoid falling back to N_NULL unless we have no choice to avoid the risk of breaking anything */ if (tty_ldisc_failto(tty, N_TTY) < 0 && tty_ldisc_failto(tty, N_NULL) < 0) - panic("Couldn't open N_NULL ldisc for %s.", - tty_name(tty)); + panic("Couldn't open N_NULL ldisc for %s.", name); } }