Re: [PATCH v2] n_tty: Protect tty->disc_data using refcount.

[Jiri Slaby <jslaby@suse.cz>]

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On 07/24/2018, 05:22 PM, Tetsuo Handa wrote:
> From 118c64e86641a97d44dec39e313a95b12d9bc3b2 Mon Sep 17 00:00:00 2001
> From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> Date: Wed, 25 Jul 2018 00:15:18 +0900
> Subject: [PATCH v2] n_tty: Protect tty->disc_data using refcount.
> 
> syzbot is reporting NULL pointer dereference at n_tty_set_termios() [1].
> This is because ioctl(TIOCVHANGUP) versus ioctl(TCSETS) can race.
> 
> Since we don't want to introduce new locking dependency, this patch
> converts "struct n_tty_data *ldata = tty->disc_data;" in individual
> function into a function argument which follows "struct tty *", and
> holds tty->disc_data at each "struct tty_ldisc_ops" hook using refcount
> in order to ensure that memory which contains "struct n_tty_data" will
> not be released while processing individual function.

This does not look correct and is way too complicated. ioctls should not
be called while changing/killing/hanging/whatever a ldisc. But there is
one missing lock in tty_reopen.

So does the attached patch helps instead?

thanks,
-- 
js
suse labs

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From a41b427c77d920cd151473361788deb80e576f6c Mon Sep 17 00:00:00 2001
From: Jiri Slaby <jslaby@suse.cz>
Date: Wed, 29 Aug 2018 15:49:51 +0200
Subject: [PATCH] tty: fix NULL ptr dereference

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
---
 drivers/tty/tty_io.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index f89939922050..160f320e0d2c 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -1255,6 +1255,7 @@ static void tty_driver_remove_tty(struct tty_driver *driver, struct tty_struct *
 static int tty_reopen(struct tty_struct *tty)
 {
 	struct tty_driver *driver = tty->driver;
+	int ret = 0;
 
 	if (driver->type == TTY_DRIVER_TYPE_PTY &&
 	    driver->subtype == PTY_TYPE_MASTER)
@@ -1268,10 +1269,12 @@ static int tty_reopen(struct tty_struct *tty)
 
 	tty->count++;
 
+	tty_ldisc_lock(tty, MAX_SCHEDULE_TIMEOUT);
 	if (!tty->ldisc)
-		return tty_ldisc_reinit(tty, tty->termios.c_line);
+		ret = tty_ldisc_reinit(tty, tty->termios.c_line);
+	tty_ldisc_unlock(tty);
 
-	return 0;
+	return ret;
 }
 
 /**
-- 
2.18.0