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To:     gregkh@linuxfoundation.org, jslaby@suse.com
Cc:     alan@llwyncelyn.cymru,
        bot+1a77aeddeaf63317949b59c3df98f139a1ca34f9@syzkaller.appspotmail.com,
        linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com,
        dvyukov@google.com, hannes@cmpxchg.org, hch@lst.de, mhocko@suse.com
Subject: [PATCH] tty: Avoid possible error pointer dereference at tty_ldisc_restore().
From:   Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
References: <001a1141f0c87da52c055d385a4d@google.com>
        <20171105103404.GB1487@kroah.com>
        <2e8fd7a6-6841-d660-8e1c-17b5a07618fa@I-love.SAKURA.ne.jp>
        <20180405142503.627552ca@alans-desktop>
In-Reply-To: <20180405142503.627552ca@alans-desktop>
Message-Id: <201804162006.GBB48461.FFJFSOMOOQHVLt@I-love.SAKURA.ne.jp>
Date:   Mon, 16 Apr 2018 20:06:34 +0900
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Greg and Jiri, are you OK with this patch?

Alan Cox wrote:
> > 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
> > 
> > Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > Cc: Jiri Slaby <jslaby@suse.com>
> > Cc: Dmitry Vyukov <dvyukov@google.com>
> > Cc: Johannes Weiner <hannes@cmpxchg.org>
> > Cc: Alan Cox <alan@llwyncelyn.cymru>
> > Cc: Christoph Hellwig <hch@lst.de>
> > Cc: Michal Hocko <mhocko@suse.com>
> 
> Seems reasonable to me
> 
> Alan
> 

>From 023cf07f799d0efd160ec1c1617d5b8902577765 Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Date: Thu, 5 Apr 2018 11:27:06 +0900
Subject: [PATCH] tty: Avoid possible error pointer dereference at tty_ldisc_restore().

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

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Alan Cox <alan@llwyncelyn.cymru>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Michal Hocko <mhocko@suse.com>
---
 drivers/tty/tty_ldisc.c | 13 +++++--------
 1 file changed, 5 insertions(+), 8 deletions(-)

diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
index 08ddb2c..de007e1 100644
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -527,19 +527,16 @@ static int tty_ldisc_failto(struct tty_struct *tty, int ld)
 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);
 	}
 }
 
-- 
1.8.3.1