linux-kernel - Re: kernel BUG at drivers/tty/tty

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Date:   Wed, 25 Apr 2018 20:12:31 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     syzbot <syzbot+40b7287c2dc987c48c81@...kaller.appspotmail.com>,
        syzkaller-bugs@...glegroups.com, Michal Hocko <mhocko@...e.com>
Cc:     linux-kernel@...r.kernel.org,
        Vegard Nossum <vegard.nossum@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        Peter Hurley <peter@...leysoftware.com>,
        One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: kernel BUG at drivers/tty/tty_ldisc.c:LINE!

>From 3606b60f60f5a2053290e7160a6fd3ee40f38a51 Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Date: Wed, 25 Apr 2018 15:26:10 +0900
Subject: [PATCH] tty: Use __GFP_NOFAIL for tty_ldisc_get()

syzbot is reporting crashes triggered by memory allocation fault injection
at tty_ldisc_get() [1]. As an attempt to handle OOM in a graceful way, we
have tried commit 5362544bebe85071 ("tty: don't panic on OOM in
tty_set_ldisc()"). But we reverted that attempt by commit a8983d01f9b7d600
("Revert "tty: don't panic on OOM in tty_set_ldisc()"") due to reproducible
crash. We should spend resource for finding and fixing race condition bugs
rather than complicate error paths for 2 * sizeof(void *) bytes allocation
failure.

[1] https://syzkaller.appspot.com/bug?id=489d33fa386453859ead58ff5171d43772b13aa3

Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Reported-by: syzbot <syzbot+40b7287c2dc987c48c81@...kaller.appspotmail.com>
Cc: Michal Hocko <mhocko@...e.com>
Cc: Vegard Nossum <vegard.nossum@...il.com>
Cc: Dmitry Vyukov <dvyukov@...gle.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Jiri Slaby <jslaby@...e.com>
Cc: Peter Hurley <peter@...leysoftware.com>
Cc: One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
---
 drivers/tty/tty_ldisc.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
index 050f4d6..971a37e 100644
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -176,12 +176,11 @@ static struct tty_ldisc *tty_ldisc_get(struct tty_struct *tty, int disc)
 			return ERR_CAST(ldops);
 	}
 
-	ld = kmalloc(sizeof(struct tty_ldisc), GFP_KERNEL);
-	if (ld == NULL) {
-		put_ldops(ldops);
-		return ERR_PTR(-ENOMEM);
-	}
-
+	/*
+	 * There is no way to handle allocation failure of only 16 bytes.
+	 * Let's simplify error handling and save more memory.
+	 */
+	ld = kmalloc(sizeof(struct tty_ldisc), GFP_KERNEL | __GFP_NOFAIL);
 	ld->ops = ldops;
 	ld->tty = tty;
 
-- 
1.8.3.1