linux-kernel - Re: sound: use-after-free in snd_timer

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Date:	Wed, 13 Jan 2016 19:34:36 +0100
From:	Dmitry Vyukov <dvyukov@...gle.com>
To:	Takashi Iwai <tiwai@...e.de>
Cc:	alsa-devel@...a-project.org, Jie Yang <yang.jie@...el.com>,
	Mark Brown <broonie@...nel.org>,
	Jaroslav Kysela <perex@...ex.cz>,
	LKML <linux-kernel@...r.kernel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Kostya Serebryany <kcc@...gle.com>,
	syzkaller <syzkaller@...glegroups.com>,
	Sasha Levin <sasha.levin@...cle.com>
Subject: Re: sound: use-after-free in snd_timer_interrupt

On Wed, Jan 13, 2016 at 5:53 PM, Takashi Iwai <tiwai@...e.de> wrote:
> This and your other relevant reports seem pointing the race of timer
> ioctls.  Although snd_timer_close() itself calls snd_timer_stop(),
> there is no other protection against the concurrent execution.
>
> If my guess is correct, a simplistic fix like below should work.  It
> basically serializes the timer ioctl by using a new mutex (and
> replacing the old tread_sem mutex).  They are no longtime blocking
> calls, so this shouldn't be a big problem.  But certainly there can be
> a less intrusive way to paper over this if this really matters.
>
> In this case for timer.c, I'd leave the final decision rather to
> Jaroslav.  Jaroslav, what do you think?


After applying this patch I still see the following WARNINGS:

------------[ cut here ]------------
WARNING: CPU: 2 PID: 30398 at lib/list_debug.c:53 __list_del_entry+0x10b/0x1e0()
list_del corruption, ffff880032d933b0->next is LIST_POISON1 (dead000000000100)
Modules linked in:
CPU: 2 PID: 30398 Comm: syz-executor Not tainted 4.4.0+ #241
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
 00000000ffffffff ffff8800627778d8 ffffffff82926eed ffff880062777948
 ffff880061c2af80 ffffffff8660b640 ffff880062777918 ffffffff81350c89
 ffffffff8298e77b ffffed000c4eef25 ffffffff8660b640 0000000000000035
Call Trace:
 [<     inline     >] __dump_stack lib/dump_stack.c:15
 [<ffffffff82926eed>] dump_stack+0x6f/0xa2 lib/dump_stack.c:50
 [<ffffffff81350c89>] warn_slowpath_common+0xd9/0x140 kernel/panic.c:483
 [<ffffffff81350d99>] warn_slowpath_fmt+0xa9/0xd0 kernel/panic.c:495
 [<ffffffff8298e77b>] __list_del_entry+0x10b/0x1e0 lib/list_debug.c:51
 [<     inline     >] list_del_init include/linux/list.h:145
 [<ffffffff84ebd199>] _snd_timer_stop+0x119/0x450 sound/core/timer.c:501
 [<ffffffff84ebd4f4>] snd_timer_stop+0x24/0x140 sound/core/timer.c:535
 [<ffffffff84ebd648>] snd_timer_close+0x38/0x5f0 sound/core/timer.c:317
 [<     inline     >] snd_timer_user_tselect sound/core/timer.c:1518
 [<     inline     >] __snd_timer_user_ioctl sound/core/timer.c:1803
 [<ffffffff84ec4362>] snd_timer_user_ioctl+0x7b2/0x25c0 sound/core/timer.c:1833
 [<     inline     >] vfs_ioctl fs/ioctl.c:43
 [<ffffffff817cbd3c>] do_vfs_ioctl+0x18c/0xfa0 fs/ioctl.c:674
 [<     inline     >] SYSC_ioctl fs/ioctl.c:689
 [<ffffffff817ccbdf>] SyS_ioctl+0x8f/0xc0 fs/ioctl.c:680
 [<ffffffff86273076>] entry_SYSCALL_64_fastpath+0x16/0x7a
arch/x86/entry/entry_64.S:185
---[ end trace bfebf27b922184a1 ]---