sound: use-after-free in snd_timer_start1

[Dmitry Vyukov]

Hello,

The following program triggers use-after-free in snd_timer_start1:

// autogenerated by syzkaller (http://github.com/google/syzkaller)
#include <pthread.h>
#include <stdint.h>
#include <string.h>
#include <sys/syscall.h>
#include <unistd.h>

int main()
{
syscall(SYS_mmap, 0x20000000ul, 0x664000ul, 0x3ul, 0x32ul,
0xfffffffffffffffful, 0x0ul);
int fd = syscall(SYS_open, "/dev/snd/timer", 0x40000ul, 0, 0, 0);
*(uint32_t*)0x20653000 = (uint32_t)0x1;
*(uint32_t*)0x20653004 = (uint32_t)0xfffffffffffffffe;
*(uint32_t*)0x20653008 = (uint32_t)0x8;
*(uint32_t*)0x2065300c = (uint32_t)0x0;
*(uint32_t*)0x20653010 = (uint32_t)0x0;
*(uint8_t*)0x20653014 = (uint8_t)0x0;
*(uint8_t*)0x20653015 = (uint8_t)0x0;
*(uint8_t*)0x20653016 = (uint8_t)0x0;
*(uint8_t*)0x20653017 = (uint8_t)0x0;
*(uint8_t*)0x20653018 = (uint8_t)0x0;
*(uint8_t*)0x20653019 = (uint8_t)0x0;
*(uint8_t*)0x2065301a = (uint8_t)0x0;
*(uint8_t*)0x2065301b = (uint8_t)0x0;
*(uint8_t*)0x2065301c = (uint8_t)0x0;
*(uint8_t*)0x2065301d = (uint8_t)0x0;
*(uint8_t*)0x2065301e = (uint8_t)0x0;
*(uint8_t*)0x2065301f = (uint8_t)0x0;
*(uint8_t*)0x20653020 = (uint8_t)0x0;
*(uint8_t*)0x20653021 = (uint8_t)0x0;
*(uint8_t*)0x20653022 = (uint8_t)0x0;
*(uint8_t*)0x20653023 = (uint8_t)0x0;
*(uint8_t*)0x20653024 = (uint8_t)0x0;
*(uint8_t*)0x20653025 = (uint8_t)0x0;
*(uint8_t*)0x20653026 = (uint8_t)0x0;
*(uint8_t*)0x20653027 = (uint8_t)0x0;
*(uint8_t*)0x20653028 = (uint8_t)0x0;
*(uint8_t*)0x20653029 = (uint8_t)0x0;
*(uint8_t*)0x2065302a = (uint8_t)0x0;
*(uint8_t*)0x2065302b = (uint8_t)0x0;
*(uint8_t*)0x2065302c = (uint8_t)0x0;
*(uint8_t*)0x2065302d = (uint8_t)0x0;
*(uint8_t*)0x2065302e = (uint8_t)0x0;
*(uint8_t*)0x2065302f = (uint8_t)0x0;
*(uint8_t*)0x20653030 = (uint8_t)0x0;
*(uint8_t*)0x20653031 = (uint8_t)0x0;
*(uint8_t*)0x20653032 = (uint8_t)0x0;
*(uint8_t*)0x20653033 = (uint8_t)0x0;
syscall(SYS_ioctl, fd, 0x40345410ul, 0x20653000ul, 0, 0, 0);
syscall(SYS_ioctl, fd, 0x54a0ul, 0, 0, 0, 0);
return 0;
}

==================================================================
BUG: KASAN: use-after-free in __list_add+0x233/0x22016/02/03 13:44:32
executing program 0:
Read of size 8 by task syz-executor/27661

INFO: Allocated in snd_timer_instance_new+0x52/0x3a0 age=7 cpu=0 pid=27660
[< inline >] slab_alloc_node mm/slub.c:2562
[< inline >] slab_alloc mm/slub.c:2604
[< none >] kmem_cache_alloc_trace+0x25c/0x300 mm/slub.c:2621
[< inline >] kmalloc include/linux/slab.h:463
[< inline >] kzalloc include/linux/slab.h:607
[< none >] snd_timer_instance_new+0x52/0x3a0 sound/core/timer.c:106
[< none >] snd_timer_open+0x522/0xd20 sound/core/timer.c:289
[< inline >] snd_timer_user_tselect sound/core/timer.c:1598
[< inline >] __snd_timer_user_ioctl sound/core/timer.c:1874
[< none >] snd_timer_user_ioctl+0x8db/0x25f0 sound/core/timer.c:1904
[< inline >] vfs_ioctl fs/ioctl.c:43
[< none >] do_vfs_ioctl+0x18c/0xfb0 fs/ioctl.c:674
[< inline >] SYSC_ioctl fs/ioctl.c:689
[< none >] SyS_ioctl+0x8f/0xc0 fs/ioctl.c:680
[< none >] entry_SYSCALL_64_fastpath+0x16/0x7a
arch/x86/entry/entry_64.S:185

INFO: Freed in snd_timer_close+0x3ee/0x750 age=9 cpu=3 pid=27661
[< inline >] slab_free mm/slub.c:2835
[< none >] kfree+0x2ac/0x2c0 mm/slub.c:3664
[< none >] snd_timer_close+0x3ee/0x750 sound/core/timer.c:375
[< inline >] snd_timer_user_tselect sound/core/timer.c:1588
[< inline >] __snd_timer_user_ioctl sound/core/timer.c:1874
[< none >] snd_timer_user_ioctl+0x7b6/0x25f0 sound/core/timer.c:1904
[< inline >] vfs_ioctl fs/ioctl.c:43
[< none >] do_vfs_ioctl+0x18c/0xfb0 fs/ioctl.c:674
[< inline >] SYSC_ioctl fs/ioctl.c:689
[< none >] SyS_ioctl+0x8f/0xc0 fs/ioctl.c:680
[< none >] entry_SYSCALL_64_fastpath+0x16/0x7a
arch/x86/entry/entry_64.S:185

Call Trace:
[< inline >] kasan_report mm/kasan/report.c:274
[<ffffffff8176640e>] __asan_report_load8_noabort+0x3e/0x40
mm/kasan/report.c:295
[<ffffffff82c4a7e3>] __list_add+0x233/0x250 lib/list_debug.c:39
[< inline >] list_move_tail include/linux/list.h:77
[<ffffffff85240fdc>] snd_timer_start1+0x6c/0x2c0 sound/core/timer.c:432
[<ffffffff85241a7b>] snd_timer_start+0x20b/0x2b0 sound/core/timer.c:499
[< inline >] snd_timer_user_start sound/core/timer.c:1799
[< inline >] __snd_timer_user_ioctl sound/core/timer.c:1883
[<ffffffff85245fb7>] snd_timer_user_ioctl+0x767/0x25f0 sound/core/timer.c:1904
[< inline >] vfs_ioctl fs/ioctl.c:43
[<ffffffff817fa9ec>] do_vfs_ioctl+0x18c/0xfb0 fs/ioctl.c:674
[< inline >] SYSC_ioctl fs/ioctl.c:689
[<ffffffff817fb89f>] SyS_ioctl+0x8f/0xc0 fs/ioctl.c:680
[<ffffffff8665ebb6>] entry_SYSCALL_64_fastpath+0x16/0x7a
arch/x86/entry/entry_64.S:185
==================================================================


Sometimes it manifests as:

------------[ cut here ]------------
WARNING: CPU: 1 PID: 6548 at lib/list_debug.c:42 __list_add+0x1db/0x250()
list_add corruption. prev->next should be next (ffff8800697c10e8), but
was ffff880030441fd0. (prev=ffff880030441fd0).
Modules linked in:
CPU: 1 PID: 6548 Comm: a.out Not tainted 4.5.0-rc2+ #309
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
00000000ffffffff ffff88002fa17918 ffffffff82be2c8d ffff88002fa17988
ffff88002fd62f80 ffffffff86acf380 ffff88002fa17958 ffffffff81355139
ffffffff82c4a78b ffffed0005f42f2d ffffffff86acf380 000000000000002a
Call Trace:
[< inline >] __dump_stack lib/dump_stack.c:15
[<ffffffff82be2c8d>] dump_stack+0x6f/0xa2 lib/dump_stack.c:50
[<ffffffff81355139>] warn_slowpath_common+0xd9/0x140 kernel/panic.c:482
[<ffffffff81355249>] warn_slowpath_fmt+0xa9/0xd0 kernel/panic.c:494
[<ffffffff82c4a78b>] __list_add+0x1db/0x250 lib/list_debug.c:39
[< inline >] list_move_tail include/linux/list.h:77
[<ffffffff85240fdc>] snd_timer_start1+0x6c/0x2c0 sound/core/timer.c:432
[<ffffffff85241a7b>] snd_timer_start+0x20b/0x2b0 sound/core/timer.c:499
[< inline >] snd_timer_user_start sound/core/timer.c:1799
[< inline >] __snd_timer_user_ioctl sound/core/timer.c:1883
[<ffffffff85245fb7>] snd_timer_user_ioctl+0x767/0x25f0 sound/core/timer.c:1904
[< inline >] vfs_ioctl fs/ioctl.c:43
[<ffffffff817fa9ec>] do_vfs_ioctl+0x18c/0xfb0 fs/ioctl.c:674
[< inline >] SYSC_ioctl fs/ioctl.c:689
[<ffffffff817fb89f>] SyS_ioctl+0x8f/0xc0 fs/ioctl.c:680
[<ffffffff8665ebb6>] entry_SYSCALL_64_fastpath+0x16/0x7a
arch/x86/entry/entry_64.S:185
---[ end trace 77da812dd399e78b ]---

On commit 34229b277480f46c1e9a19f027f30b074512e68b + recent fixes from Takashi.

[Takashi Iwai]

Thanks for the report. This is indeed an old bug, but it appears now
more easily due to the recent fix to avoid double-stop. Below is the
fix patch. topic/core-fixes branch was updated with this, too.


Takashi

-- 8< --
From: Takashi Iwai <ti...@suse.de>
Subject: [PATCH] ALSA: timer: Fix leftover link at closing

In ALSA timer core, the active timer instance is managed in
active_list linked list. Each element is added / removed dynamically
at timer start, stop and in timer interrupt. The problem is that
snd_timer_interrupt() has a thinko and leaves the element in
active_list when it's the last opened element. This eventually leads
to list corruption or use-after-free error.

This hasn't been revealed because we used to delete the list forcibly
in snd_timer_stop() in the past. However, the recent fix avoids the
double-stop behavior (in commit [f784beb75ce8: ALSA: timer: Fix link
corruption due to double start or stop]), and this leak hits reality.

This patch fixes the link management in snd_timer_interrupt(). Now it
simply unlinks no matter which stream is.

BugLink: http://lkml.kernel.org/r/CACT4Y+Yy2aukHP-EDp8-ziNqNNmb-NTf=jDWXMP7j...@mail.gmail.com
Reported-by: Dmitry Vyukov <dvy...@google.com>
Cc: <sta...@vger.kernel.org>
Signed-off-by: Takashi Iwai <ti...@suse.de>
---
sound/core/timer.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/sound/core/timer.c b/sound/core/timer.c
index b419e612f987..9b513a05765a 100644
--- a/sound/core/timer.c
+++ b/sound/core/timer.c
@@ -744,8 +744,8 @@ void snd_timer_interrupt(struct snd_timer * timer, unsigned long ticks_left)
ti->cticks = ti->ticks;
} else {
ti->flags &= ~SNDRV_TIMER_IFLG_RUNNING;
- if (--timer->running)
- list_del_init(&ti->active_list);
+ --timer->running;
+ list_del_init(&ti->active_list);
}
if ((timer->hw.flags & SNDRV_TIMER_HW_TASKLET) ||
(ti->flags & SNDRV_TIMER_IFLG_FAST))
--
2.7.0

[Dmitry Vyukov]

Fixes the crash for me.
I think this was the last one in sound that happened periodically
during fuzzing. Let's see what the fuzzer will say now :)
Thanks for quick fixes!

[Takashi Iwai]

Great, thanks for your patient reports and testing!


Takashi