On Fri, 30 Mar 2018 01:01:01 +0200, syzbot wrote:
Hello,
syzbot hit the following crash on upstream commit 0b412605ef5f5c64b31f19e2910b1d5eba9929c3 (Thu Mar 29 01:07:23 2018 +0000) Merge tag 'drm-fixes-for-v4.16-rc8' of git://people.freedesktop.org/~airlied/linux syzbot dashboard link: https://syzkaller.appspot.com/bug?extid=8e62ff4e07aa2ce87826
So far this crash happened 3 times on upstream. C reproducer: https://syzkaller.appspot.com/x/repro.c?id=6219387178582016 syzkaller reproducer: https://syzkaller.appspot.com/x/repro.syz?id=5176598244360192 Raw console output: https://syzkaller.appspot.com/x/log.txt?id=5815694612496384 Kernel config: https://syzkaller.appspot.com/x/.config?id=-8440362230543204781 compiler: gcc (GCC) 7.1.1 20170620
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================================================================== BUG: KASAN: use-after-free in snd_pcm_timer_resolution+0xa3/0xc0 sound/core/pcm_timer.c:72 Read of size 4 at addr ffff8801bdd58490 by task syzkaller702213/13238
CPU: 0 PID: 13238 Comm: syzkaller702213 Not tainted 4.16.0-rc7+ #370 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011 Call Trace: __dump_stack lib/dump_stack.c:17 [inline] dump_stack+0x194/0x24d lib/dump_stack.c:53 print_address_description+0x73/0x250 mm/kasan/report.c:256 kasan_report_error mm/kasan/report.c:354 [inline] kasan_report+0x23c/0x360 mm/kasan/report.c:412 __asan_report_load4_noabort+0x14/0x20 mm/kasan/report.c:432 snd_pcm_timer_resolution+0xa3/0xc0 sound/core/pcm_timer.c:72 snd_timer_resolution sound/core/timer.c:439 [inline] snd_timer_notify1+0x10f/0x400 sound/core/timer.c:462 snd_timer_start1+0x586/0x8f0 sound/core/timer.c:522 snd_timer_start+0x5d/0xa0 sound/core/timer.c:634 snd_timer_user_start.isra.12+0xde/0x130 sound/core/timer.c:1878 snd_timer_user_continue sound/core/timer.c:1902 [inline] __snd_timer_user_ioctl+0x1771/0x2c10 sound/core/timer.c:1977 snd_timer_user_ioctl+0x5f/0x7d sound/core/timer.c:1992
(snip)
So this looks like a race between ALSA PCM and ALSA timer interfaces. When a PCM timer is accessed during closing the corresponding PCM substream, it may lead to such a UAF.
The fix patch is below. I'll queue it after testing locally.
thanks,
Takashi
-- 8< -- From: Takashi Iwai tiwai@suse.de Subject: [PATCH] ALSA: pcm: Fix UAF at PCM release via PCM timer access
The PCM runtime object is created and freed dynamically at PCM stream open / close time. This is tracked via substream->runtime, and it's cleared at snd_pcm_detach_substream().
The runtime object assignment is protected by PCM open_mutex, so for all PCM operations, it's safely handled. However, each PCM substream provides also an ALSA timer interface, and user-space can access to this while closing a PCM substream. This may eventually lead to a UAF, as snd_pcm_timer_resolution() tries to access the runtime while clearing it in other side.
Fortunately, it's the only concurrent access from the PCM timer, and it merely reads runtime->timer_resolution field. So, we can avoid the race by reordering kfree() and wrapping the substream->runtime clearance with the corresponding timer lock.
Reported-by: syzbot+8e62ff4e07aa2ce87826@syzkaller.appspotmail.com Cc: stable@vger.kernel.org Signed-off-by: Takashi Iwai tiwai@suse.de --- sound/core/pcm.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-)
diff --git a/sound/core/pcm.c b/sound/core/pcm.c index 09ee8c6b9f75..66ac89aad681 100644 --- a/sound/core/pcm.c +++ b/sound/core/pcm.c @@ -28,6 +28,7 @@ #include <sound/core.h> #include <sound/minors.h> #include <sound/pcm.h> +#include <sound/timer.h> #include <sound/control.h> #include <sound/info.h>
@@ -1054,8 +1055,13 @@ void snd_pcm_detach_substream(struct snd_pcm_substream *substream) snd_free_pages((void*)runtime->control, PAGE_ALIGN(sizeof(struct snd_pcm_mmap_control))); kfree(runtime->hw_constraints.rules); - kfree(runtime); + /* Avoid concurrent access to runtime via PCM timer interface */ + if (substream->timer) + spin_lock_irq(&substream->timer->lock); substream->runtime = NULL; + if (substream->timer) + spin_unlock_irq(&substream->timer->lock); + kfree(runtime); put_pid(substream->pid); substream->pid = NULL; substream->pstr->substream_opened--;