Re: sound: use-after-free in snd_timer_interrupt

[Takashi Iwai]

On Fri, 15 Jan 2016 22:22:46 +0100,
Takashi Iwai wrote:
> 
> On Fri, 15 Jan 2016 20:47:05 +0100,
> Dmitry Vyukov wrote:
> > 
> > On Fri, Jan 15, 2016 at 8:18 PM, Takashi Iwai <tiwai@xxxxxxx> wrote:
> > > On Fri, 15 Jan 2016 20:13:11 +0100,
> > > Dmitry Vyukov wrote:
> > >>
> > >> On Fri, Jan 15, 2016 at 3:38 PM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> > >> > On Fri, Jan 15, 2016 at 2:51 PM, Takashi Iwai <tiwai@xxxxxxx> wrote:
> > >> >> On Fri, 15 Jan 2016 12:03:17 +0100,
> > >> >> Dmitry Vyukov wrote:
> > >> >>>
> > >> >>> On Fri, Jan 15, 2016 at 12:00 PM, Takashi Iwai <tiwai@xxxxxxx> wrote:
> > >> >>> > On Fri, 15 Jan 2016 09:06:10 +0100,
> > >> >>> > Dmitry Vyukov wrote:
> > >> >>> >>
> > >> >>> >> On Thu, Jan 14, 2016 at 5:09 PM, Takashi Iwai <tiwai@xxxxxxx> wrote:
> > >> >>> >> > On Wed, 13 Jan 2016 21:54:10 +0100,
> > >> >>> >> > Takashi Iwai wrote:
> > >> >>> >> >>
> > >> >>> >> >> OK, then this might be a possible race at the current snd_timer_stop()
> > >> >>> >> >> implementation.  There is no sync action there, so the ISR might be
> > >> >>> >> >> still alive after snd_timer_close() call.  Or might be another race.
> > >> >>> >> >> This pattern looks a bit different, as it's involved with hrtimer.
> > >> >>> >> >>
> > >> >>> >> >> I'll take a look at it tomorrow.
> > >> >>> >> >
> > >> >>> >> > I've audited the code today, but the open window doesn't look like
> > >> >>> >> > what I expected.  I found only some possible cases with slave timer
> > >> >>> >> > instances.
> > >> >>> >> >
> > >> >>> >> > In anyway, below is a test fix patch.  Since I couldn't reproduce the
> > >> >>> >> > issue on my local machines, it's hard to say whether this covers the
> > >> >>> >> > holes you fell.  Let's see...
> > >> >>> >>
> > >> >>> >>
> > >> >>> >> Hi Takashi,
> > >> >>> >>
> > >> >>> >> I would be interested to understand why other people can't reproduce
> > >> >>> >> issues that I hit pretty reliably.
> > >> >>> >> I suspect that it can be due to .config. Please try with the following
> > >> >>> >> config values.
> > >> >>> >
> > >> >>> > I guess rather other config, e.g. the kernel debug options.
> > >> >>> > I suppose you enabled KASAN and DEBUG_LIST.  What else?
> > >> >>>
> > >> >>> I've attached my config (you will need to disable CONFIG_KCOV, it is
> > >> >>> not upstreamed).
> > >> >>
> > >> >> Hm, that has lots of other drivers built-in...
> > >> >>
> > >> >>> >> I also start qemu with "-soundhw all" arg.
> > >> >>> >
> > >> >>> > OK, so you're testing with VM?  This makes easier to recheck.
> > >> >>>
> > >> >>> Yes, I start qemu as:
> > >> >>>
> > >> >>> qemu-system-x86_64 -hda wheezy.img -net
> > >> >>> user,host=10.0.2.10,hostfwd=tcp::10022-:22 -net nic -nographic -kernel
> > >> >>> arch/x86/boot/bzImage -append "console=ttyS0 root=/dev/sda debug
> > >> >>> earlyprintk=serial slub_debug=UZ" -enable-kvm -m 2G -numa
> > >> >>> node,nodeid=0,cpus=0-1 -numa node,nodeid=1,cpus=2-3 -smp
> > >> >>> sockets=2,cores=2,threads=1 -usb -usbdevice mouse -usbdevice tablet
> > >> >>> -soundhw all
> > >> >>
> > >> >> And which test did trigger use-after-free, even with all previous
> > >> >> patches?
> > >> >
> > >> > I will try to extract a new reproducer now.
> > >>
> > >> Ok, I does not seem to see any crashes except the timer hangs below.
> > >> Let's consider all other bugs as fixed. I will report anything new
> > >> that I see separately.
> > >
> > > OK, good to hear.
> > >
> > >> > Meanwhile, can you try to reproduce this one:
> > >> > https://groups.google.com/forum/#!msg/syzkaller/bbtG9_h1ONU/CPLblMC6FAAJ
> > >> > ? I run the program in a tight parallel loop.
> > >
> > > I could reproduce this after your suggestion with parallel runs.
> > >
> > > This seems specific to hrtimer.  Possibly it's not about the snd-timer
> > > core itself.  Could you check whether this doesn't happen when
> > > CONFIG_SND_HRTIMER isn't set?
> > 
> > 
> > Does not happen without CONFIG_SND_HRTIMER.
> > Do you mean that this is hrtimer bug?
> 
> I guess rather it's a bug in snd-hrtimer driver.
> Will check it later.

The patch below *might* fix the issue.  There was a deadlock problem
and the current code has a weird workaround for it.  I suspect it
being the cause.

If this works, I'll happily apply it before submitting the next pull
request for 4.5.  If not, I'll take a closer look at it in the next
week :)


thanks,

Takashi

---
diff --git a/sound/core/hrtimer.c b/sound/core/hrtimer.c
index f845ecf7e172..a8027350520d 100644
--- a/sound/core/hrtimer.c
+++ b/sound/core/hrtimer.c
@@ -39,6 +39,7 @@ struct snd_hrtimer {
 	struct snd_timer *timer;
 	struct hrtimer hrt;
 	atomic_t running;
+	bool cancel_pending;
 };
 
 static enum hrtimer_restart snd_hrtimer_callback(struct hrtimer *hrt)
@@ -62,7 +63,7 @@ static int snd_hrtimer_open(struct snd_timer *t)
 {
 	struct snd_hrtimer *stime;
 
-	stime = kmalloc(sizeof(*stime), GFP_KERNEL);
+	stime = kzalloc(sizeof(*stime), GFP_KERNEL);
 	if (!stime)
 		return -ENOMEM;
 	hrtimer_init(&stime->hrt, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
@@ -90,7 +91,10 @@ static int snd_hrtimer_start(struct snd_timer *t)
 	struct snd_hrtimer *stime = t->private_data;
 
 	atomic_set(&stime->running, 0);
-	hrtimer_cancel(&stime->hrt);
+	if (stime->cancel_pending) {
+		hrtimer_cancel(&stime->hrt);
+		stime->cancel_pending = false;
+	}
 	hrtimer_start(&stime->hrt, ns_to_ktime(t->sticks * resolution),
 		      HRTIMER_MODE_REL);
 	atomic_set(&stime->running, 1);
@@ -101,6 +105,8 @@ static int snd_hrtimer_stop(struct snd_timer *t)
 {
 	struct snd_hrtimer *stime = t->private_data;
 	atomic_set(&stime->running, 0);
+	if (hrtimer_try_to_cancel(&stime->hrt) < 0)
+		stime->cancel_pending = true;
 	return 0;
 }