linux-kernel - Re: sound: spinlock lockup in snd_timer_user

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Date:   Tue, 28 Feb 2017 14:58:20 +0100
From:   Takashi Iwai <tiwai@...e.de>
To:     "Dmitry Vyukov" <dvyukov@...gle.com>
Cc:     <alsa-devel@...a-project.org>, "Jaroslav Kysela" <perex@...ex.cz>,
        "LKML" <linux-kernel@...r.kernel.org>,
        "syzkaller" <syzkaller@...glegroups.com>
Subject: Re: sound: spinlock lockup in snd_timer_user_tinterrupt

On Mon, 27 Feb 2017 21:46:48 +0100,
Dmitry Vyukov wrote:
> 
> Hello,
> 
> The following program locks up system:
> https://gist.githubusercontent.com/dvyukov/1b5cdca7f0fc6254afd4816901160e4c/raw/024c53be2112c83e5d64f3e4d5e8aa38ded727e3/gistfile1.txt

Thanks.  It doesn't look like the hard lockup, but it keeps the IRQs
too busy, resulting in a quasi-lockup.

The problem is that the timer accepted the too low resolution.  A
simple fix would be to give an error for a too short parameter.
The fix patch is attached below.  Please give it a try.


thanks,

Takashi

-- 8< --
From: Takashi Iwai <tiwai@...e.de>
Subject: [PATCH] ALSA: timer: Reject user params with too small ticks

When a user sets a too small ticks with a fine-grained timer like
hrtimer, the kernel tries to fire up the timer irq too frequently.
This may lead to the condensed locks, eventually the kernel spinlock
lockup with warnings.

For avoiding such a situation, we define a lower limit of the
resolution, namely 1ms.  When the user passes a too small tick value
that results in less than that, the kernel returns -EINVAL now.

Reported-by: Dmitry Vyukov <dvyukov@...gle.com>
Cc: <stable@...r.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@...e.de>
---
 sound/core/timer.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/sound/core/timer.c b/sound/core/timer.c
index fc144f43faa6..ad153149b231 100644
--- a/sound/core/timer.c
+++ b/sound/core/timer.c
@@ -1702,9 +1702,21 @@ static int snd_timer_user_params(struct file *file,
 		return -EBADFD;
 	if (copy_from_user(&params, _params, sizeof(params)))
 		return -EFAULT;
-	if (!(t->hw.flags & SNDRV_TIMER_HW_SLAVE) && params.ticks < 1) {
-		err = -EINVAL;
-		goto _end;
+	if (!(t->hw.flags & SNDRV_TIMER_HW_SLAVE)) {
+		u64 resolution;
+
+		if (params.ticks < 1) {
+			err = -EINVAL;
+			goto _end;
+		}
+
+		/* Don't allow resolution less than 1ms */
+		resolution = snd_timer_resolution(tu->timeri);
+		resolution *= params.ticks;
+		if (resolution < 1000000) {
+			err = -EINVAL;
+			goto _end;
+		}
 	}
 	if (params.queue_size > 0 &&
 	    (params.queue_size < 32 || params.queue_size > 1024)) {
-- 
2.11.1