Re: [PATCH] ALSA: cmipci: Fix possible a data race in snd_cmipci_interrupt()

From: Jia-Ju Bai
Date: Mon Jan 13 2020 - 03:20:38 EST




On 2020/1/12 16:20, Takashi Iwai wrote:
On Sat, 11 Jan 2020 17:30:27 +0100,
Jia-Ju Bai wrote:
The functions snd_cmipci_interrupt() and snd_cmipci_capture_trigger()
may be concurrently executed.

The function snd_cmipci_capture_trigger() calls
snd_cmipci_pcm_trigger(). In snd_cmipci_pcm_trigger(), the variable
rec->running is written with holding a spinlock cm->reg_lock. But in
snd_cmipci_interrupt(), the identical variable cm->channel[0].running
or cm->channel[1].running is read without holding this spinlock. Thus,
a possible data race may occur.

To fix this data race, in snd_cmipci_interrupt(), the variables
cm->channel[0].running and cm->channel[1].running are read with holding
the spinlock cm->reg_lock.

This data race is found by the runtime testing of our tool DILP-2.

Signed-off-by: Jia-Ju Bai <baijiaju1990@xxxxxxxxx>
Thanks for the patch.

That's indeed a kind of race, but this change won't fix anything in
practice, though. The inconsistent running flag between those places,
there are two cases:

- running became 0 to 1; this cannot happen, as the irq isn't issued
before the stream gets started

- running became 1 to 0; this means that the stream gets stopped
between two points, and it's not better to call
snd_pcm_period_elapsed() for an already stopped stream.

Thanks for the reply :)

I am not sure to understand your words.

Do you mean that this code should be also protected by the spinlock?
ÂÂÂ if (cm->pcm) {
ÂÂÂ ÂÂÂ if ((status & CM_CHINT0) && cm->channel[0].running)
ÂÂÂ ÂÂÂ ÂÂÂ snd_pcm_period_elapsed(cm->channel[0].substream);
ÂÂÂ ÂÂÂ if ((status & CM_CHINT1) && cm->channel[1].running)
ÂÂÂ ÂÂÂ ÂÂÂ snd_pcm_period_elapsed(cm->channel[1].substream);
ÂÂÂ }


Best wishes,
Jia-Ju Bai