Re: [alsa-devel] [PATCH] ALSA: pcm: Fix possible inconsistent appl_ptr update via mmap

Hi,

On Jun 17 2017 05:40, Takashi Iwai wrote:

The ALSA PCM core refers to the appl_ptr value stored on the mmapped page that is shared between kernel and user-space. Although the reference is performed in the PCM stream lock, it doesn't guarantee the atomic access when the value gets updated concurrently from the user-space on another CPU.

In most of codes, this is no big problem, but still there are a few places that may result in slight inconsistencies because they access runtime->control->appl_ptr multiple times; that is, the second read might be a different value from the first value. It can be even backward or jumping, as we have no control for it. Hence, the calculation may give an unexpected value. Luckily, there is no security vulnerability by that, as far as I've checked. But still we should address it.

This patch tries to reduce such possible cases. The fix is simple -- we just read once, store it to a local variable and use it for the rest calculations.

Signed-off-by: Takashi Iwai tiwai@suse.de

sound/core/pcm_lib.c | 13 +++++++------ 1 file changed, 7 insertions(+), 6 deletions(-)

This looks good to me by itself.

Reviewed-by: Takashi Sakamoto o-takashi@sakamocchi.jp

This function, 'snd_pcm_playback_silence()' can be called without acquiring stream lock by 'snd_pcm_reset()' when runtime of PCM substream is configured to have a larger value than zero to its 'silence_size' member. When a task or any IRQ handler operates the runtime and another task operates the reset in the same time, both of them got expected result.

I'll post a patch to take 'snd_pcm_reset()' to use 'snd_pcm_action_lock_irq()' instead of 'snd_pcm_action_nonatomic()'. How do you think about it?

diff --git a/sound/core/pcm_lib.c b/sound/core/pcm_lib.c index 631cd598ba6c..cda258d91490 100644 --- a/sound/core/pcm_lib.c +++ b/sound/core/pcm_lib.c @@ -65,15 +65,16 @@ void snd_pcm_playback_silence(struct snd_pcm_substream *substream, snd_pcm_ufram

if (runtime->silence_size < runtime->boundary) { snd_pcm_sframes_t noise_dist, n;

• if (runtime->silence_start != runtime->control->appl_ptr) {
  

• 	n = runtime->control->appl_ptr - runtime->silence_start;
  

• snd_pcm_uframes_t appl_ptr = runtime->control->appl_ptr;
  

• if (runtime->silence_start != appl_ptr) {
  

• 	n = appl_ptr - runtime->silence_start;
  if (n < 0)
  	n += runtime->boundary;
  if ((snd_pcm_uframes_t)n < runtime->silence_filled)
  	runtime->silence_filled -= n;
  else
  	runtime->silence_filled = 0;
  

• 	runtime->silence_start = runtime->control->appl_ptr;
  

• 	runtime->silence_start = appl_ptr;
  

  } if (runtime->silence_filled >= runtime->buffer_size) return;

@@ -2224,7 +2225,9 @@ snd_pcm_sframes_t __snd_pcm_lib_xfer(struct snd_pcm_substream *substream, continue; /* draining */ } frames = size > avail ? avail : size;

• cont = runtime->buffer_size - runtime->control->appl_ptr % runtime->buffer_size;
  

• appl_ptr = runtime->control->appl_ptr;
  

• appl_ofs = appl_ptr % runtime->buffer_size;
  

• cont = runtime->buffer_size - appl_ofs;
  

  if (frames > cont) frames = cont; if (snd_BUG_ON(!frames)) {

@@ -2232,8 +2235,6 @@ snd_pcm_sframes_t __snd_pcm_lib_xfer(struct snd_pcm_substream *substream, snd_pcm_stream_unlock_irq(substream); return -EINVAL; }

• appl_ptr = runtime->control->appl_ptr;
  

• appl_ofs = appl_ptr % runtime->buffer_size;
  

  snd_pcm_stream_unlock_irq(substream); err = writer(substream, appl_ofs, data, offset, frames, transfer);

Regards

Takashi Sakamoto