ALSA: pcm: Fix possible inconsistent appl_ptr update via mmap

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.  The READ_ONCE() macro is used for it in order to
avoid the ill-effect by possible compiler optimizations.

Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --git a/sound/core/pcm_lib.c b/sound/core/pcm_lib.c
index e8131c060c8689eb94b26e14020ddee6408a4135..e76d55a4d1b265aaf7dfda1628e30dbba0788c3e 100644 (file)
--- 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 = READ_ONCE(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;
@@ -2203,7 +2204,9 @@ 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 = READ_ONCE(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)) {
@@ -2211,8 +2214,6 @@ 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);