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.
It is likely that the compiler's optimizer already merged multiple accesses and used a temporary register.
However, if the compiler can do this transformation, it can also do the reverse transformation. So this new code does not actually guarantee that there is a single atomic access. That can be enforced only with READ_ONCE():
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;
snd_pcm_uframes_t appl_ptr = READ_ONCE(runtime->control->appl_ptr); etc.
Regards, Clemens