CVE-2026-89979: ALSA: pcm: Fix race between non-atomic ops and trigger-start

In the Linux kernel, the following vulnerability has been resolved:

ALSA: pcm: Fix race between non-atomic ops and trigger-start

We protect the races of the concurrent state transitions between atomic PCM ops, but the checks between the non-atomic ops (hwparams, hwfree and prepare) and the atomic ops aren't perfect; there is a check of the conflicting PCM state at the beginning of hwparams & co, but the atomic PCM ops can be still issued during the non-atomic PCM operations. An example such scenario is that a thread A re-issues the PREPARE or HWPARAMS for the already prepared stream, while another thread B triggers the PCM start in the middle of the prepare operation. Although this usually doesn't lead to much serious issues, it can give some inconsistency as reported by syzkaller (such as ODEBUG warning).

There are various atomic PCM ops, and basically the only problem is the PCM start as it operates from the PREPARED state. Other trigger commands (stop, etc) are for the running or the other special state, hence they are filtered as pre-condition.

This patch is for preventing the PCM trigger-start during the non- atomic operations in order to address the problems above. Fortunately, the hwparams, hwfree and prepare operations call sndpcmbufferaccesslock(), and this can be used for checking the concurrent operations at the PCM trigger -- which sets the runtime->bufferaccessing to a negative (if possible), so the PCM trigger just needs to check the runtime->bufferaccessing value; if it's negative, it means the concurrent non-atomic PCM ops is running.