CVE-2026-43437: ALSA: pcm: fix use-after-free on linked stream runtime in snd_pcm_drain()

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

ALSA: pcm: fix use-after-free on linked stream runtime in sndpcmdrain()

In the drain loop, the local variable 'runtime' is reassigned to a linked stream's runtime (runtime = s->runtime at line 2157). After releasing the stream lock at line 2169, the code accesses runtime->noperiodwakeup, runtime->rate, and runtime->buffersize (lines 2170-2178) — all referencing the linked stream's runtime without any lock or refcount protecting its lifetime.

A concurrent close() on the linked stream's fd triggers sndpcmreleasesubstream() → sndpcmdrop() → pcmreleaseprivate() → sndpcmunlink() → sndpcmdetachsubstream() → kfree(runtime). No synchronization prevents kfree(runtime) from completing while the drain path dereferences the stale pointer.

Fix by caching the needed runtime fields (noperiodwakeup, rate, buffersize) into local variables while still holding the stream lock, and using the cached values after the lock is released.