REDHAT-BUG-2439947: Medium severity Linux ALSA aloop driver vulnerability

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

ALSA: aloop: Fix racy access at PCM trigger

The PCM trigger callback of aloop driver tries to check the PCM state and stop the stream of the tied substream in the corresponding cable. Since both check and stop operations are performed outside the cable lock, this may result in UAF when a program attempts to trigger frequently while opening/closing the tied stream, as spotted by fuzzers.

For addressing the UAF, this patch changes two things: - It covers the most of code in loopbackcheckformat() with cable->lock spinlock, and add the proper NULL checks. This avoids already some racy accesses. - In addition, now we try to check the state of the capture PCM stream that may be stopped in this function, which was the major pain point leading to UAF.