AZL-90660

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

ALSA: timer: Fix UAF at sndtimeruser_params()

At releasing a timer object, e.g. when a userspace timer (CONFIGSNDUTIMER) gets closed and sndtimerfree() is called, it tries to detach the timer instances and release the resources. However, it's still possible that other in-flight tasks are holding the timer instance where the to-be-deleted timer object is associated, and this may lead to racy accesses.

Fortunately, most of ioctls dealing with the timer instance list already have the protection with registermutex, and this also avoids such races. But, SNDRVTIMERIOCTLPARAMS isn't protected, hence the concurrent ioctl may lead to use-after-free.

This patch just adds the guard with registermutex to protect sndtimeruserparams() for covering the code path as a quick workaround. It's no hot-path but rather a rarely issued ioctl, so the performance penalty doesn't matter.