Problem Description:
When an audio channel's buffer needs to be resized, the sound(4)
driver checks that the buffer is not memory-mapped and then drops
the channel lock to allocate a new buffer. An mmap(2) call on
another thread can complete during this window, after which the
resize path frees the now-mapped buffer without rechecking.
Impact:
An unprivileged local user with access to a sound device can
exploit this race to trigger a use-after-free in the kernel,
potentially escalating privileges.