CVE-2021-47281

In the Linux kernel, the following vulnerability has been resolved: ALSA:
seq: Fix race of snd_seq_timer_open() The timer instance per queue is
exclusive, and snd_seq_timer_open() should have managed the concurrent
accesses. It looks as if it’s checking the already existing timer instance
at the beginning, but it’s not right, because there is no protection, hence
any later concurrent call of snd_seq_timer_open() may override the timer
instance easily. This may result in UAF, as the leftover timer instance can
keep running while the queue itself gets closed, as spotted by syzkaller
recently. For avoiding the race, add a proper check at the assignment of
tmr->timeri again, and return -EBUSY if it’s been already registered.