CVE-2021-47281: Use After Free

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.