CVE-2021-47281

In the Linux kernel, the following vulnerability has been resolved:<br /> <br /> ALSA: seq: Fix race of snd_seq_timer_open()<br /> <br /> The timer instance per queue is exclusive, and snd_seq_timer_open()<br /> should have managed the concurrent accesses. It looks as if it&amp;#39;s<br /> checking the already existing timer instance at the beginning, but<br /> it&amp;#39;s not right, because there is no protection, hence any later<br /> concurrent call of snd_seq_timer_open() may override the timer<br /> instance easily. This may result in UAF, as the leftover timer<br /> instance can keep running while the queue itself gets closed, as<br /> spotted by syzkaller recently.<br /> <br /> For avoiding the race, add a proper check at the assignment of<br /> tmr-&gt;timeri again, and return -EBUSY if it&amp;#39;s been already registered.