CVE-2021-47281 Information

Description

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.

Reference

https://git.kernel.org/stable/c/bd7d88b0874f82f7b29d1a53e574cedaf23166ba https://git.kernel.org/stable/c/536a7646c00a0f14fee49e5e313109e5da2f6031 https://git.kernel.org/stable/c/83e197a8414c0ba545e7e3916ce05f836f349273