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 its checking the already existing timer instance at the beginning, but its 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 its been already registered.

Weakness

Referencing memory after it has been freed can cause a program to crash, use unexpected values, or execute code.

Affected Software

Extended Description

The use of previously-freed memory can have any number of adverse consequences, ranging from the corruption of valid data to the execution of arbitrary code, depending on the instantiation and timing of the flaw. The simplest way data corruption may occur involves the system’s reuse of the freed memory. Use-after-free errors have two common and sometimes overlapping causes:

In this scenario, the memory in question is allocated to another pointer validly at some point after it has been freed. The original pointer to the freed memory is used again and points to somewhere within the new allocation. As the data is changed, it corrupts the validly used memory; this induces undefined behavior in the process. If the newly allocated data happens to hold a class, in C++ for example, various function pointers may be scattered within the heap data. If one of these function pointers is overwritten with an address to valid shellcode, execution of arbitrary code can be achieved.

Potential Mitigations

References

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