In the Linux kernel, the following vulnerability has been resolved:

userfaultfd: fix a race between writeprotect and exit_mmap()

A race is possible when a process exits, its VMAs are removed by
exit_mmap() and at the same time userfaultfd_writeprotect() is called.

The race was detected by KASAN on a development kernel, but it appears
to be possible on vanilla kernels as well.

Use mmget_not_zero() to prevent the race as done in other userfaultfd
operations.

References

• https://nvd.nist.gov/vuln/detail/CVE-2021-47461
• https://git.kernel.org/stable/c/149958ecd0627a9f1e9c678c25c665400054cd6a
• https://git.kernel.org/stable/c/3cda4bfffd4f755645577aaa9e96a606657b4525
• https://git.kernel.org/stable/c/cb185d5f1ebf900f4ae3bf84cee212e6dd035aca