userfaultfd: fix a race between writeprotect and exit_mmap()

Vulnerability Description

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://git.kernel.org/stable/c/3cda4bfffd4f755645577aaa9e96a606657b4525
• https://git.kernel.org/stable/c/149958ecd0627a9f1e9c678c25c665400054cd6a
• https://git.kernel.org/stable/c/cb185d5f1ebf900f4ae3bf84cee212e6dd035aca