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.