CVE-2025-38017

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

fs/eventpoll: fix endless busy loop after timeout has expired

After commit 0a65bc27bd64 ("eventpoll: Set epoll timeout if it's in the future"), the following program would immediately enter a busy loop in the kernel:

int main() {
  int e = epoll_create1(0);
  struct epoll_event event = {.events = EPOLLIN};
  epoll_ctl(e, EPOLL_CTL_ADD, 0, &event);
  const struct timespec timeout = {.tv_nsec = 1};
  epoll_pwait2(e, &event, 1, &timeout, 0);
}

This happens because the given (non-zero) timeout of 1 nanosecond usually expires before eppoll() is entered and then epschedule_timeout() returns false, but timed_out is never set because the code line that sets it is skipped. This quickly turns into a soft lockup, RCU stalls and deadlocks, inflicting severe headaches to the whole system.

When the timeout has expired, we don't need to schedule a hrtimer, but we should set the timed_out variable. Therefore, I suggest moving the epscheduletimeout() check into the timed_out expression instead of skipping it.

brauner: Note that there was an earlier fix by Joe Damato in response to my bug report in [1].