6.14-stable review patch. If anyone has any objections, please let me know.
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From: Max Kellermann max.kellermann@ionos.com
commit d9ec73301099ec5975505e1c3effbe768bab9490 upstream.
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 ep_poll() is entered and then ep_schedule_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 ep_schedule_timeout() 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].
Fixes: 0a65bc27bd64 ("eventpoll: Set epoll timeout if it's in the future") Cc: Joe Damato jdamato@fastly.com Cc: stable@vger.kernel.org Signed-off-by: Max Kellermann max.kellermann@ionos.com Link: https://lore.kernel.org/20250429153419.94723-1-jdamato@fastly.com [1] Link: https://lore.kernel.org/20250429185827.3564438-1-max.kellermann@ionos.com Reviewed-by: Jan Kara jack@suse.cz Signed-off-by: Christian Brauner brauner@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- fs/eventpoll.c | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-)
--- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -2111,9 +2111,10 @@ static int ep_poll(struct eventpoll *ep,
write_unlock_irq(&ep->lock);
- if (!eavail && ep_schedule_timeout(to)) - timed_out = !schedule_hrtimeout_range(to, slack, - HRTIMER_MODE_ABS); + if (!eavail) + timed_out = !ep_schedule_timeout(to) || + !schedule_hrtimeout_range(to, slack, + HRTIMER_MODE_ABS); __set_current_state(TASK_RUNNING);
/*