[PATCH RFC v1] pidfd: fix a race in setting exit_state for pidfd polling

[Joel Fernandes (Google)]

From: Suren Baghdasaryan <surenb@xxxxxxxxxx>

There is a race between reading task->exit_state in pidfd_poll and writing
it after do_notify_parent calls do_notify_pidfd. Expected sequence of
events is:

CPU 0                            CPU 1
------------------------------------------------
exit_notify
  do_notify_parent
    do_notify_pidfd
  tsk->exit_state = EXIT_DEAD
                                  pidfd_poll
                                     if (tsk->exit_state)

However nothing prevents the following sequence:

CPU 0                            CPU 1
------------------------------------------------
exit_notify
  do_notify_parent
    do_notify_pidfd
                                   pidfd_poll
                                      if (tsk->exit_state)
  tsk->exit_state = EXIT_DEAD

This causes a polling task to wait forever, since poll blocks because
exit_state is 0 and the waiting task is not notified again. A stress
test continuously doing pidfd poll and process exits uncovered this bug,
and the below patch fixes it.

To fix this, we set tsk->exit_state before calling do_notify_pidfd.

Cc: kernel-team@xxxxxxxxxxx
Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>

---
 kernel/exit.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index a75b6a7f458a..740ceacb4b76 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -720,6 +720,7 @@ static void exit_notify(struct task_struct *tsk, int group_dead)
 	if (group_dead)
 		kill_orphaned_pgrp(tsk->group_leader, NULL);
 
+	tsk->exit_state = EXIT_ZOMBIE;
 	if (unlikely(tsk->ptrace)) {
 		int sig = thread_group_leader(tsk) &&
 				thread_group_empty(tsk) &&
@@ -1156,10 +1157,11 @@ static int wait_task_zombie(struct wait_opts *wo, struct task_struct *p)
 		ptrace_unlink(p);
 
 		/* If parent wants a zombie, don't release it now */
-		state = EXIT_ZOMBIE;
+		p->exit_state = EXIT_ZOMBIE;
 		if (do_notify_parent(p, p->exit_signal))
-			state = EXIT_DEAD;
-		p->exit_state = state;
+			p->exit_state = EXIT_DEAD;
+
+		state = p->exit_state;
 		write_unlock_irq(&tasklist_lock);
 	}
 	if (state == EXIT_DEAD)
-- 
2.22.0.657.g960e92d24f-goog