From: Suren Baghdasaryan <sur...@google.com>
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-t...@android.com
Signed-off-by: Suren Baghdasaryan <sur...@google.com>
Signed-off-by: Joel Fernandes (Google) <j...@joelfernandes.org>
---
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
