From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
To: linux-kernel@vger.kernel.org
Cc: Suren Baghdasaryan <surenb@google.com>,
kernel-team@android.com, Joel Fernandes <joel@joelfernandes.org>,
Andrea Arcangeli <aarcange@redhat.com>,
Andrew Morton <akpm@linux-foundation.org>,
Christian Brauner <christian@brauner.io>,
"Eric W. Biederman" <ebiederm@xmission.com>,
Oleg Nesterov <oleg@redhat.com>, Tejun Heo <tj@kernel.org>
Subject: [PATCH RFC v1] pidfd: fix a race in setting exit_state for pidfd polling
Date: Wed, 17 Jul 2019 13:21:00 -0400 [thread overview]
Message-ID: <20190717172100.261204-1-joel@joelfernandes.org> (raw)
From: Suren Baghdasaryan <surenb@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-team@android.com
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Joel Fernandes (Google) <joel@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
next reply other threads:[~2019-07-17 17:21 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-17 17:21 Joel Fernandes (Google) [this message]
2019-07-17 17:55 ` [PATCH RFC v1] pidfd: fix a race in setting exit_state for pidfd polling Christian Brauner
2019-07-17 18:09 ` Suren Baghdasaryan
2019-07-17 20:47 ` Joel Fernandes
2019-07-18 10:09 ` Christian Brauner
2019-07-17 20:51 ` Joel Fernandes
2019-07-18 10:17 ` Christian Brauner
2019-07-18 16:05 ` Suren Baghdasaryan
2019-07-18 15:00 ` Oleg Nesterov
2019-07-19 16:14 ` Oleg Nesterov
2019-07-19 16:27 ` Christian Brauner
2019-07-19 16:33 ` Suren Baghdasaryan
2019-07-19 16:51 ` Joel Fernandes
2019-07-19 16:53 ` Christian Brauner
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