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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
Message-Id: <20190717172100.261204-1-joel@joelfernandes.org>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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