linux-kernel - Re: KASAN: use-after-free Read in task_is

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Message-ID: <20181025162554.GG3725@redhat.com>
Date:   Thu, 25 Oct 2018 18:25:54 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     serge@...lyn.com,
        syzbot <syzbot+a9ac39bf55329e206219@...kaller.appspotmail.com>,
        jmorris@...ei.org, keescook@...omium.org,
        linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        syzkaller-bugs@...glegroups.com
Subject: Re: KASAN: use-after-free Read in task_is_descendant

On 10/25, Oleg Nesterov wrote:
>
> Because our rcu read-lock critical section extends beyond the return from
> synchronize_rcu(), and thus we must have a full memory barrier _between_
> that synchronize_rcu() and our rcu_read_lock(). We must see all memory updates,
> including thread_pid = NULL which makes pid_alive() == F.

In case I was not clear....

Suppose we have int X = 0. If some CPU does

	X = 1;
	synchronize_rcu();

and another CPU does

	rcu_read_lock();
	x = X;
	rcu_read_unlock();

then x should be == 1 in case when rcu_read_unlock() happens _after_ return
from synchronize_rcu().

Oleg.