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

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Date:   Fri, 26 Oct 2018 17:09:17 +0100
From:   Kees Cook <keescook@...omium.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        syzbot <syzbot+a9ac39bf55329e206219@...kaller.appspotmail.com>,
        James Morris <jmorris@...ei.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        syzkaller-bugs@...glegroups.com
Subject: Re: KASAN: use-after-free Read in task_is_descendant

On Thu, Oct 25, 2018 at 2:01 PM, Oleg Nesterov <oleg@...hat.com> wrote:
> On 10/25, Oleg Nesterov wrote:
>> perhaps it needs some changes too. I even have a vague feeling that I have already
>> blamed this function some time ago...
>
> Heh, yes, 3 years ago ;)
>
> https://lore.kernel.org/lkml/20150106184427.GA18153@redhat.com/
>
> I can't understand my email today, but note that I tried to point out that
> task_is_descendant() can dereference the freed mem.

Instead of:

        while (walker->pid > 0) {

should it simply be "while (pid_liave(walker)) {"? And add a
pid_alive(parent) after rcu_read_lock()?

> And yes, task_is_descendant() is overcompicated for no reason, afaics.

Yeah, agreed. I'll fix this up. Just to make sure I'm not crazy: the
real_parent of all tasks in a thread group are the same, yes? The
trouble I was trying to deal with for the complication was where a
non-leader thread would add an exception to the checking, and the
tasks wouldn't match. (As far as I can see, though, using
same_thread_group() should fix it.)

-Kees

-- 
Kees Cook