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

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Message-ID: <20181026143937.GA18848@redhat.com>
Date:   Fri, 26 Oct 2018 16:39:37 +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 10/26, Tetsuo Handa wrote:
>
> On 2018/10/26 22:04, Oleg Nesterov wrote:
> >> Suppose p1 == p2->real_parent and p2 == p3->real_parent, and p1 exited
> >> when p2 tried to attach on p1, p2->real_parent was pointing to already
> >> (or about to be) freed p1.
> >
> > No, p2->real_parent will be updated. If p1 exits it will re-parent its
> > children including p2.
>
> My error.
>
> Suppose p1 == p2->real_parent and p2 == p3->real_parent, and p1 exited
> when someone tried to attach on p2, p2->real_parent was pointing to already
> (or about to be) freed p1.

I don't see a difference.

If p1 exits it will re-parent p2, p2->real_parent will be updated.

> So, the puzzle part is why p2->real_parent was still pointing p1 even after
> p1 was freed...

I don't understand the question.

Once again. TASK->real_parent can point to the freed mem only if a) TASK exits,
and b) _after_ that its parent TASK->real_parent exits too.

> > Again, did you read my previous email?
>
> Yes. But I still can't be convinced that pid_alive() test helps.

Well, I don't understand which part of my explanations is not clear to you.

OR. Perhaps I am wrong and do not understand your concerns.

Oleg.