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

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Message-ID: <522c4993-4d98-be98-def2-452ed28098f9@i-love.sakura.ne.jp>
Date:   Fri, 26 Oct 2018 22:51:42 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Oleg Nesterov <oleg@...hat.com>
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 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.

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

> 
> Again, did you read my previous email?

Yes. But I still can't be convinced that pid_alive() test helps.