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

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Date:   Sat, 27 Oct 2018 00:04:32 +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 23:39, Oleg Nesterov wrote:
> On 10/26, Tetsuo Handa wrote:
>> 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.

Oh, p2 exited and then p1 also exited when someone tried to attach on p2.
Then, p2->real_parent can point to already (or about to be) freed p1.

> 
>>> 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.

OK. Checking pid_alive() should help.

(By the way, if p->real_parent were updated to point to init_task when p exits,
we could omit pid_alive() check?)