linux-kernel - Re: Unkillable processes due to PTRACE

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Date:	Fri, 4 Dec 2015 20:02:50 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Pavel Machek <pavel@....cz>
Cc:	Dmitry Vyukov <dvyukov@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Roland McGrath <roland@...k.frob.com>,
	syzkaller@...glegroups.com, Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Robert Swiecki <swiecki@...gle.com>,
	Kees Cook <keescook@...gle.com>,
	Julien Tinnes <jln@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: Unkillable processes due to PTRACE_TRACEME

Hi Pavel,

On 12/03, Pavel Machek wrote:
>
> > You can't. This is one of historical oddities. You need to reap the
> > traced sub-thread first. And PTRACE_DETACH doesn't work.
>
> If kill -9 does not take out the process,

Just in case, "kill -9" can't help because the task is already killed and
zombie. The problem is that /sbin/init can't reap it without __WALL unless
we change the kernel.

> surely that sounds like a
> security problem?
>
> I know ptrace is old and tricky and ugly, but ....?

Yes this should be fixed. I'll resend the patches next week, I am a bit busy
now.

And, Dmitry, I didn't forget about another problem you reported ;) I'll try
to redo/resend the fixes for WARN_ON() in in task_participate_group_stop()
as well.

Oleg.

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