linux-kernel - Re: Trouble with ptrace self-attach rule since kernel

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Date:	Mon, 4 Sep 2006 15:00:27 -0700 (PDT)
From:	Linus Torvalds <torvalds@...l.org>
To:	Andreas Hobein <ah2@...air.de>
cc:	Oleg Nesterov <oleg@...sign.ru>, Andrew Morton <akpm@...l.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Roland McGrath <roland@...hat.com>,
	Markus Gutschke <markus@...gle.com>
Subject: Re: Trouble with ptrace self-attach rule since kernel > 2.6.14

On Mon, 4 Sep 2006, Andreas Hobein wrote:
> 
> I've tested tracing child threads from the parent thread as well as tracing 
> siblings and parent threads from a child. All tests where successful when 
> reverting the above mentioned changes.

The problems tend to happen when the thread leader exits while one of the 
sub-threads is being traced, and the tracer thread ends up being 
re-parented to be the child of the traced thread (or something like that - 
I forget the exact details).

There was also some problem with the tracer doing an exit() without 
detaching, or something. 

We may have fixed most of the problems since - Oleg has certainly been 
cleaning up some of this, and it's possible that the problems we had are 
ok now. 

Even back when it was broken, _normal_ use never showed the problem (ie no 
well-behaved ptrace use would cause anything bad to happen). But the 
breakage was a local DoS attack, where you could either force an oops or a 
unkillable process, I forget which.

There was an exploit for at least one of the exploits, so maybe somebody 
could test that exploit together with the one-line revert.

That said, it sounds like both of the people who ever noticed this are 
reasonably happy with their work-arounds, so I'm hoping we can simply 
decide not to care, and just keep doing the simpler "you can't ptrace your 
own thread group" thing. That rule simply avoids a lot of special cases.

			Linus
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