linux-kernel - Re: [PATCH v5 3.1.0-rc4-tip 26/26] uprobes: queue signals while thread is singlestepping.

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Date:	Wed, 12 Oct 2011 21:34:17 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linux-mm <linux-mm@...ck.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Andi Kleen <andi@...stfloor.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jonathan Corbet <corbet@....net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Roland McGrath <roland@...k.frob.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 3.1.0-rc4-tip 26/26]   uprobes: queue signals while
	thread is singlestepping.

On 10/12, Srikar Dronamraju wrote:
>
> I think we should be okay if the test exits in UTASK_SSTEP state.

Yes, and afaics we can't avoid this case, at least currently.

But we should move free_uprobe_utask() to mm_release(), or somewhere
else before mm->core_state check in exit_mm().

My main concern is stop/freeze in UTASK_SSTEP state. If nothing else,
debugger can attach to the stopped task and disable the stepping. Or
SIGKILL, it should work in this case.

> > Great. I'll think a bit more and send you the "final" version tomorrow.
> > Assuming we can change sstep_complete() as we discussed, it doesn't need
> > fatal_signal_pending().
>
> Okay.

Sorry. I was busy today. Tomorrow ;)

> > HOWEVER. There is yet another problem. Another thread can, say, unmap()
> > xol_vma. In this case we should ensure that the task can't fault in an
> > endless loop.
>
> Hmm should we add a check in unmap() to see if the vma that we are
> trying to unmap is the xol_vma and if so return?

Oh, I am not sure. You know, I _think_ that perhaps we should do something
diferent in the long term. In particular, this xol page should not have
vma at all. This way we shouldn't worry about unmap/remap/mprotect.
But even if this is possible (I am not really sure), I do not think we
should do this right now.

> Our assumption has been that once an xol_vma has been created, it should
> be around till the process gets killed.

Yes, I see. But afaics this assumption is currently wrong. This means
that we should ensure the evil application can't exploit this fact.

Oleg.

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