linux-kernel - Re: [PATCH RFC 0/5] uprobes: kill xol vma

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Date:	Mon, 12 Dec 2011 18:30:19 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-mm <linux-mm@...ck.org>, Ingo Molnar <mingo@...e.hu>,
	Andi Kleen <andi@...stfloor.org>,
	Christoph Hellwig <hch@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Roland McGrath <roland@...k.frob.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Anton Arapov <anton@...hat.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Stephen Wilson <wilsons@...rt.ca>,
	Josh Stone <jistone@...hat.com>
Subject: Re: [PATCH RFC 0/5] uprobes: kill xol vma

On 11/28, Oleg Nesterov wrote:
>
> On top of this series, not for inclusion yet, just to explain what
> I mean. May be someone can test it ;)
>
> This series kills xol_vma. Instead we use the per_cpu-like xol slots.
>
> This is much more simple and efficient. And this of course solves
> many problems we currently have with xol_vma.
>
> For example, we simply can not trust it. We do not know what actually
> we are going to execute in UTASK_SSTEP mode. An application can unmap
> this area and then do mmap(PROT_EXEC|PROT_WRITE, MAP_FIXED) to fool
> uprobes.
>
> The only disadvantage is that this adds a bit more arch-dependant
> code.
>
> The main question, can this work?

OK, it almost works.

But, this way we can't probe the compat tasks. A __USER32_CS task can't
access the fix_to_virt() area, so it can't use uprobe_xol_slots[].

Many thanks to Josh who noticed this.

I'll try to think more, but so far I do not see any simple solution.

Oleg.

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