linux-kernel - Re: [PATCH 4/6] ftrace, x86: make kernel text writable only for conversions

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Date:	Mon, 23 Feb 2009 10:02:13 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	"H. Peter Anvin" <hpa@...or.com>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [PATCH 4/6] ftrace, x86: make kernel text writable only for
	conversions


* Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> wrote:

> Can you provide numbers to indicate why it's required to be so 
> intrusive in the kernel mappings while doing these 
> modifications ? I think opening such time window where 
> standard code mapping is writeable globally in config RO_DATA 
> kernels could open the door to unexpected side-effects, so 
> ideally going through the "backdoor" page mapped by text_poke 
> seems safer. Given similar performance, I would tend to use a 
> text_poke-like approach.

It's not really an issue - this code is only called during 
normal operation if the admin does it.

As far as scare mongering goes a "backdoor" page is in fact more 
attackable because it's at a more predictable position and due 
to text-poke's slowness the window of vulnerability is longer.

Anyway, this is all pretty theoretical and irrelevant. The 
purpose of RODATA is mainly to protect against benign/unintended 
sources of kernel text corruption. An attacker, if he can modify 
arbitrary kernel text address can already modify other critical 
data structures to gain access.

	Ingo
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