linux-kernel - Re: [PATCH] arm: don't allow CONFIG_DEBUG_SET_MODULE_RONX if CONFIG_JUMP

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Date:	Tue, 01 Apr 2014 20:53:31 +0200
From:	Alexander Holler <holler@...oftware.de>
To:	Kees Cook <keescook@...omium.org>,
	Laura Abbott <lauraa@...eaurora.org>
CC:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	LKML <linux-kernel@...r.kernel.org>,
	Russell King <linux@....linux.org.uk>
Subject: Re: [PATCH] arm: don't allow CONFIG_DEBUG_SET_MODULE_RONX if CONFIG_JUMP_LABEL
 is enabled

Am 01.04.2014 20:36, schrieb Kees Cook:
> On Tue, Apr 1, 2014 at 11:03 AM, Laura Abbott <lauraa@...eaurora.org> wrote:
>> On 4/1/2014 3:04 AM, Alexander Holler wrote:
>>> CONFIG_DEBUG_SET_MODULE_RONX sounds like a nice security feature, but
>>> things might fail late (and unexpected) if module code is set to read-only
>>> while CONFIG_JUMP_LABEL is enabled (e.g. modprobe bridge).
>
> Isn't this a ordering problem? I thought jump labels got set up once,
> and then after that, the memory could be made RO?

I basically just run into that and looked up what happened. But the 
problem appears e.g. in netfiler/core.c function nf_register_hook() 
which calls static_key_slow_inc(). So you would have to make sure 
nf_register_hook() will be called before the code is set ro. Something 
that doesn't look easy to do.

I would have to look up when that might be called, but I assume there 
are many ways to register and unregister hooks in netfilter and some of 
them might happen outside any init, probe or whatever one might set the 
code read-only afterwards. You would have to set the code rw too, before 
nf_unregister_hook() happens.

Maybe it's possible to mark some modules to not become ro at all, I 
don't know. And doing so would make them a prefered target for exploits. 
So I'm not sure if it would make sense.

Regards,

Alexander Holler
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