linux-kernel - Re: [PATCH v2] x86/mm: warn on W+x mappings

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Message-ID: <20151021143651.GE3575@pd.tnic>
Date:	Wed, 21 Oct 2015 16:36:51 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Matt Fleming <matt@...eblueprint.co.uk>,
	Stephen Smalley <sds@...ho.nsa.gov>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Kees Cook <keescook@...omium.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andy Lutomirski <luto@...nel.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Brian Gerst <brgerst@...il.com>,
	"linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>
Subject: Re: [PATCH v2] x86/mm: warn on W+x mappings

On Wed, Oct 21, 2015 at 03:28:56PM +0200, Ard Biesheuvel wrote:
> In theory, yes. In practice, since this is supposed to be a security
> enhancement, we need some kind of ground truth to tell us which pages
> can be legally modified *and* executed, so that we can detect the
> illegal cases. My point was that, since a multitude of PE/COFF images
> can be covered by a single EfiRuntimeServicesCode region, the UEFI
> memory map does not give us enough information to make the distinction
> between a page that sits on the text/data boundary of some PE/COFF
> image and a page that sits wholly in either.

Well, we're going to simply allow the accesses to in-kernel users which
fault on those ranges, assuming that in-kernel modifiers are legit and
DTRT. Which means, we don't really need to know which pages can be
legally modified - we simply trust the in-kernel users.

The moment you're able to load an evil kernel module, guarding against
those writes is the last thing you need to worry about...

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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