linux-kernel - Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only memory

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Message-ID: <5655F059.4010801@zytor.com>
Date:	Wed, 25 Nov 2015 09:31:05 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Mathias Krause <minipli@...glemail.com>,
	kernel-hardening@...ts.openwall.com
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Kees Cook <keescook@...omium.org>,
	Andy Lutomirski <luto@...capital.net>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>, x86-ml <x86@...nel.org>,
	Arnd Bergmann <arnd@...db.de>,
	Michael Ellerman <mpe@...erman.id.au>,
	linux-arch@...r.kernel.org, PaX Team <pageexec@...email.hu>,
	Emese Revfy <re.emese@...il.com>
Subject: Re: [kernel-hardening] [PATCH 0/2] introduce post-init read-only
 memory

On 11/25/15 01:13, Mathias Krause wrote:
> 
> While having that annotation makes perfect sense, not only from a
> security perspective but also from a micro-optimization point of view
> (much like the already existing __read_mostly annotation), it has its
> drawbacks. Violating the "r/o after init" rule by writing to such
> annotated variables from non-init code goes unnoticed as far as it
> concerns the toolchain. Neither the compiler nor the linker will flag
> that incorrect use. It'll just trap at runtime and that's bad.
> 
> I myself had some educating experience seeing my machine triple fault
> when resuming from a S3 sleep. The root cause was a variable that was
> annotated __read_only but that was (unnecessarily) modified during CPU
> bring-up phase. Debugging that kind of problems is sort of a PITA, you
> could imagine.
> 
> So, prior extending the usage of the __read_only annotation some
> toolchain support is needed. Maybe a gcc plugin that'll warn/error on
> code that writes to such a variable but is not __init itself. The
> initify and checker plugins from the PaX patch might be worth to look
> at for that purpose, as they're doing similar things already. Adding
> such a check to sparse might be worth it, too.
> A modpost check probably won't work as it's unable to tell if it's a
> legitimate access (r/o) or a violation (/w access). So the gcc plugin
> is the way to go, IMHO.
> 

We should not wait for compile-time support, that doesn't make any
sense.  What would be useful would be a way to override this on the
command line -- that way, if disabling RO or RO-after-init memory makes
something work, we have an instant diagnosis.

	-hpa


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