linux-kernel - Re: [PATCH] x86: Lock down MSR writing in secure boot

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Date:	Fri, 8 Feb 2013 11:21:39 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Matthew Garrett <matthew.garrett@...ula.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
	"linux-security-module@...r.kernel.org" 
	<linux-security-module@...r.kernel.org>
Subject: Re: [PATCH] x86: Lock down MSR writing in secure boot

On Fri, Feb 8, 2013 at 11:17 AM, Matthew Garrett
<matthew.garrett@...ula.com> wrote:
> On Fri, 2013-02-08 at 11:12 -0800, Kees Cook wrote:
>> Writing to MSRs should not be allowed unless CAP_COMPROMISE_KERNEL is
>> set since it could lead to execution of arbitrary code in kernel mode.
>
> Willing to buy this, but do you have a description of one potential
> approach? We should probably also figure out what's writing to MSRs at
> the moment (anything other than energy_perf_bias?) and decide what the
> best thing to do there is.

Yes, change the SYSENTER entry point to where-ever you like. There are
examples already written:
http://grsecurity.net/~spender/msr32.c

IMO, _writing_ an MSR from userspace should be considered a bug. If
writing is needed, a kernel driver should be mediating the change.
wrmsr (and rdmsr) are ring-0 only for good reason. :)

-Kees

--
Kees Cook
Chrome OS Security
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