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

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Message-ID: <CAGXu5j+_mGzR+72YqORhErMUY1aF1H2urczZ8SwuJHZK-oZL6g@mail.gmail.com>
Date:	Fri, 8 Feb 2013 12:14:02 -0800
From:	Kees Cook <keescook@...omium.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Matthew Garrett <matthew.garrett@...ula.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 <linux-security-module@...r.kernel.org>
Subject: Re: [PATCH] x86: Lock down MSR writing in secure boot

On Fri, Feb 8, 2013 at 11:42 AM, H. Peter Anvin <hpa@...or.com> wrote:
> On 02/08/2013 11:18 AM, Kees Cook wrote:
>>
>> No. CAP_RAWIO is for reading. Writing needs a much stronger check.
>
> If so, I suspect we need to do this for *all* raw I/O... but I keep
> wondering how much more sensitive writing really is than reading.

Well, I think there's a reasonable distinction between systems that
expect to strictly enforce user-space/kernel-space separation
(CAP_COMPROMISE_KERNEL) and things that are fiddling with hardware
(CAP_SYS_RAWIO).

For example, even things like /dev/mem already have this separation
(although it is stronger). You can't open /dev/mem without
CAP_SYS_RAWIO, but if you do, you still can't write to RAM in
/dev/mem. This might be one of the earliest examples of this
distinction, actually.

I think it's likely that after a while, we can convert some of these
proposed CAP_COMPROMISE_KERNEL checks in always-deny once we figure
out how to deal with those areas more safely.

-Kees

--
Kees Cook
Chrome OS Security
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