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

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Date:	Fri, 08 Feb 2013 12:18:27 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Kees Cook <keescook@...omium.org>
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

Analogy fail.  The /dev/mem lockout applies to system RAM, not MMIO.

I fear COMPROMISE_KERNEL is becoming the new SYS_ADMIN, which in turn is the new root.  Why?  Because it is inhebtly about a usage model, not about a specific resource.

Kees Cook <keescook@...omium.org> wrote:

>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

-- 
Sent from my mobile phone. Please excuse brevity and lack of formatting.
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