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

From: Kees Cook
Date: Fri Feb 08 2013 - 15:14:09 EST

On Fri, Feb 8, 2013 at 11:42 AM, H. Peter Anvin <hpa@xxxxxxxxx> 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

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 Cook
Chrome OS Security
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