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

From: H. Peter Anvin
Date: Fri Feb 08 2013 - 15:18:58 EST

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@xxxxxxxxxxxx> wrote:

>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

Sent from my mobile phone. Please excuse brevity and lack of formatting.
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at