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

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Date:	Wed, 13 Feb 2013 09:27:13 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Matthew Garrett <matthew.garrett@...ula.com>,
	Borislav Petkov <bp@...en8.de>,
	Kees Cook <keescook@...omium.org>,
	LKML <linux-kernel@...r.kernel.org>,
	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

Il 13/02/2013 07:33, H. Peter Anvin ha scritto:
>>
>>> Sounds like you are thinking of CAP_SYS_ADMIN, but I don't really see a
>>> huge difference between MSRs and I/O control registers... just different
>>> address spaces.
>>
>> Not having CAP_SYS_RAWIO blocks various SCSI commands, for instance.
>> These might result in the ability to write individual blocks or destroy
>> the device firmware, but do any of them permit modifying the running
>> kernel?

No, they cannot.

> That is just batshit crazy.  If you have CAP_SYS_RAWIO you can do iopl()
> which means you can reprogram your northbridge, at which point you most
> definitely *can* modify the running kernel.
> 
> And some SCSI driver requires this??!

No, and that's why there is a patchset floating that lets you toggle
this ability with a sysfs control.  This way you do not need
CAP_SYS_RAWIO anymore.

On non-x86 machines CAP_SYS_RAWIO is much less dangerous, especially
when coupled with file DAC.

Paolo
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