linux-kernel - Re: [PATCH 2/2] cpu: intel, amd: mask cleared cpuid features

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <2764CF11-A5E5-4C8D-A2EE-69491D2864C3@parallels.com>
Date:	Fri, 20 Jul 2012 22:21:58 +0400
From:	Vladimir Davydov <VDavydov@...allels.com>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Borislav Petkov <borislav.petkov@....com>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] cpu: intel, amd: mask cleared cpuid features

On Jul 20, 2012, at 9:20 PM, H. Peter Anvin wrote:

> On 07/20/2012 09:37 AM, Vladimir Davydov wrote:
>> If 'clearcpuid=N' is specified in boot options, CPU feature #N won't be
>> reported in /proc/cpuinfo and used by the kernel. However, if a
>> userpsace process checks CPU features directly using the cpuid
>> instruction, it will be reported about all features supported by the CPU
>> irrespective of what features are cleared.
>> 
>> The patch makes the clearcpuid boot option not only clear CPU features
>> in kernel but also mask them in hardware for Intel and AMD CPUs that
>> support it so that the features cleared won't be reported even by the
>> cpuid instruction.
>> 
>> This can be useful for migration of virtual machines managed by
>> hypervisors that do not support/use Intel VT/AMD-V hardware-assisted
>> virtualization technology.
>> 
>> If CPUID masking is supported, this will be reported in
>> /proc/cpuinfo:flags as 'cpuidmask'.
> 
> I am a bit concerned about this patch:
> 
> 1. it silently changes existing behavior.

Yes, but who needs the current implementation of 'clearcpuid' which, in fact, just hides flags in /proc/cpuinfo while userspace apps will see and consequently use all CPU features?

So, I think it logically extends the existing behavior.

> 2. even on enabled hardware, only some of the bits are maskable.

The patch makes only words 0, 1, 4, 6 maskable, but words 3, 7, 8 are Linux-defined, words 2 and 5 are Transmeta-, Centaur-, etc- defined, and word 9 contains some bizarre Intel CPU features. Thus, it is words 0, 1, 4, 6 that contain useful information for most hardware models.

If you ask about some Intel CPUs that can't mask CPUID function 0x80000001, this function describes AMD-specific features, and I bet those Intel CPUs just don't have them at all and thus have nothing to mask.

> 
> 	-hpa
> 
> 

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/