linux-kernel - Re: [PATCH] x86/fpu: Parse comma separated list passed in clearcpuid

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: <20190221185806.GC16922@tassilo.jf.intel.com>
Date:   Thu, 21 Feb 2019 10:58:06 -0800
From:   Andi Kleen <ak@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Prarit Bhargava <prarit@...hat.com>, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH] x86/fpu: Parse comma separated list passed in clearcpuid

On Thu, Feb 21, 2019 at 02:37:45PM +0100, Peter Zijlstra wrote:
> On Thu, Feb 21, 2019 at 08:12:25AM -0500, Prarit Bhargava wrote:
> > Users cannot disable multiple CPU features with the kernel parameter
> > clearcpuid=.  For example, "clearcpuid=154 clearcpuid=227" only disables
> > CPUID bit 154.
> > 
> > Previous to commit 0c2a3913d6f5 ("x86/fpu: Parse clearcpuid= as early XSAVE
> > argument") it was possible to pass multiple clearcpuid options as kernel
> > parameters using individual entries.  With the new code it isn't easy to
> > replicate exactly that behaviour but a comma separated list can be easily
> > implemented, eg) "clearcpuid=154,227"
> > 
> > Make the clearcpuid parse a comma-separated list of values instead of only
> > a single value.
> 
> So I think the feature is broken as is; because it doesn't clear the
> CPUID bits for userspace.

Usually it's enough to make the kernel stop using something. I used it many 
times for this.

People who want to affect user space usually run VMs anyways.

-Andi