linux-kernel - Re: [RESEND PATCH v3] KVM: x86: Add Intel CPUID.1F cpuid emulation support

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: <20190603191818.GF13384@linux.intel.com>
Date:   Mon, 3 Jun 2019 12:18:18 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     Like Xu <like.xu@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [RESEND PATCH v3] KVM: x86: Add Intel CPUID.1F cpuid emulation
 support

On Mon, Jun 03, 2019 at 06:56:17PM +0200, Radim Krčmář wrote:
> > +			break;
> > +		}
> >  		entry->eax = min(entry->eax, (u32)(f_intel_pt ? 0x14 : 0xd));
> 
> Similarly in the existing code.  If we don't have f_intel_pt, then we
> should make sure that leaf 0x14 is not being filled, but we don't really
> have to limit the maximal index.
> 
> Adding a single clamping like
> 
> 		/* Limited to the highest leaf implemented in KVM. */
> 		entry->eax = min(entry->eax, 0x1f);
> 
> seems sufficient.
>
> (Passing the hardware value is ok in theory, but it is a cheap way to
>  avoid future leaves that cannot be simply zeroed for some weird reason.)

I don't have a strong opinion regarding the code itself, but whatever ends
up getting committed should have a big beefy changelog explaining why the
clamping exists, or at least extolling its virtues.  I had a hell of a
time understanding the intent of this one line of code because as your
response shows, there is no one right answer.