linux-kernel - Re: [PATCH 4/4] KVM: x86: Simplify logic to handle lack of host NX 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: <YMk40yLeyV1DHpYp@google.com>
Date:   Tue, 15 Jun 2021 23:33:39 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/4] KVM: x86: Simplify logic to handle lack of host NX
 support

On Tue, Jun 15, 2021, Jim Mattson wrote:
> On Tue, Jun 15, 2021 at 9:45 AM Sean Christopherson <seanjc@...gle.com> wrote:
> > @@ -226,7 +224,7 @@ static void cpuid_fix_nx_cap(struct kvm_vcpu *vcpu)
> >                         break;
> >                 }
> >         }
> > -       if (entry && cpuid_entry_has(entry, X86_FEATURE_NX) && !is_efer_nx()) {
> > +       if (entry && cpuid_entry_has(entry, X86_FEATURE_NX)) {
> >                 cpuid_entry_clear(entry, X86_FEATURE_NX);
> >                 printk(KERN_INFO "kvm: guest NX capability removed\n");
> >         }
> 
> It would be nice if we chose one consistent approach to dealing with
> invalid guest CPUID information and stuck with it. Silently modifying
> the table provided by userspace seems wrong to me. I much prefer the
> kvm_check_cpuid approach of telling userspace that the guest CPUID
> information is invalid. (Of course, once we return -EINVAL for more
> than one field, good luck figuring out which field is invalid!)

Yeah.  I suspect this one can be dropped if EFER.NX is required for everything
except EPT, but I didn't fully grok the problem that this was fixing, and it's
such an esoteric case that I both don't care and am terrified of breaking some
bizarre case.