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From:   Jim Mattson <jmattson@google.com>
Date:   Fri, 16 Sep 2022 14:00:26 -0700
Message-ID: <CALMp9eQXroxQYiWUCejd0Cj7kD5g5navWY_E2O_vzbVAQjLyNg@mail.gmail.com>
Subject: Re: [PATCH 2/5] KVM: svm: Disallow EFER.LMSLE on hardware that
 doesn't support it
To:     Sean Christopherson <seanjc@google.com>
Cc:     Avi Kivity <avi@redhat.com>, Babu Moger <babu.moger@amd.com>,
        Borislav Petkov <bp@alien8.de>,
        "Chang S. Bae" <chang.seok.bae@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
        Joerg Roedel <joerg.roedel@amd.com>,
        Josh Poimboeuf <jpoimboe@kernel.org>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Wyes Karny <wyes.karny@amd.com>, x86@kernel.org
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On Fri, Sep 16, 2022 at 1:14 PM Sean Christopherson <seanjc@google.com> wrote:
>
> On Thu, Sep 15, 2022, Jim Mattson wrote:
> > KVM has never properly virtualized EFER.LMSLE. When the "nested"
> > module parameter is true, it allows an SVM guest to set EFER.LMSLE,
> > and it passes the bit through in the VMCB, but the KVM emulator
> > doesn't perform the required data segment limit checks in 64-bit mode.
> >
> > With Zen3, AMD has dropped support for EFER.LMSLE. Hence, if a Zen3
> > guest sets EFER.LMSLE, the next VMRUN will fail with "invalid VMCB."
> >
> > When the host reports X86_FEATURE_NO_LMSLE, treat EFER.LMSLE as a
> > reserved bit in the guest. Now, if a guest tries to set EFER.LMSLE on
> > a host without support for EFER.LMSLE, the WRMSR will raise a #GP.
> >
> > At the moment, the #GP may come as a surprise, but it's an improvement
> > over the failed VMRUN. The #GP will be vindicated anon.
> >
> > Fixes: eec4b140c924 ("KVM: SVM: Allow EFER.LMSLE to be set with nested svm")
> > Signed-off-by: Jim Mattson <jmattson@google.com>
> > ---
> >  arch/x86/kvm/svm/svm.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > index f3813dbacb9f..7c4fd594166c 100644
> > --- a/arch/x86/kvm/svm/svm.c
> > +++ b/arch/x86/kvm/svm/svm.c
> > @@ -5012,7 +5012,9 @@ static __init int svm_hardware_setup(void)
> >
> >       if (nested) {
> >               printk(KERN_INFO "kvm: Nested Virtualization enabled\n");
> > -             kvm_enable_efer_bits(EFER_SVME | EFER_LMSLE);
> > +             kvm_enable_efer_bits(EFER_SVME);
> > +             if (!boot_cpu_has(X86_FEATURE_NO_LMSLE))
> > +                     kvm_enable_efer_bits(EFER_LMSLE);
>
> Since KVM doesn't correctly virtualize EFER.LMSLE, I wonder if we can get away with
> dropping support entirely.  I.e. delete the reference to EFER_LMSLE and unconditionally
> set F(NO_LMSLE) in KVM_GET_SUPPORTED_CPUID.

It's possible that SLES11 Xen 4.0 sets the bit, but never actually
uses truncated segments in 64-bit mode. In any case, according to the
original commit, it won't boot if setting EFER.LMSLE is not allowed.