Re: [PATCH 2/5] KVM: svm: Disallow EFER.LMSLE on hardware that doesn't support it

[Jim Mattson]

On Fri, Sep 16, 2022 at 1:14 PM Sean Christopherson <seanjc@xxxxxxxxxx> 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@xxxxxxxxxx>
> > ---
> >  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.