Re: [PATCH v2 2/5] KVM: nSVM: Ignore EFER.LMA if EFER.LME=0 when preparing L2 state

[Sean Christopherson <seanjc@google.com>]

On Wed, Sep 02, 2026, Yosry Ahmed wrote:
> On Wed, Sep 2, 2026 at 4:20 PM Sean Christopherson <seanjc@google.com> wrote:
> >
> > Force EFER.LMA=0 if EFER.LME=0 when preparing L2 state for VMRUN, i.e.
> > mimic real hardware's behavior of ignoring EFER.LMA if EFER.LME=0.  VMRUN
> > unfortunately allows the nonsensical combination, i.e. doesn't fail, but
> > KVM itself has an invariant EFER.LMA can be set et if and only if EFER.LME
> > is set.   Breaking that invariant can lead to a variety of issue,
> > particularly in MMU code that keys off EFER.LMA when determining whether to
> > emulate/virtualization 4/5-level paging versus PAE paging.
> >
> > Cc: stable@vger.kernel.org
> > Cc: Yosry Ahmed <yosry@kernel.org>
> > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > ---
> >  arch/x86/kvm/svm/nested.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> >
> > diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> > index 49fb10ad1f9f..23d29597d6bf 100644
> > --- a/arch/x86/kvm/svm/nested.c
> > +++ b/arch/x86/kvm/svm/nested.c
> > @@ -789,6 +789,10 @@ static void nested_vmcb02_prepare_save(struct vcpu_svm *svm)
> >
> >         kvm_set_rflags(vcpu, save->rflags | X86_EFLAGS_FIXED);
> >
> > +       /* SVM ignores EFER.LMA if EFER.LME=0 (instead of failing VMRUN). */
> > +       if (!(svm->nested.save.efer & EFER_LME))
> > +               svm->nested.save.efer &= ~EFER_LMA;
> 
> We sanitize control fields in __nested_copy_vmcb_control_to_cache().
> Should we similarly sanitize this in __nested_copy_vmcb_save_to_cache()?

Ideally, yes?  In practice, it doesn't work because svm_set_nested_state() loads
state from "save", not from "save_cached".  And even if we fixed that, it would
then allow userspace to pass in garbage (that is then ignored), i.e. would undo
patch 1, and I don't want to do that.