Re: [PATCH] KVM: X86: Raise #GP when clearing CR0_PG in 64 bit mode

[Jim Mattson]

On Mon, Jun 9, 2025 at 7:23 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Fri, Jun 06, 2025, H. Peter Anvin wrote:
> > On 2021-12-09 09:55, Paolo Bonzini wrote:
> > > On 12/7/21 10:52, Lai Jiangshan wrote:
> > > > From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> > > >
> > > > In the SDM:
> > > > If the logical processor is in 64-bit mode or if CR4.PCIDE = 1, an
> > > > attempt to clear CR0.PG causes a general-protection exception (#GP).
> > > > Software should transition to compatibility mode and clear CR4.PCIDE
> > > > before attempting to disable paging.
> > > >
> > > > Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> > > > ---
> > > >   arch/x86/kvm/x86.c | 3 ++-
> > > >   1 file changed, 2 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > > index 00f5b2b82909..78c40ac3b197 100644
> > > > --- a/arch/x86/kvm/x86.c
> > > > +++ b/arch/x86/kvm/x86.c
> > > > @@ -906,7 +906,8 @@ int kvm_set_cr0(struct kvm_vcpu *vcpu, unsigned
> > > > long cr0)
> > > >           !load_pdptrs(vcpu, kvm_read_cr3(vcpu)))
> > > >           return 1;
> > > > -    if (!(cr0 & X86_CR0_PG) && kvm_read_cr4_bits(vcpu, X86_CR4_PCIDE))
> > > > +    if (!(cr0 & X86_CR0_PG) &&
> > > > +        (is_64_bit_mode(vcpu) || kvm_read_cr4_bits(vcpu,
> > > > X86_CR4_PCIDE)))
> > > >           return 1;
> > > >       static_call(kvm_x86_set_cr0)(vcpu, cr0);
> > > >

Isn't this redundant with the "if (cs_l)" check above?

> > > Queued, thanks.
> > >
> >
> > Have you actually checked to see what real CPUs do in this case?
>
> I have now, and EMR at least behaves as the SDM describes.  Why do you ask?
>
>
> kvm_intel: Clearing CR0.PG faulted (vector = 13)
>
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index f79604bc0127..f90ad464ab7e 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -8637,6 +8637,23 @@ void vmx_exit(void)
>         kvm_x86_vendor_exit();
>  }
>
> +static noinline void vmx_disable_paging(void)
> +{
> +       unsigned long cr0 = native_read_cr0();
> +       long vector = -1;
> +
> +       asm volatile("1: mov %1, %%cr0\n\t"
> +                    "   mov %2, %%cr0\n\t"
> +                    "2:"
> +                    _ASM_EXTABLE_FAULT(1b, 2b)
> +                    : "+a" (vector)
> +                    : "r" (cr0 & ~X86_CR0_PG), "r" (cr0)
> +                    : "cc", "memory" );
> +
> +       pr_warn("Clearing CR0.PG %s (vector = %ld)\n",
> +               vector < 0 ? "succeeded" : "faulted", vector);
> +}
> +
>  int __init vmx_init(void)
>  {
>         int r, cpu;
> @@ -8644,6 +8661,8 @@ int __init vmx_init(void)
>         if (!kvm_is_vmx_supported())
>                 return -EOPNOTSUPP;
>
> +       vmx_disable_paging();
> +
>         /*
>          * Note, hv_init_evmcs() touches only VMX knobs, i.e. there's nothing
>          * to unwind if a later step fails.
>