Re: [PATCH 1/4] KVM: nVMX: Check GUEST_DEBUGCTL on vmentry of nested guests

[Jim Mattson <jmattson@google.com>]

On Thu, Aug 29, 2019 at 2:25 PM Krish Sadhukhan
<krish.sadhukhan@oracle.com> wrote:
>
> According to section "Checks on Guest Control Registers, Debug Registers, and
> and MSRs" in Intel SDM vol 3C, the following checks are performed on vmentry
> of nested guests:
>
>     If the "load debug controls" VM-entry control is 1, bits reserved in the
>     IA32_DEBUGCTL MSR must be 0 in the field for that register. The first
>     processors to support the virtual-machine extensions supported only the
>     1-setting of this control and thus performed this check unconditionally.
>
> Signed-off-by: Krish Sadhukhan <krish.sadhukhan@oracle.com>
> Reviewed-by: Karl Heubaum <karl.heubaum@oracle.com>
> ---
>  arch/x86/kvm/vmx/nested.c | 4 ++++
>  arch/x86/kvm/x86.h        | 6 ++++++
>  2 files changed, 10 insertions(+)
>
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 46af3a5e9209..0b234e95e0ed 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -2677,6 +2677,10 @@ static int nested_vmx_check_guest_state(struct kvm_vcpu *vcpu,
>             !nested_guest_cr4_valid(vcpu, vmcs12->guest_cr4))
>                 return -EINVAL;
>
> +       if ((vmcs12->vm_entry_controls & VM_ENTRY_LOAD_DEBUG_CONTROLS) &&
> +           !kvm_debugctl_valid(vmcs12->guest_ia32_debugctl))
> +               return -EINVAL;
> +
>         if ((vmcs12->vm_entry_controls & VM_ENTRY_LOAD_IA32_PAT) &&
>             !kvm_pat_valid(vmcs12->guest_ia32_pat))
>                 return -EINVAL;
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index a470ff0868c5..28ba6d0c359f 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -354,6 +354,12 @@ static inline bool kvm_pat_valid(u64 data)
>         return (data | ((data & 0x0202020202020202ull) << 1)) == data;
>  }
>
> +static inline bool kvm_debugctl_valid(u64 data)
> +{
> +       /* Bits 2, 3, 4, 5, 13 and [31:16] are reserved */
> +       return ((data & 0xFFFFFFFFFFFF203Cull) ? false : true);
> +}

This should actually be consistent with the constraints in kvm_set_msr_common:

case MSR_IA32_DEBUGCTLMSR:
        if (!data) {
                /* We support the non-activated case already */
                break;
        } else if (data & ~(DEBUGCTLMSR_LBR | DEBUGCTLMSR_BTF)) {
                /* Values other than LBR and BTF are vendor-specific,
                   thus reserved and should throw a #GP */
                return 1;
        }

Also, as I said earlier...

I'd rather see this built on an interface like:

bool kvm_valid_msr_value(u32 msr_index, u64 value);

Strange that we allow IA32_DEBUGCTL.BTF, since kvm_vcpu_do_singlestep
ignores it. And vLBR still isn't a thing, is it?

It's a bit scary to me that we allow any architecturally legal
IA32_DEBUGCTL bits to be set today. There's probably a CVE in there
somewhere.