Re: [PATCH] KVM: x86: avoid incorrect writes to host MSR_IA32_SPEC_CTRL

[Xiaoyao Li]

On 1/21/2020 9:48 PM, Paolo Bonzini wrote:
> If the guest is configured to have SPEC_CTRL but the host does not
> (which is a nonsensical configuration but these are not explicitly
> forbidden) then a host-initiated MSR write can write vmx->spec_ctrl
> (respectively svm->spec_ctrl) and trigger a #GP when KVM tries to
> restore the host value of the MSR.  Add a more comprehensive check
> for valid bits of SPEC_CTRL, covering host CPUID flags and,
> since we are at it and it is more correct that way, guest CPUID
> flags too.
>
> For AMD, remove the unnecessary is_guest_mode check around setting
> the MSR interception bitmap, so that the code looks the same as
> for Intel.
>
> Cc: Jim Mattson <jmattson@xxxxxxxxxx>
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> ---
>   arch/x86/kvm/svm.c     |  9 +++------
>   arch/x86/kvm/vmx/vmx.c |  7 +++----
>   arch/x86/kvm/x86.c     | 22 ++++++++++++++++++++++
>   arch/x86/kvm/x86.h     |  1 +
>   4 files changed, 29 insertions(+), 10 deletions(-)
>
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index b7c5369c7998..235a7e51de96 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -4324,12 +4324,10 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
>   		    !guest_cpuid_has(vcpu, X86_FEATURE_AMD_SSBD))
>   			return 1;
>   
> -		/* The STIBP bit doesn't fault even if it's not advertised */
> -		if (data & ~(SPEC_CTRL_IBRS | SPEC_CTRL_STIBP | SPEC_CTRL_SSBD))
> +		if (data & ~kvm_spec_ctrl_valid_bits(vcpu))
>   			return 1;
>   
>   		svm->spec_ctrl = data;
> -
>   		if (!data)
>   			break;
>   
> @@ -4353,13 +4351,12 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
>   
>   		if (data & ~PRED_CMD_IBPB)
>   			return 1;
> -
> +		if (!boot_cpu_has(X86_FEATURE_AMD_IBPB))
> +			return 1;
>   		if (!data)
>   			break;
>   
>   		wrmsrl(MSR_IA32_PRED_CMD, PRED_CMD_IBPB);
> -		if (is_guest_mode(vcpu))
> -			break;
>   		set_msr_interception(svm->msrpm, MSR_IA32_PRED_CMD, 0, 1);
>   		break;
>   	case MSR_AMD64_VIRT_SPEC_CTRL:
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index bdbf27e92851..112d2314231d 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1998,12 +1998,10 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>   		    !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL))
>   			return 1;
>   
> -		/* The STIBP bit doesn't fault even if it's not advertised */
> -		if (data & ~(SPEC_CTRL_IBRS | SPEC_CTRL_STIBP | SPEC_CTRL_SSBD))
> +		if (data & ~kvm_spec_ctrl_valid_bits(vcpu))
>   			return 1;
>   
>   		vmx->spec_ctrl = data;
> -
>   		if (!data)
>   			break;
>   
> @@ -2037,7 +2035,8 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>   
>   		if (data & ~PRED_CMD_IBPB)
>   			return 1;
> -
> +		if (!boot_cpu_has(X86_FEATURE_SPEC_CTRL))
> +			return 1;
>   		if (!data)
>   			break;
>   
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 9f24f5d16854..141fb129c6bf 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -10389,6 +10389,28 @@ bool kvm_arch_no_poll(struct kvm_vcpu *vcpu)
>   }
>   EXPORT_SYMBOL_GPL(kvm_arch_no_poll);
>   
> +bool kvm_spec_ctrl_valid_bits(struct kvm_vcpu *vcpu)

The return type should be u64.

> +{
> +	uint64_t bits = SPEC_CTRL_IBRS | SPEC_CTRL_STIBP | SPEC_CTRL_SSBD;
> +
> +	/* The STIBP bit doesn't fault even if it's not advertised */
> +	if (!guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL) &&
> +	    !guest_cpuid_has(vcpu, X86_FEATURE_AMD_IBRS))
> +		bits &= ~(SPEC_CTRL_IBRS | SPEC_CTRL_STIBP);
> +	if (!boot_cpu_has(X86_FEATURE_SPEC_CTRL) &&
> +	    !boot_cpu_has(X86_FEATURE_AMD_IBRS))
> +		bits &= ~(SPEC_CTRL_IBRS | SPEC_CTRL_STIBP);
> +
> +	if (!guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL_SSBD) &&
> +	    !guest_cpuid_has(vcpu, X86_FEATURE_AMD_SSBD))
> +		bits &= ~SPEC_CTRL_SSBD;
> +	if (!boot_cpu_has(X86_FEATURE_SPEC_CTRL_SSBD) &&
> +	    !boot_cpu_has(X86_FEATURE_AMD_SSBD))
> +		bits &= ~SPEC_CTRL_SSBD;
> +
> +	return bits;
> +}
> +EXPORT_SYMBOL_GPL(kvm_spec_ctrl_valid_bits);
>   
>   EXPORT_TRACEPOINT_SYMBOL_GPL(kvm_exit);
>   EXPORT_TRACEPOINT_SYMBOL_GPL(kvm_fast_mmio);
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index ab715cee3653..bc38ac695776 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -367,5 +367,6 @@ static inline bool kvm_pat_valid(u64 data)
>   
>   void kvm_load_guest_xsave_state(struct kvm_vcpu *vcpu);
>   void kvm_load_host_xsave_state(struct kvm_vcpu *vcpu);
> +bool kvm_spec_ctrl_valid_bits(struct kvm_vcpu *vcpu);
>   
>   #endif