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

[Paolo Bonzini]

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)
+{
+	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
-- 
1.8.3.1