linux-kernel - [PATCH] KVM: x86: reinstate vendor-agnostic check on SPEC

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Date:   Thu,  3 Dec 2020 10:15:16 -0500
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     stable@...r.kernel.org, "Denis V . Lunev" <den@...nvz.org>
Subject: [PATCH] KVM: x86: reinstate vendor-agnostic check on SPEC_CTRL cpuid bits

Until commit e7c587da1252 ("x86/speculation: Use synthetic bits for IBRS/IBPB/STIBP",
2018-05-17), KVM was testing both Intel and AMD CPUID bits before allowing the
guest to write MSR_IA32_SPEC_CTRL and MSR_IA32_PRED_CMD.  Testing only Intel bits
on VMX processors, or only AMD bits on SVM processors, fails if the guests are
created with the "opposite" vendor as the host.

While at it, also tweak the host CPU check to use the vendor-agnostic feature bit
X86_FEATURE_IBPB, since we only care about the availability of the MSR on the host
here and not about specific CPUID bits.

Fixes: e7c587da1252 ("x86/speculation: Use synthetic bits for IBRS/IBPB/STIBP")
Cc: stable@...r.kernel.org
Reported-by: Denis V. Lunev <den@...nvz.org>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 arch/x86/kvm/svm/svm.c |  3 ++-
 arch/x86/kvm/vmx/vmx.c | 10 +++++++---
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 62390fbc9233..0b4aa60b2754 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -2686,12 +2686,13 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
 		break;
 	case MSR_IA32_PRED_CMD:
 		if (!msr->host_initiated &&
+		    !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL) &&
 		    !guest_cpuid_has(vcpu, X86_FEATURE_AMD_IBPB))
 			return 1;
 
 		if (data & ~PRED_CMD_IBPB)
 			return 1;
-		if (!boot_cpu_has(X86_FEATURE_AMD_IBPB))
+		if (!boot_cpu_has(X86_FEATURE_IBPB))
 			return 1;
 		if (!data)
 			break;
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index c3441e7e5a87..b74d2105ced7 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -2028,7 +2028,10 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		break;
 	case MSR_IA32_SPEC_CTRL:
 		if (!msr_info->host_initiated &&
-		    !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL))
+                    !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL) &&
+                    !guest_cpuid_has(vcpu, X86_FEATURE_AMD_STIBP) &&
+                    !guest_cpuid_has(vcpu, X86_FEATURE_AMD_IBRS) &&
+                    !guest_cpuid_has(vcpu, X86_FEATURE_AMD_SSBD))
 			return 1;
 
 		if (kvm_spec_ctrl_test_value(data))
@@ -2063,12 +2066,13 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		goto find_uret_msr;
 	case MSR_IA32_PRED_CMD:
 		if (!msr_info->host_initiated &&
-		    !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL))
+		    !guest_cpuid_has(vcpu, X86_FEATURE_SPEC_CTRL) &&
+		    !guest_cpuid_has(vcpu, X86_FEATURE_AMD_IBPB))
 			return 1;
 
 		if (data & ~PRED_CMD_IBPB)
 			return 1;
-		if (!boot_cpu_has(X86_FEATURE_SPEC_CTRL))
+		if (!boot_cpu_has(X86_FEATURE_IBPB))
 			return 1;
 		if (!data)
 			break;
-- 
2.26.2