[PATCH] KVM: SVM: don't apply host security mitigations to the guest

[Fares Mehanna]

Support of virtual SPEC_CTRL caused a new behavior in KVM which made host
security mitigations applying by default to the guests.

We noticed a regression after applying the patch, primarily because of the
enablement of SSBD on AMD Milan.

This patch keeps the new behavior of applying host security mitigations to
the guests, but adds an option to disable it so the guests are free to pick
their own security mitigations.

Fixes: d00b99c514b3 ("KVM: SVM: Add support for Virtual SPEC_CTRL")
Signed-off-by: Fares Mehanna <faresx@xxxxxxxxx>
Reviewed-by: Benjamin Serebrin <serebrin@xxxxxxxxxx>
Reviewed-by: Filippo Sironi <sironi@xxxxxxxxx>
---
 arch/x86/kvm/svm/svm.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index f3813dbacb9f..c6ea24685301 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -225,6 +225,10 @@ module_param(avic, bool, 0444);
 bool __read_mostly dump_invalid_vmcb;
 module_param(dump_invalid_vmcb, bool, 0644);
 
+/* enable/disable applying host security mitigations on the guest */
+static bool host_mitigations_on_guest = true;
+module_param(host_mitigations_on_guest, bool, 0444);
+
 
 bool intercept_smi = true;
 module_param(intercept_smi, bool, 0444);
@@ -4000,6 +4004,12 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu)
 	 */
 	if (!static_cpu_has(X86_FEATURE_V_SPEC_CTRL))
 		x86_spec_ctrl_set_guest(svm->spec_ctrl, svm->virt_spec_ctrl);
+	else if (!host_mitigations_on_guest)
+		/*
+		 * Clear the host MSR before vm-enter to avoid applying host
+		 * security mitigations to the guest.
+		 */
+		x86_spec_ctrl_set_guest(0, 0);
 
 	svm_vcpu_enter_exit(vcpu);
 
@@ -4025,7 +4035,7 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu)
 	if (!sev_es_guest(vcpu->kvm))
 		reload_tss(vcpu);
 
-	if (!static_cpu_has(X86_FEATURE_V_SPEC_CTRL))
+	if (!static_cpu_has(X86_FEATURE_V_SPEC_CTRL) || !host_mitigations_on_guest)
 		x86_spec_ctrl_restore_host(svm->spec_ctrl, svm->virt_spec_ctrl);
 
 	if (!sev_es_guest(vcpu->kvm)) {
-- 
2.37.1




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