linux-kernel - [PATCH] KVM: SVM: relax conditions for allowing MSR_IA32_SPEC

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Message-Id: <1580915628-42930-1-git-send-email-pbonzini@redhat.com>
Date:   Wed,  5 Feb 2020 16:13:48 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     dgilbert@...hat.com, jmattson@...gle.com
Subject: [PATCH] KVM: SVM: relax conditions for allowing MSR_IA32_SPEC_CTRL accesses

Userspace that does not know about the AMD_IBRS bit might still
allow the guest to protect itself with MSR_IA32_SPEC_CTRL using
the Intel SPEC_CTRL bit.  However, svm.c disallows this and will
cause a #GP in the guest when writing to the MSR.  Fix this by
loosening the test and allowing the Intel CPUID bit, and in fact
allow the AMD_STIBP bit as well since it allows writing to
MSR_IA32_SPEC_CTRL too.

Reported-by: Zhiyi Guo <zhguo@...hat.com>
Analyzed-by: Dr. David Alan Gilbert <dgilbert@...hat.com>
Analyzed-by: Laszlo Ersek <lersek@...hat.com>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 arch/x86/kvm/svm.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index bf0556588ad0..a3e32d61d60c 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -4225,6 +4225,8 @@ static int svm_get_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_AMD_STIBP) &&
 		    !guest_cpuid_has(vcpu, X86_FEATURE_AMD_IBRS) &&
 		    !guest_cpuid_has(vcpu, X86_FEATURE_AMD_SSBD))
 			return 1;
@@ -4310,6 +4312,8 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
 		break;
 	case MSR_IA32_SPEC_CTRL:
 		if (!msr->host_initiated &&
+		    !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;
-- 
1.8.3.1