[PATCH 4.19 09/29] KVM: SVM: relax conditions for allowing MSR_IA32_SPEC_CTRL accesses

Greg Kroah-Hartman
2021-01-05T09:30:36Z

From: Paolo Bonzini <pbonzini@redhat.com>

[ Upstream commit df7e8818926eb4712b67421442acf7d568fe2645 ]

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@redhat.com>
Analyzed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Analyzed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
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 a0c3d1b4b295b..f513110983d4c 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -4209,6 +4209,8 @@ 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;
@@ -4312,6 +4314,8 @@ 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;
--
2.27.0