Re: [PATCH] KVM: VMX: Intercept FS/GS_BASE MSR accesses for 32-bit KVM

From: Paolo Bonzini
Date: Thu Apr 22 2021 - 02:47:52 EST

On 22/04/21 04:38, Sean Christopherson wrote:
Disable pass-through of the FS and GS base MSRs for 32-bit KVM. Intel's
SDM unequivocally states that the MSRs exist if and only if the CPU
supports x86-64. FS_BASE and GS_BASE are mostly a non-issue; a clever
guest could opportunistically use the MSRs without issue. KERNEL_GS_BASE
is a bigger problem, as a clever guest would subtly be broken if it were
migrated, as KVM disallows software access to the MSRs, and unlike the
direct variants, KERNEL_GS_BASE needs to be explicitly migrated as it's
not captured in the VMCS.

Fixes: 25c5f225beda ("KVM: VMX: Enable MSR Bitmap feature")
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>

I added an explicit note that this is not for stable kernels. The clever guest breaking after migration is the clever guest's problem.


Note, this breaks kvm-unit-tests on 32-bit KVM VMX due to the boot code
using WRMSR(MSR_GS_BASE). But, the tests are already broken on SVM, and
have always been broken on SVM, which is honestly the main reason I
didn't just turn a blind eye. :-) I post the fix shortly.

Fair enough. Queued, thanks.

arch/x86/kvm/vmx/nested.c | 2 ++
arch/x86/kvm/vmx/vmx.c | 4 ++++
2 files changed, 6 insertions(+)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 8b111682fe5c..0f8c118ebc35 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -614,6 +614,7 @@ static inline bool nested_vmx_prepare_msr_bitmap(struct kvm_vcpu *vcpu,
/* KVM unconditionally exposes the FS/GS base MSRs to L1. */
+#ifdef CONFIG_X86_64
nested_vmx_disable_intercept_for_msr(msr_bitmap_l1, msr_bitmap_l0,
@@ -622,6 +623,7 @@ static inline bool nested_vmx_prepare_msr_bitmap(struct kvm_vcpu *vcpu,
nested_vmx_disable_intercept_for_msr(msr_bitmap_l1, msr_bitmap_l0,
* Checking the L0->L1 bitmap is trying to verify two things:
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 6501d66167b8..b58dc2d454f1 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -157,9 +157,11 @@ static u32 vmx_possible_passthrough_msrs[MAX_POSSIBLE_PASSTHROUGH_MSRS] = {
+#ifdef CONFIG_X86_64
@@ -6969,9 +6971,11 @@ static int vmx_create_vcpu(struct kvm_vcpu *vcpu)
bitmap_fill(vmx->shadow_msr_intercept.write, MAX_POSSIBLE_PASSTHROUGH_MSRS);
vmx_disable_intercept_for_msr(vcpu, MSR_IA32_TSC, MSR_TYPE_R);
+#ifdef CONFIG_X86_64
vmx_disable_intercept_for_msr(vcpu, MSR_FS_BASE, MSR_TYPE_RW);
vmx_disable_intercept_for_msr(vcpu, MSR_GS_BASE, MSR_TYPE_RW);
vmx_disable_intercept_for_msr(vcpu, MSR_KERNEL_GS_BASE, MSR_TYPE_RW);
vmx_disable_intercept_for_msr(vcpu, MSR_IA32_SYSENTER_CS, MSR_TYPE_RW);
vmx_disable_intercept_for_msr(vcpu, MSR_IA32_SYSENTER_ESP, MSR_TYPE_RW);
vmx_disable_intercept_for_msr(vcpu, MSR_IA32_SYSENTER_EIP, MSR_TYPE_RW);