KVM: VMX: Intercept FS/GS_BASE MSR accesses for 32-bit KVM

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@google.com>
Message-Id: <20210422023831.3473491-1-seanjc@google.com>
[*NOT* for stable kernels. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 8b11168..0f8c118 100644 (file)
--- 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,
                                             MSR_FS_BASE, MSR_TYPE_RW);
 
@@ -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,
                                             MSR_KERNEL_GS_BASE, MSR_TYPE_RW);
+#endif
 
        /*
         * 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 026fe77..edc23c7 100644 (file)
--- 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] = {
        MSR_IA32_SPEC_CTRL,
        MSR_IA32_PRED_CMD,
        MSR_IA32_TSC,
+#ifdef CONFIG_X86_64
        MSR_FS_BASE,
        MSR_GS_BASE,
        MSR_KERNEL_GS_BASE,
+#endif
        MSR_IA32_SYSENTER_CS,
        MSR_IA32_SYSENTER_ESP,
        MSR_IA32_SYSENTER_EIP,
@@ -6956,9 +6958,11 @@ 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);
+#endif
        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);