[PATCH 5.10 502/530] KVM: VMX: Intercept FS/GS_BASE MSR accesses for 32-bit KVM

[Greg Kroah-Hartman]

From: Sean Christopherson <seanjc@xxxxxxxxxx>

[ Upstream commit dbdd096a5a74b94f6b786a47baef2085859b0dce ]

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>
Message-Id: <20210422023831.3473491-1-seanjc@xxxxxxxxxx>
[*NOT* for stable kernels. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 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 44ab9df0b21b..e8882715735a 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -618,6 +618,7 @@ 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);
 
@@ -626,6 +627,7 @@ 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 8635413cc649..fca4f452827b 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -156,9 +156,11 @@ 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,
@@ -6904,9 +6906,11 @@ 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);
-- 
2.30.2