linux-kernel - [PATCH 1/2] KVM: nVMX: Truncate writes to vmcs.SYSENTER

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Date:   Tue, 28 Apr 2020 16:10:24 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH 1/2] KVM: nVMX: Truncate writes to vmcs.SYSENTER_EIP/ESP for 32-bit vCPU

Explicitly truncate the data written to vmcs.SYSENTER_EIP/ESP on WRMSR
if the virtual CPU doesn't support 64-bit mode.  The SYSENTER address
fields in the VMCS are natural width, i.e. bits 63:32 are dropped if the
CPU doesn't support Intel 64 architectures.  This behavior is visible to
the guest after a VM-Exit/VM-Exit roundtrip, e.g. if the guest sets bits
63:32 in the actual MSR.

Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
---
 arch/x86/kvm/vmx/vmx.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 3ab6ca6062ce..bc91ce499a7a 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -1936,6 +1936,16 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 	return 0;
 }
 
+static u64 nested_vmx_truncate_sysenter_addr(struct kvm_vcpu *vcpu,
+						    u64 data)
+{
+#ifdef CONFIG_X86_64
+	if (!guest_cpuid_has(vcpu, X86_FEATURE_LM))
+		return (u32)data;
+#endif
+	return (unsigned long)data;
+}
+
 /*
  * Writes msr value into the appropriate "register".
  * Returns 0 on success, non-0 otherwise.
@@ -1973,13 +1983,17 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		vmcs_write32(GUEST_SYSENTER_CS, data);
 		break;
 	case MSR_IA32_SYSENTER_EIP:
-		if (is_guest_mode(vcpu))
+		if (is_guest_mode(vcpu)) {
+			data = nested_vmx_truncate_sysenter_addr(vcpu, data);
 			get_vmcs12(vcpu)->guest_sysenter_eip = data;
+		}
 		vmcs_writel(GUEST_SYSENTER_EIP, data);
 		break;
 	case MSR_IA32_SYSENTER_ESP:
-		if (is_guest_mode(vcpu))
+		if (is_guest_mode(vcpu)) {
+			data = nested_vmx_truncate_sysenter_addr(vcpu, data);
 			get_vmcs12(vcpu)->guest_sysenter_esp = data;
+		}
 		vmcs_writel(GUEST_SYSENTER_ESP, data);
 		break;
 	case MSR_IA32_DEBUGCTLMSR:
-- 
2.26.0