linux-kernel - [PATCH 2/3] KVM: nVMX: restore L1's EFER prior to setting the nested state

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Message-Id: <20211110100018.367426-3-mlevitsk@redhat.com>
Date:   Wed, 10 Nov 2021 12:00:17 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     kvm@...r.kernel.org
Cc:     Wanpeng Li <wanpengli@...cent.com>, Borislav Petkov <bp@...en8.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        Sean Christopherson <seanjc@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        x86@...nel.org (maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)),
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Maxim Levitsky <mlevitsk@...hat.com>
Subject: [PATCH 2/3] KVM: nVMX: restore L1's EFER prior to setting the nested state

It is known that some kvm's users (e.g. qemu) load part of L2's register
state prior to setting the nested state after a migration.

If a 32 bit L2 guest is running in a 64 bit L1 guest, and nested migration
happens, Qemu will restore L2's EFER, and then the nested state load
function will use it as if it was L1's EFER.

Signed-off-by: Maxim Levitsky <mlevitsk@...hat.com>
---
 arch/x86/kvm/vmx/nested.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 49ae96c0cc4d1..28e270824e5b1 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -6404,6 +6404,17 @@ static int vmx_set_nested_state(struct kvm_vcpu *vcpu,
 			kvm_state->hdr.vmx.preemption_timer_deadline;
 	}
 
+	/*
+	 * The vcpu might currently contain L2's IA32_EFER, due to the way
+	 * some userspace kvm users (e.g qemu) restore nested state.
+	 *
+	 * To fix this, restore its IA32_EFER to the value it would have
+	 * after VM exit from the nested guest.
+	 *
+	 */
+
+	vcpu->arch.efer = nested_vmx_get_vmcs12_host_efer(vcpu, vmcs12);
+
 	if (nested_vmx_check_controls(vcpu, vmcs12) ||
 	    nested_vmx_check_host_state(vcpu, vmcs12) ||
 	    nested_vmx_check_guest_state(vcpu, vmcs12, &ignored))
-- 
2.26.3