linux-kernel - [PATCH] KVM: x86: always allow writing '0' to MSR_KVM_ASYNC_PF

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Date:   Fri, 11 Sep 2020 11:31:47 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: [PATCH] KVM: x86: always allow writing '0' to MSR_KVM_ASYNC_PF_EN

Even without in-kernel LAPIC we should allow writing '0' to
MSR_KVM_ASYNC_PF_EN as we're not enabling the mechanism. In
particular, QEMU with 'kernel-irqchip=off' fails to start
a guest with

qemu-system-x86_64: error: failed to set MSR 0x4b564d02 to 0x0

Fixes: 9d3c447c72fb2 ("KVM: X86: Fix async pf caused null-ptr-deref")
Reported-by: Dr. David Alan Gilbert <dgilbert@...hat.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
---
 arch/x86/kvm/x86.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index d39d6cf1d473..44a86f7f2397 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2730,9 +2730,6 @@ static int kvm_pv_enable_async_pf(struct kvm_vcpu *vcpu, u64 data)
 	if (data & 0x30)
 		return 1;
 
-	if (!lapic_in_kernel(vcpu))
-		return 1;
-
 	vcpu->arch.apf.msr_en_val = data;
 
 	if (!kvm_pv_async_pf_enabled(vcpu)) {
@@ -2741,6 +2738,9 @@ static int kvm_pv_enable_async_pf(struct kvm_vcpu *vcpu, u64 data)
 		return 0;
 	}
 
+	if (!lapic_in_kernel(vcpu))
+		return 1;
+
 	if (kvm_gfn_to_hva_cache_init(vcpu->kvm, &vcpu->arch.apf.data, gpa,
 					sizeof(u64)))
 		return 1;
-- 
2.25.4