[PATCH 5.8 060/177] KVM: x86: always allow writing 0 to MSR_KVM_ASYNC_PF_EN

[Greg Kroah-Hartman]

From: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>

[ Upstream commit d831de177217cd494bfb99f2c849a0d40c2a7890 ]

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@xxxxxxxxxx>
Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
Message-Id: <20200911093147.484565-1-vkuznets@xxxxxxxxxx>
[Actually commit the version proposed by Sean Christopherson. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 arch/x86/kvm/x86.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index f7304132d5907..f5481ae588aff 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2696,7 +2696,7 @@ static int kvm_pv_enable_async_pf(struct kvm_vcpu *vcpu, u64 data)
 		return 1;
 
 	if (!lapic_in_kernel(vcpu))
-		return 1;
+		return data ? 1 : 0;
 
 	vcpu->arch.apf.msr_en_val = data;
 
-- 
2.25.1