KVM: x86: Bug the VM if an accelerated x2APIC trap occurs on a "bad" reg

Bug the VM if retrieving the x2APIC MSR/register while processing an
accelerated vAPIC trap VM-Exit fails.  In theory it's impossible for the
lookup to fail as hardware has already validated the register, but bugs
happen, and not checking the result of kvm_lapic_msr_read() would result
in consuming the uninitialized "val" if a KVM or hardware bug occurs.

Fixes: 1bd9dfec ("KVM: x86: Do not block APIC write for non ICR registers")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220804235028.1766253-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/lapic.c

@@ -2284,10 +2284,12 @@ void kvm_apic_write_nodecode(struct kvm_vcpu *vcpu, u32 offset)
 	struct kvm_lapic *apic = vcpu->arch.apic;
 	u64 val;
 
-	if (apic_x2apic_mode(apic))
-		kvm_lapic_msr_read(apic, offset, &val);
-	else
+	if (apic_x2apic_mode(apic)) {
+		if (KVM_BUG_ON(kvm_lapic_msr_read(apic, offset, &val), vcpu->kvm))
+			return;
+	} else {
 		val = kvm_lapic_get_reg(apic, offset);
+	}
 
 	/*
 	 * ICR is a single 64-bit register when x2APIC is enabled.  For legacy