[PATCH] KVM: LAPIC: Fix SynIC Timers inject timer interrupt w/o LAPIC present

[Wanpeng Li <kernellwp@gmail.com>]

From: Wanpeng Li <wanpengli@tencent.com>

Reported by syzkaller:

	kasan: GPF could be caused by NULL-ptr deref or user memory access
	general protection fault: 0000 [#1] PREEMPT SMP KASAN
	RIP: 0010:__apic_accept_irq+0x46/0x740 arch/x86/kvm/lapic.c:1029
	Call Trace:
	kvm_apic_set_irq+0xb4/0x140 arch/x86/kvm/lapic.c:558
	stimer_notify_direct arch/x86/kvm/hyperv.c:648 [inline]
	stimer_expiration arch/x86/kvm/hyperv.c:659 [inline]
	kvm_hv_process_stimers+0x594/0x1650 arch/x86/kvm/hyperv.c:686
	vcpu_enter_guest+0x2b2a/0x54b0 arch/x86/kvm/x86.c:7896
	vcpu_run+0x393/0xd40 arch/x86/kvm/x86.c:8152
	kvm_arch_vcpu_ioctl_run+0x636/0x900 arch/x86/kvm/x86.c:8360
	kvm_vcpu_ioctl+0x6cf/0xaf0 arch/x86/kvm/../../../virt/kvm/kvm_main.c:2765

The testcase programs HV_X64_MSR_STIMERn_CONFIG/HV_X64_MSR_STIMERn_COUNT,
in addition, there is no lapic in the kernel, the counters value are small 
enough in order that kvm_hv_process_stimers() inject this already-expired 
timer interrupt into the guest through lapic in the kernel which triggers 
the NULL deferencing. This patch fixes it by checking lapic_in_kernel, 
discarding the inject if it is 0.

Reported-by: syzbot+dff25ee91f0c7d5c1695@syzkaller.appspotmail.com
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
---
 arch/x86/kvm/hyperv.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index c10a8b1..461fcc5 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -645,7 +645,9 @@ static int stimer_notify_direct(struct kvm_vcpu_hv_stimer *stimer)
 		.vector = stimer->config.apic_vector
 	};
 
-	return !kvm_apic_set_irq(vcpu, &irq, NULL);
+	if (lapic_in_kernel(vcpu))
+		return !kvm_apic_set_irq(vcpu, &irq, NULL);
+	return 0;
 }
 
 static void stimer_expiration(struct kvm_vcpu_hv_stimer *stimer)
-- 
2.7.4