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Subject[PATCH] KVM: LAPIC: Fix SynIC Timers inject timer interrupt w/o LAPIC present
From: Wanpeng Li <>

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.

Cc: Paolo Bonzini <>
Cc: Radim Krčmář <>
Cc: Vitaly Kuznetsov <>
Signed-off-by: Wanpeng Li <>
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)
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