[PATCH 5.3 164/166] KVM: hyperv: Fix Direct Synthetic timers assert an interrupt w/o lapic_in_kernel

Greg Kroah-Hartman
2019-10-06T17:55:14Z

From: Wanpeng Li <wanpengli@tencent.com>

commit a073d7e3ad687a7ef32b65affe80faa7ce89bf92 upstream.

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 don't advertise direct mode
synthetic timers and discarding the inject when lapic is not in kernel.

syzkaller source: https://syzkaller.appspot.com/x/repro.c?x=1752fe0a600000

Reported-by: syzbot+dff25ee91f0c7d5c1695@syzkaller.appspotmail.com
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
arch/x86/kvm/hyperv.c | 12 ++++++++++--
1 file changed, 10 insertions(+), 2 deletions(-)

--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -645,7 +645,9 @@ static int stimer_notify_direct(struct k
.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)
@@ -1852,7 +1854,13 @@ int kvm_vcpu_ioctl_get_hv_cpuid(struct k

ent->edx |= HV_FEATURE_FREQUENCY_MSRS_AVAILABLE;
ent->edx |= HV_FEATURE_GUEST_CRASH_MSR_AVAILABLE;
- ent->edx |= HV_STIMER_DIRECT_MODE_AVAILABLE;
+
+ /*
+ * Direct Synthetic timers only make sense with in-kernel
+ * LAPIC
+ */
+ if (lapic_in_kernel(vcpu))
+ ent->edx |= HV_STIMER_DIRECT_MODE_AVAILABLE;

break;