Re: [PATCH] KVM: nVMX: Fix IRQs inject to L2 which belong to L1 since race

Wanpeng Li
2014-07-04T06:17:01Z

On Thu, Jul 03, 2014 at 01:15:26AM -0400, Bandan Das wrote:
>Jan Kiszka <jan.kiszka@siemens.com> writes:
>
>> On 2014-07-02 08:54, Wanpeng Li wrote:
>>> This patch fix bug https://bugzilla.kernel.org/show_bug.cgi?id=72381
>>>
>>> If we didn't inject a still-pending event to L1 since nested_run_pending,
>>> KVM_REQ_EVENT should be requested after the vmexit in order to inject the
>>> event to L1. However, current log blindly request a KVM_REQ_EVENT even if
>>> there is no still-pending event to L1 which blocked by nested_run_pending.
>>> There is a race which lead to an interrupt will be injected to L2 which
>>> belong to L1 if L0 send an interrupt to L1 during this window.
>>>
>>> VCPU0 another thread
>>>
>>> L1 intr not blocked on L2 first entry
>>> vmx_vcpu_run req event
>>> kvm check request req event
>>> check_nested_events don't have any intr
>>> not nested exit
>>> intr occur (8254, lapic timer etc)
>>> inject_pending_event now have intr
>>> inject interrupt
>>>
>>> This patch fix this race by introduced a l1_events_blocked field in nested_vmx
>>> which indicates there is still-pending event which blocked by nested_run_pending,
>>> and smart request a KVM_REQ_EVENT if there is a still-pending event which blocked
>>> by nested_run_pending.
>>
>> There are more, unrelated reasons why KVM_REQ_EVENT could be set. Why
>> aren't those able to trigger this scenario?
>>
>> In any case, unconditionally setting KVM_REQ_EVENT seems strange and
>> should be changed.
>
>
>Ugh! I think I am hitting another one but this one's probably because
>we are not setting KVM_REQ_EVENT for something we should.
>
>Before this patch, I was able to hit this bug everytime with
>"modprobe kvm_intel ept=0 nested=1 enable_shadow_vmcs=0" and then booting
>L2. I can verify that I was indeed hitting the race in inject_pending_event.
>
>After this patch, I believe I am hitting another bug - this happens
>after I boot L2, as above, and then start a Linux kernel compilation
>and then wait and watch :) It's a pain to debug because this happens
>almost once in three times; it never happens if I run with ept=1, however,
>I think that's only because the test completes sooner. But I can confirm
>that I don't see it if I always set REQ_EVENT if nested_run_pending is set instead of
>the approach this patch takes.
>(Any debug hints help appreciated!)
>
>So, I am not sure if this is the right fix. Rather, I think the safer thing
>to do is to have the interrupt pending check for injection into L1 at
>the "same site" as the call to kvm_queue_interrupt() just like we had before
>commit b6b8a1451fc40412c57d1. Is there any advantage to having all the
>nested events checks together ?
>

How about revert commit b6b8a1451 and try if the bug which you mentioned
is still there?

Regards,
Wanpeng Li

>PS - Actually, a much easier fix (or rather hack) is to return 1 in
>vmx_interrupt_allowed() (as I mentioned elsewhere) only if
>!is_guest_mode(vcpu) That way, the pending interrupt interrupt
>can be taken care of correctly during the next vmexit.
>
>Bandan
>
>> Jan
>>
>>>
>>> Signed-off-by: Wanpeng Li <wanpeng.li@linux.intel.com>
>>> ---
>>> arch/x86/kvm/vmx.c | 20 +++++++++++++++-----
>>> 1 file changed, 15 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>>> index f4e5aed..fe69c49 100644
>>> --- a/arch/x86/kvm/vmx.c
>>> +++ b/arch/x86/kvm/vmx.c
>>> @@ -372,6 +372,7 @@ struct nested_vmx {
>>> u64 vmcs01_tsc_offset;
>>> /* L2 must run next, and mustn't decide to exit to L1. */
>>> bool nested_run_pending;
>>> + bool l1_events_blocked;
>>> /*
>>> * Guest pages referred to in vmcs02 with host-physical pointers, so
>>> * we must keep them pinned while L2 runs.
>>> @@ -7380,8 +7381,10 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
>>> * we did not inject a still-pending event to L1 now because of
>>> * nested_run_pending, we need to re-enable this bit.
>>> */
>>> - if (vmx->nested.nested_run_pending)
>>> + if (to_vmx(vcpu)->nested.l1_events_blocked) {
>>> + to_vmx(vcpu)->nested.l1_events_blocked = false;
>>> kvm_make_request(KVM_REQ_EVENT, vcpu);
>>> + }
>>>
>>> vmx->nested.nested_run_pending = 0;
>>>
>>> @@ -8197,15 +8200,20 @@ static int vmx_check_nested_events(struct kvm_vcpu *vcpu, bool external_intr)
>>>
>>> if (nested_cpu_has_preemption_timer(get_vmcs12(vcpu)) &&
>>> vmx->nested.preemption_timer_expired) {
>>> - if (vmx->nested.nested_run_pending)
>>> + if (vmx->nested.nested_run_pending) {
>>> + vmx->nested.l1_events_blocked = true;
>>> return -EBUSY;
>>> + }
>>> nested_vmx_vmexit(vcpu, EXIT_REASON_PREEMPTION_TIMER, 0, 0);
>>> return 0;
>>> }
>>>
>>> if (vcpu->arch.nmi_pending && nested_exit_on_nmi(vcpu)) {
>>> - if (vmx->nested.nested_run_pending ||
>>> - vcpu->arch.interrupt.pending)
>>> + if (vmx->nested.nested_run_pending) {
>>> + vmx->nested.l1_events_blocked = true;
>>> + return -EBUSY;
>>> + }
>>> + if (vcpu->arch.interrupt.pending)
>>> return -EBUSY;
>>> nested_vmx_vmexit(vcpu, EXIT_REASON_EXCEPTION_NMI,
>>> NMI_VECTOR | INTR_TYPE_NMI_INTR |
>>> @@ -8221,8 +8229,10 @@ static int vmx_check_nested_events(struct kvm_vcpu *vcpu, bool external_intr)
>>>
>>> if ((kvm_cpu_has_interrupt(vcpu) || external_intr) &&
>>> nested_exit_on_intr(vcpu)) {
>>> - if (vmx->nested.nested_run_pending)
>>> + if (vmx->nested.nested_run_pending) {
>>> + vmx->nested.l1_events_blocked = true;
>>> return -EBUSY;
>>> + }
>>> nested_vmx_vmexit(vcpu, EXIT_REASON_EXTERNAL_INTERRUPT, 0, 0);
>>> }
>>>
>>>