Re: [PATCH 3/5] KVM: nSVM: Don't forget about L1-injected events

[Maxim Levitsky]

On Thu, 2022-03-10 at 22:38 +0100, Maciej S. Szmigiero wrote:
> From: "Maciej S. Szmigiero" <maciej.szmigiero@xxxxxxxxxx>
> 
> In SVM synthetic software interrupts or INT3 or INTO exception that L1
> wants to inject into its L2 guest are forgotten if there is an intervening
> L0 VMEXIT during their delivery.
> 
> They are re-injected correctly with VMX, however.
> 
> This is because there is an assumption in SVM that such exceptions will be
> re-delivered by simply re-executing the current instruction.
> Which might not be true if this is a synthetic exception injected by L1,
> since in this case the re-executed instruction will be one already in L2,
> not the VMRUN instruction in L1 that attempted the injection.
> 
> Leave the pending L1 -> L2 event in svm->nested.ctl.event_inj{,err} until
> it is either re-injected successfully or returned to L1 upon a nested
> VMEXIT.
> Make sure to always re-queue such event if returned in EXITINTINFO.
> 
> The handling of L0 -> {L1, L2} event re-injection is left as-is to avoid
> unforeseen regressions.

Some time ago I noticed this too, but haven't dug into this too much.
I rememeber I even had some half-baked patch for this I never posted,
because I didn't think about the possibility of this syntetic injection. 

Just to be clear that I understand this correctly:

1. What is happening is that L1 is injecting INTn/INTO/INT3 but L2 code
   doesn't actualy contain an INTn/INTO/INT3 instruction.
   This is wierd but legal thing to do.
   Again, if L2 actually contained the instruction, it would have worked?


2. When actual INTn/INT0/INT3 are intercepted on SVM, then
   save.RIP points to address of the instruction, and control.next_rip
   points to address of next instruction after (as expected)

3. When EVENTINJ is used with '(TYPE = 3) with vectors 3 or 4'
   or 'TYPE=4', then next_rip is pushed on the stack, while save.RIP is
   pretty much ignored, and exectution jumps to the handler in the IDT.
   also at least for INT3/INTO, PRM states that IDT's DPL field is checked
   before dispatch, meaning that we can get legit #GP during delivery.
   (this looks like another legit reason to fix exception merging in KVM)


Best regards,
	Maxim Levitsky


> 
> Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@xxxxxxxxxx>
> ---
>  arch/x86/kvm/svm/nested.c | 65 +++++++++++++++++++++++++++++++++++++--
>  arch/x86/kvm/svm/svm.c    | 17 ++++++++--
>  arch/x86/kvm/svm/svm.h    | 47 ++++++++++++++++++++++++++++
>  3 files changed, 125 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index 9656f0d6815c..75017bf77955 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -420,8 +420,17 @@ void nested_copy_vmcb_save_to_cache(struct vcpu_svm *svm,
>  void nested_sync_control_from_vmcb02(struct vcpu_svm *svm)
>  {
>  	u32 mask;
> -	svm->nested.ctl.event_inj      = svm->vmcb->control.event_inj;
> -	svm->nested.ctl.event_inj_err  = svm->vmcb->control.event_inj_err;
> +
> +	/*
> +	 * Leave the pending L1 -> L2 event in svm->nested.ctl.event_inj{,err}
> +	 * if its re-injection is needed
> +	 */
> +	if (!exit_during_event_injection(svm, svm->nested.ctl.event_inj,
> +					 svm->nested.ctl.event_inj_err)) {
> +		WARN_ON_ONCE(svm->vmcb->control.event_inj & SVM_EVTINJ_VALID);
> +		svm->nested.ctl.event_inj      = svm->vmcb->control.event_inj;
> +		svm->nested.ctl.event_inj_err  = svm->vmcb->control.event_inj_err;
> +	}

Beware that this could backfire in regard to nested migration.

I once chased a nasty bug related to this.

The bug was:

- L1 does VMRUN with injection (EVENTINJ set)

- VMRUN exit handler is 'executed' by KVM,
  This copies EVENINJ fields from VMCB12 to VMCB02

- Once VMRUN exit handler is done executing, we exit to userspace to start migration
  (basically static_call(kvm_x86_handle_exit)(...) handles the SVM_EXIT_VMRUN,
   and that is all, vcpu_enter_guest isn't called again, so injection is not canceled)

- migration happens and it migrates the control area of vmcb02 with EVENTINJ fields set.

- on migration target, we inject another interrupt to the guest via EVENTINJ
  because svm_check_nested_events checks nested_run_pending to avoid doing this
  but nested_run_pending was not migrated correctly, 
  and overwrites the EVENTINJ - injection is lost.

Paolo back then proposed to me that instead of doing direct copy from VMCB12 to VMCB02
we should instead go through 'vcpu->arch.interrupt' and such.
I had a prototype of this but never gotten to clean it up to be accepted upstream,
knowing that current way also works.

  


>  
>  	/* Only a few fields of int_ctl are written by the processor.  */
>  	mask = V_IRQ_MASK | V_TPR_MASK;
> @@ -669,6 +678,54 @@ static void nested_svm_copy_common_state(struct vmcb *from_vmcb, struct vmcb *to
>  	to_vmcb->save.spec_ctrl = from_vmcb->save.spec_ctrl;
>  }
>  
> +void nested_svm_maybe_reinject(struct kvm_vcpu *vcpu)

A personal taste note: I don't like the 'maybe' for some reason.
But I won't fight over this.

> +{
> +	struct vcpu_svm *svm = to_svm(vcpu);
> +	unsigned int vector, type;
> +	u32 exitintinfo = svm->vmcb->control.exit_int_info;
> +
> +	if (WARN_ON_ONCE(!is_guest_mode(vcpu)))
> +		return;
> +
> +	/*
> +	 * No L1 -> L2 event to re-inject?
> +	 *
> +	 * In this case event_inj will be cleared by
> +	 * nested_sync_control_from_vmcb02().
> +	 */
> +	if (!(svm->nested.ctl.event_inj & SVM_EVTINJ_VALID))
> +		return;
> +
> +	/* If the last event injection was successful there shouldn't be any pending event */
> +	if (WARN_ON_ONCE(!(exitintinfo & SVM_EXITINTINFO_VALID)))
> +		return;
> +
> +	kvm_make_request(KVM_REQ_EVENT, vcpu);
> +
> +	vector = exitintinfo & SVM_EXITINTINFO_VEC_MASK;
> +	type = exitintinfo & SVM_EXITINTINFO_TYPE_MASK;
> +
> +	switch (type) {
> +	case SVM_EXITINTINFO_TYPE_NMI:
> +		vcpu->arch.nmi_injected = true;
> +		break;
> +	case SVM_EXITINTINFO_TYPE_EXEPT:
> +		if (exitintinfo & SVM_EXITINTINFO_VALID_ERR)
> +			kvm_requeue_exception_e(vcpu, vector,
> +						svm->vmcb->control.exit_int_info_err);
> +		else
> +			kvm_requeue_exception(vcpu, vector);
> +		break;
> +	case SVM_EXITINTINFO_TYPE_SOFT:

Note that AFAIK, SVM_EXITINTINFO_TYPE_SOFT is only for INTn instructions,
while INT3 and INTO are considered normal exceptions but EVENTINJ
handling has special case for them.


On VMX it is a bit cleaner:
It has:

3 - normal stock exception caused by CPU itself, like #PF and such
      
4 - INTn
      * does DPL check and uses VM_EXIT_INSTRUCTION_LEN
       
5 - ICEBP/INT1, which SVM doesnt support to re-inject
      * doesn't do DPL check, but uses VM_EXIT_INSTRUCTION_LEN I think

6 - software exception (INT3/INTO)
      * does DPL check and uses VM_EXIT_INSTRUCTION_LEN as well

I don't know if there is any difference between 4 and 6.




> +	case SVM_EXITINTINFO_TYPE_INTR:
> +		kvm_queue_interrupt(vcpu, vector, type == SVM_EXITINTINFO_TYPE_SOFT);
> +		break;
> +	default:
> +		vcpu_unimpl(vcpu, "unknown L1 -> L2 exitintinfo type 0x%x\n", type);
> +		break;
> +	}
> +}
> +
>  int enter_svm_guest_mode(struct kvm_vcpu *vcpu, u64 vmcb12_gpa,
>  			 struct vmcb *vmcb12, bool from_vmrun)
>  {
> @@ -898,6 +955,10 @@ int nested_svm_vmexit(struct vcpu_svm *svm)
>  	if (svm->nrips_enabled)
>  		vmcb12->control.next_rip  = vmcb->control.next_rip;
>  
> +	/* Forget about any pending L1 event injection since it's a L1 worry now */
> +	svm->nested.ctl.event_inj = 0;
> +	svm->nested.ctl.event_inj_err = 0;
> +
>  	vmcb12->control.int_ctl           = svm->nested.ctl.int_ctl;
>  	vmcb12->control.tlb_ctl           = svm->nested.ctl.tlb_ctl;
>  	vmcb12->control.event_inj         = svm->nested.ctl.event_inj;
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 1e5d904aeec3..5b128baa5e57 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -3322,13 +3322,18 @@ static void svm_inject_irq(struct kvm_vcpu *vcpu)
>  {
>  	struct vcpu_svm *svm = to_svm(vcpu);
>  
> -	WARN_ON(!gif_set(svm));
> +	WARN_ON(!(vcpu->arch.interrupt.soft || gif_set(svm)));
>  
>  	trace_kvm_inj_virq(vcpu->arch.interrupt.nr);
>  	++vcpu->stat.irq_injections;
>  
>  	svm->vmcb->control.event_inj = vcpu->arch.interrupt.nr |
> -		SVM_EVTINJ_VALID | SVM_EVTINJ_TYPE_INTR;
> +		SVM_EVTINJ_VALID;
> +	if (vcpu->arch.interrupt.soft) {
> +		svm->vmcb->control.event_inj |= SVM_EVTINJ_TYPE_SOFT;
> +	} else {
> +		svm->vmcb->control.event_inj |= SVM_EVTINJ_TYPE_INTR;
> +	}
>  }
>  
>  void svm_complete_interrupt_delivery(struct kvm_vcpu *vcpu, int delivery_mode,
> @@ -3627,6 +3632,14 @@ static void svm_complete_interrupts(struct kvm_vcpu *vcpu)
>  	if (!(exitintinfo & SVM_EXITINTINFO_VALID))
>  		return;
>  
> +	/* L1 -> L2 event re-injection needs a different handling */
> +	if (is_guest_mode(vcpu) &&
> +	    exit_during_event_injection(svm, svm->nested.ctl.event_inj,
> +					svm->nested.ctl.event_inj_err)) {
> +		nested_svm_maybe_reinject(vcpu);
> +		return;
> +	}
> +
>  	kvm_make_request(KVM_REQ_EVENT, vcpu);
>  
>  	vector = exitintinfo & SVM_EXITINTINFO_VEC_MASK;
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index f757400fc933..7cafc2e6c82a 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -488,6 +488,52 @@ static inline bool nested_npt_enabled(struct vcpu_svm *svm)
>  	return svm->nested.ctl.nested_ctl & SVM_NESTED_CTL_NP_ENABLE;
>  }
>  
> +static inline bool event_inj_same(u32 event_inj1, u32 event_inj_err1,
> +				  u32 event_inj2, u32 event_inj_err2)
> +{
> +	unsigned int vector_1, vector_2, type_1, type_2;
> +
> +	/* Either of them not valid? */
> +	if (!(event_inj1 & SVM_EVTINJ_VALID) ||
> +	    !(event_inj2 & SVM_EVTINJ_VALID))
> +		return false;
> +
> +	vector_1 = event_inj1 & SVM_EVTINJ_VEC_MASK;
> +	type_1 = event_inj1 & SVM_EVTINJ_TYPE_MASK;
> +	vector_2 = event_inj2 & SVM_EVTINJ_VEC_MASK;
> +	type_2 = event_inj2 & SVM_EVTINJ_TYPE_MASK;
> +
> +	/* Different vector or type? */
> +	if (vector_1 != vector_2 || type_1 != type_2)
> +		return false;
> +
> +	/* Different error code presence flag? */
> +	if ((event_inj1 & SVM_EVTINJ_VALID_ERR) !=
> +	    (event_inj2 & SVM_EVTINJ_VALID_ERR))
> +		return false;
> +
> +	/* No error code? */
> +	if (!(event_inj1 & SVM_EVTINJ_VALID_ERR))
> +		return true;
> +
> +	/* Same error code? */
> +	return event_inj_err1 == event_inj_err2;
> +}
> +
> +/* Did the last VMEXIT happen when attempting to inject that event? */
> +static inline bool exit_during_event_injection(struct vcpu_svm *svm,
> +					       u32 event_inj, u32 event_inj_err)
> +{
> +	BUILD_BUG_ON(SVM_EXITINTINFO_VEC_MASK != SVM_EVTINJ_VEC_MASK ||
> +		     SVM_EXITINTINFO_TYPE_MASK != SVM_EVTINJ_TYPE_MASK ||
> +		     SVM_EXITINTINFO_VALID != SVM_EVTINJ_VALID ||
> +		     SVM_EXITINTINFO_VALID_ERR != SVM_EVTINJ_VALID_ERR);
> +
> +	return event_inj_same(svm->vmcb->control.exit_int_info,
> +			      svm->vmcb->control.exit_int_info_err,
> +			      event_inj, event_inj_err);
> +}
> +
>  /* svm.c */
>  #define MSR_INVALID				0xffffffffU
>  
> @@ -540,6 +586,7 @@ static inline bool nested_exit_on_nmi(struct vcpu_svm *svm)
>  	return vmcb12_is_intercept(&svm->nested.ctl, INTERCEPT_NMI);
>  }
>  
> +void nested_svm_maybe_reinject(struct kvm_vcpu *vcpu);
>  int enter_svm_guest_mode(struct kvm_vcpu *vcpu,
>  			 u64 vmcb_gpa, struct vmcb *vmcb12, bool from_vmrun);
>  void svm_leave_nested(struct kvm_vcpu *vcpu);
> 


I will also review Sean's take on this, let see which one is simplier.


Best regards,
	Maxim Levitsky