Re: [PATCH 1/2] KVM: VMX: avoid running vmx_handle_exit_irqoff in case of emulation

[Sean Christopherson]

On Thu, Aug 26, 2021, Maxim Levitsky wrote:
> If we are emulating an invalid guest state, we don't have a correct
> exit reason, and thus we shouldn't do anything in this function.
> 
> Signed-off-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>

This should have Cc: stable.  I believe userspace could fairly easily trick KVM
into "handling" a spurious IRQ, e.g. trigger SIGALRM and stuff invalid state.
For all those evil folks running CPUs that are almost old enough to drive :-)

> ---
>  arch/x86/kvm/vmx/vmx.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index fada1055f325..0c2c0d5ae873 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6382,6 +6382,9 @@ static void vmx_handle_exit_irqoff(struct kvm_vcpu *vcpu)
>  {
>  	struct vcpu_vmx *vmx = to_vmx(vcpu);
>  
> +	if (vmx->emulation_required)
> +		return;

Rather than play whack-a-mole with flows consuming stale state, I'd much prefer
to synthesize a VM-Exit(INVALID_GUEST_STATE).  Alternatively, just skip ->run()
entirely by adding hooks in vcpu_enter_guest(), but that's a much larger change
and probably not worth the risk at this juncture.

---
 arch/x86/kvm/vmx/vmx.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 32e3a8b35b13..12fe63800889 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6618,10 +6618,21 @@ static fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu)
 		     vmx->loaded_vmcs->soft_vnmi_blocked))
 		vmx->loaded_vmcs->entry_time = ktime_get();
 
-	/* Don't enter VMX if guest state is invalid, let the exit handler
-	   start emulation until we arrive back to a valid state */
-	if (vmx->emulation_required)
+	/*
+	 * Don't enter VMX if guest state is invalid, let the exit handler
+	 * start emulation until we arrive back to a valid state.  Synthesize a
+	 * consistency check VM-Exit due to invalid guest state and bail.
+	 */
+	if (unlikely(vmx->emulation_required)) {
+		vmx->fail = 0;
+		vmx->exit_reason.full = EXIT_REASON_INVALID_STATE;
+		vmx->exit_reason.failed_vmentry = 1;
+		kvm_register_mark_available(vcpu, VCPU_EXREG_EXIT_INFO_1);
+		vmx->exit_qualification = ENTRY_FAIL_DEFAULT;
+		kvm_register_mark_available(vcpu, VCPU_EXREG_EXIT_INFO_2);
+		vmx->exit_intr_info = 0;
 		return EXIT_FASTPATH_NONE;
+	}
 
 	trace_kvm_entry(vcpu);
 
--

or the beginnings of an aggressive refactor...

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index cf8fb6eb676a..a4fe0f78898a 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9509,6 +9509,9 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
                goto cancel_injection;
        }

+       if (unlikely(static_call(kvm_x86_emulation_required)(vcpu)))
+               return static_call(kvm_x86_emulate_invalid_guest_state)(vcpu);
+
        preempt_disable();

        static_call(kvm_x86_prepare_guest_switch)(vcpu);

> +
>  	if (vmx->exit_reason.basic == EXIT_REASON_EXTERNAL_INTERRUPT)
>  		handle_external_interrupt_irqoff(vcpu);
>  	else if (vmx->exit_reason.basic == EXIT_REASON_EXCEPTION_NMI)
> -- 
> 2.26.3
>