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Date:   Tue, 20 Sep 2022 00:55:56 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        mlevitsk@redhat.com, vkuznets@redhat.com
Subject: Re: [PATCH v2 9/9] KVM: x86: never write to memory from
 kvm_vcpu_check_block
Message-ID: <YykPnPrjqann+L2i@google.com>
References: <20220811210605.402337-1-pbonzini@redhat.com>
 <20220811210605.402337-10-pbonzini@redhat.com>
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On Thu, Aug 11, 2022, Paolo Bonzini wrote:
> kvm_vcpu_check_block() is called while not in TASK_RUNNING, and therefore
> it cannot sleep.  Writing to guest memory is therefore forbidden, but it
> can happen on AMD processors if kvm_check_nested_events() causes a vmexit.
> 
> Fortunately, all events that are caught by kvm_check_nested_events() are
> also recognized by kvm_vcpu_has_events() through vendor callbacks such as
> kvm_x86_interrupt_allowed() or kvm_x86_ops.nested_ops->has_events(), so
> remove the call and postpone the actual processing to vcpu_block().
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  arch/x86/kvm/x86.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 5e9358ea112b..9226fd536783 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -10639,6 +10639,17 @@ static inline int vcpu_block(struct kvm_vcpu *vcpu)
>  			return 1;
>  	}
>  
> +	if (is_guest_mode(vcpu)) {
> +		/*
> +		 * Evaluate nested events before exiting the halted state.
> +		 * This allows the halt state to be recorded properly in
> +		 * the VMCS12's activity state field (AMD does not have
> +		 * a similar field and a vmexit always causes a spurious
> +		 * wakeup from HLT).
> +		 */
> +		kvm_check_nested_events(vcpu);

Continuing the conversation with myself, this "needs" to check the return of
kvm_check_nested_events().  In quotes because KVM just signals "internal error"
if vmx_complete_nested_posted_interrupt() fails, i.e. the VM is likely dead anyways,
but again it's odd that the return of kvm_apic_accept_events() is checked but the
direct call to kvm_check_nested_events() is not.

> +	}
> +
>  	if (kvm_apic_accept_events(vcpu) < 0)
>  		return 0;
>  	switch(vcpu->arch.mp_state) {
> @@ -10662,9 +10673,6 @@ static inline int vcpu_block(struct kvm_vcpu *vcpu)
>  
>  static inline bool kvm_vcpu_running(struct kvm_vcpu *vcpu)
>  {
> -	if (is_guest_mode(vcpu))
> -		kvm_check_nested_events(vcpu);
> -
>  	return (vcpu->arch.mp_state == KVM_MP_STATE_RUNNABLE &&
>  		!vcpu->arch.apf.halted);
>  }
> -- 
> 2.31.1
>