linux-kernel - Re: [PATCH 3/3] KVM: x86: never write to memory from kvm_vcpu_check

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Date:   Wed, 27 Apr 2022 23:42:51 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     seanjc@...gle.com, stable@...r.kernel.org
Subject: Re: [PATCH 3/3] KVM: x86: never write to memory from
 kvm_vcpu_check_block

On Wed, 2022-04-27 at 13:37 -0400, Paolo Bonzini wrote:
> kvm_vcpu_check_block is called while not in TASK_RUNNING, and therefore
> cannot sleep.  Writing to guest memory is therefore forbidden, but it
> can happen if kvm_check_nested_events causes a vmexit.
> 
> Fortunately, all events that are caught by kvm_check_nested_events are
> also handled 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.
> 
> Cc: stable@...r.kernel.org
> Reported-by: Maxim Levitsky <mlevitsk@...hat.com>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/x86/kvm/x86.c | 3 ---
>  1 file changed, 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index d563812ca229..90b4f50b9a84 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -10341,9 +10341,6 @@ 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);
>  }

Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>

I tested this on AMD, and it seems to work fine, and my nested AVIC test
works as good as was before.

Note that I forgot to mention, that I had to apply most of the patches
manually, they don't apply to kvm/queue.

Best regards,
	Maxim Levitsky