linux-kernel - Re: [PATCH 4/4] KVM: Nullify async #PF worker's "apf" pointer as soon as it might be freed

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Date: Fri, 26 Jan 2024 17:30:24 +0100
From: Vitaly Kuznetsov <vkuznets@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini
 <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, David Matlack
 <dmatlack@...gle.com>, Xu Yilun <yilun.xu@...ux.intel.com>, Sean
 Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH 4/4] KVM: Nullify async #PF worker's "apf" pointer as
 soon as it might be freed

Sean Christopherson <seanjc@...gle.com> writes:

> Nullify the async #PF worker's local "apf" pointer immediately after the
> point where the structure can be freed by the vCPU.  The existing comment
> is helpful, but easy to overlook as there is no associated code.
>
> Update the comment to clarify that it can be freed by as soon as the lock
> is dropped, as "after this point" isn't strictly accurate, nor does it
> help understand what prevents the structure from being freed earlier.
>

"No functional change intended." must be made a requirement, especially
for those who made it their trademark)

> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  virt/kvm/async_pf.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/virt/kvm/async_pf.c b/virt/kvm/async_pf.c
> index c3f4f351a2ae..1088c6628de9 100644
> --- a/virt/kvm/async_pf.c
> +++ b/virt/kvm/async_pf.c
> @@ -83,13 +83,14 @@ static void async_pf_execute(struct work_struct *work)
>  	apf->vcpu = NULL;
>  	spin_unlock(&vcpu->async_pf.lock);
>  
> -	if (!IS_ENABLED(CONFIG_KVM_ASYNC_PF_SYNC) && first)
> -		kvm_arch_async_page_present_queued(vcpu);
> -
>  	/*
> -	 * apf may be freed by kvm_check_async_pf_completion() after
> -	 * this point
> +	 * The apf struct may freed by kvm_check_async_pf_completion() as soon

Nit: "may be freed"/"may get freed" maybe?

> +	 * as the lock is dropped.  Nullify it to prevent improper usage.
>  	 */
> +	apf = NULL;
> +
> +	if (!IS_ENABLED(CONFIG_KVM_ASYNC_PF_SYNC) && first)
> +		kvm_arch_async_page_present_queued(vcpu);
>  
>  	trace_kvm_async_pf_completed(addr, cr2_or_gpa);

Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

-- 
Vitaly