Re: [PATCH] KVM: arm64: pkvm: Rollback refcount on hyp share/unshare error

[Quentin Perret <qperret@google.com>]

Hey Vincent,

On Tuesday 24 Mar 2026 at 17:27:57 (+0000), Vincent Donnefort wrote:
> If one of the HVC __pkvm_host_share_hyp or __pkvm_host_unshare_hyp fails,
> rollback the refcount to ensure the hyp_shared_pfns tracking reflects
> the actual sharing status.

If any of these hypercalls fail I think we're still in trouble as
kvm_{un}share_hyp() work on multi-page ranges and we could leak pages in
a borked state if we fail halfway through. And failing any of these
hypercalls is also sign of a bigger problem somewhere else so I wasn't
too worried.

But if we're going to fix this properly, I'd suggest also improving the
error handling in kvm_share_hyp(). 'Fixing' kvm_unshare_hyp() is a bit
harder because we must tell the caller to leak the data structure that
was shared I presume, so maybe we just keep the WARN and cross our
fingers :)

Cheers,
Quentin

> Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
> 
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 17d64a1e11e5..0fb41d2c8b44 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -493,11 +493,17 @@ static int share_pfn_hyp(u64 pfn)
>  		goto unlock;
>  	}
>  
> +	ret = kvm_call_hyp_nvhe(__pkvm_host_share_hyp, pfn);
> +	if (ret) {
> +		kfree(this);
> +		goto unlock;
> +	}
> +
>  	this->pfn = pfn;
>  	this->count = 1;
>  	rb_link_node(&this->node, parent, node);
>  	rb_insert_color(&this->node, &hyp_shared_pfns);
> -	ret = kvm_call_hyp_nvhe(__pkvm_host_share_hyp, pfn);
> +
>  unlock:
>  	mutex_unlock(&hyp_shared_pfns_lock);
>  
> @@ -521,9 +527,15 @@ static int unshare_pfn_hyp(u64 pfn)
>  	if (this->count)
>  		goto unlock;
>  
> +	ret = kvm_call_hyp_nvhe(__pkvm_host_unshare_hyp, pfn);
> +	if (ret) {
> +		this->count++;
> +		goto unlock;
> +	}
> +
>  	rb_erase(&this->node, &hyp_shared_pfns);
>  	kfree(this);
> -	ret = kvm_call_hyp_nvhe(__pkvm_host_unshare_hyp, pfn);
> +
>  unlock:
>  	mutex_unlock(&hyp_shared_pfns_lock);
>  
> 
> base-commit: c369299895a591d96745d6492d4888259b004a9e
> -- 
> 2.53.0.1018.g2bb0e51243-goog
>