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To:     Claudio Imbrenda <imbrenda@linux.ibm.com>, kvm@vger.kernel.org
Cc:     cohuck@redhat.com, borntraeger@de.ibm.com, thuth@redhat.com,
        pasic@linux.ibm.com, david@redhat.com, linux-s390@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20210728142631.41860-1-imbrenda@linux.ibm.com>
 <20210728142631.41860-3-imbrenda@linux.ibm.com>
From:   Janosch Frank <frankja@linux.ibm.com>
Subject: Re: [PATCH v2 02/13] KVM: s390: pv: leak the ASCE page when destroy
 fails
Message-ID: <21eb4861-f118-cf3b-409e-ea31694582a5@linux.ibm.com>
Date:   Thu, 29 Jul 2021 12:41:05 +0200
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On 7/28/21 4:26 PM, Claudio Imbrenda wrote:
> When a protected VM is created, the topmost level of page tables of its
> ASCE is marked by the Ultravisor; any attempt to use that memory for
> protected virtualization will result in failure.
> 
> Only a successful Destroy Configuration UVC will remove the marking.
> 
> When the Destroy Configuration UVC fails, the topmost level of page
> tables of the VM does not get its marking cleared; to avoid issues it
> must not be used again.
> 
> Since the page becomes in practice unusable, we set it aside and leak it.
> 
> Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
> ---
>  arch/s390/kvm/pv.c | 53 +++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 52 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/s390/kvm/pv.c b/arch/s390/kvm/pv.c
> index e007df11a2fe..1ecdc1769ed9 100644
> --- a/arch/s390/kvm/pv.c
> +++ b/arch/s390/kvm/pv.c
> @@ -155,6 +155,55 @@ static int kvm_s390_pv_alloc_vm(struct kvm *kvm)
>  	return -ENOMEM;
>  }
>  
> +/*
> + * Remove the topmost level of page tables from the list of page tables of
> + * the gmap.
> + * This means that it will not be freed when the VM is torn down, and needs
> + * to be handled separately by the caller, unless an intentional leak is
> + * intended.
> + */
> +static void kvm_s390_pv_remove_old_asce(struct kvm *kvm)
> +{
> +	struct page *old;
> +
> +	old = virt_to_page(kvm->arch.gmap->table);
> +	list_del(&old->lru);
> +	/* in case the ASCE needs to be "removed" multiple times */
> +	INIT_LIST_HEAD(&old->lru);
> +}
> +
> +/*
> + * Try to replace the current ASCE with another equivalent one.
> + * If the allocation of the new top level page table fails, the ASCE is not
> + * replaced.
> + * In any case, the old ASCE is removed from the list, therefore the caller
> + * has to make sure to save a pointer to it beforehands, unless an
> + * intentional leak is intended.
> + */
> +static int kvm_s390_pv_replace_asce(struct kvm *kvm)
> +{
> +	unsigned long asce;
> +	struct page *page;
> +	void *table;
> +
> +	kvm_s390_pv_remove_old_asce(kvm);
> +
> +	page = alloc_pages(GFP_KERNEL_ACCOUNT, CRST_ALLOC_ORDER);
> +	if (!page)
> +		return -ENOMEM;
> +	list_add(&page->lru, &kvm->arch.gmap->crst_list);
> +
> +	table = page_to_virt(page);
> +	memcpy(table, kvm->arch.gmap->table, 1UL << (CRST_ALLOC_ORDER + PAGE_SHIFT));

Don't we want to memcpy first and then add it to the list?
The gmap is still active per-se so I think we want to take the
guest_table_lock for the list_add here.

> +
> +	asce = (kvm->arch.gmap->asce & ~PAGE_MASK) | __pa(table);
> +	WRITE_ONCE(kvm->arch.gmap->asce, asce);
> +	WRITE_ONCE(kvm->mm->context.gmap_asce, asce);
> +	WRITE_ONCE(kvm->arch.gmap->table, table);

If I remember correctly those won't need locks but I'm not 100% sure so
please have a look at that.

> +
> +	return 0;
> +}

That should both be in gmap.c

> +
>  /* this should not fail, but if it does, we must not free the donated memory */
>  int kvm_s390_pv_deinit_vm(struct kvm *kvm, u16 *rc, u16 *rrc)
>  {
> @@ -169,9 +218,11 @@ int kvm_s390_pv_deinit_vm(struct kvm *kvm, u16 *rc, u16 *rrc)
>  	atomic_set(&kvm->mm->context.is_protected, 0);
>  	KVM_UV_EVENT(kvm, 3, "PROTVIRT DESTROY VM: rc %x rrc %x", *rc, *rrc);
>  	WARN_ONCE(cc, "protvirt destroy vm failed rc %x rrc %x", *rc, *rrc);
> -	/* Inteded memory leak on "impossible" error */
> +	/* Intended memory leak on "impossible" error */
>  	if (!cc)
>  		kvm_s390_pv_dealloc_vm(kvm);
> +	else
> +		kvm_s390_pv_replace_asce(kvm);
>  	return cc ? -EIO : 0;
>  }
>  
>