linux-kernel - Re: [PATCH v1 01/11] KVM: s390: pv: leak the ASCE page when destroy fails

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Message-ID: <62e16545-d078-6d91-2370-e4e5677306c7@linux.ibm.com>
Date:   Tue, 18 May 2021 14:00:09 +0200
From:   Janosch Frank <frankja@...ux.ibm.com>
To:     Claudio Imbrenda <imbrenda@...ux.ibm.com>
Cc:     kvm@...r.kernel.org, cohuck@...hat.com, borntraeger@...ibm.com,
        thuth@...hat.com, pasic@...ux.ibm.com, david@...hat.com,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 01/11] KVM: s390: pv: leak the ASCE page when destroy
 fails

On 5/18/21 12:40 PM, Claudio Imbrenda wrote:
> On Tue, 18 May 2021 12:26:51 +0200
> Janosch Frank <frankja@...ux.ibm.com> wrote:
> 
>> On 5/17/21 10:07 PM, Claudio Imbrenda wrote:
>>> When the destroy configuration UVC fails, the page pointed to by the
>>> ASCE of the VM becomes poisoned, and, to avoid issues it must not be
>>> used again.
>>>
>>> Since the page becomes in practice unusable, we set it aside and
>>> leak it.  
>>
>> I think we need something a bit more specific.
>>
>> On creation of a protected guest the top most level of page tables are
>> marked by the Ultravisor and can only be used as top level page tables
>> for the protected guest that was created. If another protected guest
>> would re-use those pages for its top level page tables the UV would
>> throw errors.
>>
>> When a destroy fails the UV will not remove the markings so these
>> pages are basically unusable since we can't guarantee that they won't
>> be used for a guest ASCE in the future.
>>
>> Hence we choose to leak those pages in the very unlikely event that a
>> destroy fails.
> 
> it's more than that. the top level page, once marked, also cannot be
> used as backing for the virtual and real memory areas donated with the
> create secure configuration and create secure cpu UVCs.
> 
> and there might also other circumstances in which that page cannot be
> used that I am not aware of
> 

Even more reason to document it :)

>>
>> LGTM
>>
>>>
>>> Signed-off-by: Claudio Imbrenda <imbrenda@...ux.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 813b6e93dc83..e0532ab725bf 100644
>>> --- a/arch/s390/kvm/pv.c
>>> +++ b/arch/s390/kvm/pv.c
>>> @@ -150,6 +150,55 @@ 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)); +
>>> +	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);
>>> +
>>> +	return 0;
>>> +}
>>> +
>>>  /* 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) {
>>> @@ -164,9 +213,11 @@ 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;
>>>  }
>>>  
>>>   
>>
>