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To:     Claudio Imbrenda <imbrenda@linux.ibm.com>
Cc:     kvm@vger.kernel.org, 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>
 <21eb4861-f118-cf3b-409e-ea31694582a5@linux.ibm.com>
 <20210729145414.62b568cf@p-imbrenda>
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: <9bc312f4-6105-8661-bea4-9538b4a05010@linux.ibm.com>
Date:   Thu, 29 Jul 2021 15:45:45 +0200
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On 7/29/21 2:54 PM, Claudio Imbrenda wrote:
> On Thu, 29 Jul 2021 12:41:05 +0200
> Janosch Frank <frankja@linux.ibm.com> wrote:
> 
>> 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.
> 
> doesn't really make a difference, it is not actually used until a few
> lines later

Still when you touch the patch, then just move that around please :)

> 
> also, the list is only ever touched here, during guest creation and
> destruction; IIRC in all those cases we hold kvm->lock

The crst_list is also used in gmap_alloc_table() coming from
__gmap_link() when touching the top level entries. To be fair there
shouldn't be asynchronous things that trigger a fault/link but we have
valid references to the gmap so at the very least I want a comment why
this is totally fine like we have for gmap_free().

Hrm, I'll need to brood over this a bit more, it has been too long.

> 
>>> +
>>> +	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.
> 
> it should not need locks, the VM is in use, so it can't disappear under
> our feet.
> 
>>> +
>>> +	return 0;
>>> +}  
>>
>> That should both be in gmap.c
> 
> why?

Because we do gmap stuff and I don't want to pollute pv.c with memory
things if we don't have to.

Btw. s390_reset_acc() is also 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;
>>>  }
>>>  
>>>   
>>
>