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

Janosch Frank
2021-05-18T12:00:18Z

On 5/18/21 12:40 PM, Claudio Imbrenda wrote:
> On Tue, 18 May 2021 12:26:51 +0200
> Janosch Frank <frankja@linux.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@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 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;
>>> }
>>>
>>>
>>
>