linux-kernel - Re: [PATCH v2 02/13] KVM: s390: pv: leak the ASCE page when destroy fails

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Message-ID: <20210729145414.62b568cf@p-imbrenda>
Date:   Thu, 29 Jul 2021 14:54:14 +0200
From:   Claudio Imbrenda <imbrenda@...ux.ibm.com>
To:     Janosch Frank <frankja@...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 v2 02/13] KVM: s390: pv: leak the ASCE page when destroy
 fails

On Thu, 29 Jul 2021 12:41:05 +0200
Janosch Frank <frankja@...ux.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@...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 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

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

> > +
> > +	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?

> > +
> >  /* 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;
> >  }
> >  
> >   
>