Re: [PATCH v5 05/14] KVM: s390: pv: leak the topmost page table when destroy fails

[Claudio Imbrenda <imbrenda@linux.ibm.com>]

On Tue, 12 Oct 2021 09:58:19 +0200
Janosch Frank <frankja@linux.ibm.com> wrote:

> On 9/20/21 15:24, Claudio Imbrenda wrote:
> > Each secure guest must have a unique address space control element and
> > we must avoid that new guests use the same ASCE, to avoid errors.
> > Since the ASCE mostly consists of the topmost page table address (and
> > flags), we must not return that memory to the pool unless the ASCE is
> > no longer in use.
> > 
> > Only a successful Destroy Secure Configuration UVC will make the ASCE
> > reusable again. If the Destroy Configuration UVC fails, the ASCE
> > cannot be reused for a secure guest (either for the ASCE or for other
> > memory areas). To avoid a collision, it must not be used again.
> > 
> > This is a permanent error and the page becomes in practice unusable, so
> > we set it aside and leak it. On failure we already leak other memory
> > that belongs to the ultravisor (i.e. the variable and base storage for
> > a guest) and not leaking the topmost page table was an oversight.
> > 
> > This error should not happen unless the hardware is broken or KVM has
> > some unknown serious bug.
> > 
> > Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>  
> 
> Fixes tag?

will add

> 
> > ---
> >   arch/s390/include/asm/gmap.h |  2 ++
> >   arch/s390/kvm/pv.c           |  4 ++-
> >   arch/s390/mm/gmap.c          | 55 ++++++++++++++++++++++++++++++++++++
> >   3 files changed, 60 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/s390/include/asm/gmap.h b/arch/s390/include/asm/gmap.h
> > index 40264f60b0da..746e18bf8984 100644
> > --- a/arch/s390/include/asm/gmap.h
> > +++ b/arch/s390/include/asm/gmap.h
> > @@ -148,4 +148,6 @@ void gmap_sync_dirty_log_pmd(struct gmap *gmap, unsigned long dirty_bitmap[4],
> >   			     unsigned long gaddr, unsigned long vmaddr);
> >   int gmap_mark_unmergeable(void);
> >   void s390_reset_acc(struct mm_struct *mm);
> > +void s390_remove_old_asce(struct gmap *gmap);
> > +int s390_replace_asce(struct gmap *gmap);
> >   #endif /* _ASM_S390_GMAP_H */
> > diff --git a/arch/s390/kvm/pv.c b/arch/s390/kvm/pv.c
> > index 00d272d134c2..76b0d64ce8fa 100644
> > --- a/arch/s390/kvm/pv.c
> > +++ b/arch/s390/kvm/pv.c
> > @@ -168,9 +168,11 @@ 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 */  
> 
> Rather unrelated

it's a typo, might as well fix it here, since I'm touching this function

> >   	if (!cc)
> >   		kvm_s390_pv_dealloc_vm(kvm);
> > +	else
> > +		s390_replace_asce(kvm->arch.gmap);
> >   	return cc ? -EIO : 0;  
> 
> Might make more sense now to do an early return so we don't have the 
> ternary if here.

will do

> >   }
> >   
> > diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c
> > index 9bb2c7512cd5..5a138f6220c4 100644
> > --- a/arch/s390/mm/gmap.c
> > +++ b/arch/s390/mm/gmap.c
> > @@ -2706,3 +2706,58 @@ void s390_reset_acc(struct mm_struct *mm)
> >   	mmput(mm);
> >   }
> >   EXPORT_SYMBOL_GPL(s390_reset_acc);
> > +
> > +/*
> > + * 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.
> > + */
> > +void s390_remove_old_asce(struct gmap *gmap)
> > +{
> > +	struct page *old;
> > +
> > +	old = virt_to_page(gmap->table);
> > +	spin_lock(&gmap->guest_table_lock);
> > +	list_del(&old->lru);
> > +	spin_unlock(&gmap->guest_table_lock);
> > +	/* in case the ASCE needs to be "removed" multiple times */
> > +	INIT_LIST_HEAD(&old->lru);
> > +}
> > +EXPORT_SYMBOL_GPL(s390_remove_old_asce);  
> 
> Is this used anywhere else than below?
> This can be static, no?

it's used in KVM in the subsequent patches of this series.

should I add the export when KVM needs the funcion, instead?

> > +
> > +/*
> > + * 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.
> > + */
> > +int s390_replace_asce(struct gmap *gmap)
> > +{
> > +	unsigned long asce;
> > +	struct page *page;
> > +	void *table;
> > +
> > +	s390_remove_old_asce(gmap);
> > +
> > +	page = alloc_pages(GFP_KERNEL_ACCOUNT, CRST_ALLOC_ORDER);
> > +	if (!page)
> > +		return -ENOMEM;
> > +	table = page_to_virt(page);
> > +	memcpy(table, gmap->table, 1UL << (CRST_ALLOC_ORDER + PAGE_SHIFT));
> > +
> > +	spin_lock(&gmap->guest_table_lock);
> > +	list_add(&page->lru, &gmap->crst_list);
> > +	spin_unlock(&gmap->guest_table_lock);
> > +
> > +	asce = (gmap->asce & ~PAGE_MASK) | __pa(table);
> > +	WRITE_ONCE(gmap->asce, asce);  
> 
> Are you sure we don't need the mm in write lock?
> 
> > +	WRITE_ONCE(gmap->mm->context.gmap_asce, asce);  
> 
> This is usually changed with the context lock held.

I had thought about it, and I realized that probably we would not need
it. The guest is not running at this point, and we are replacing an
ASCE with a different one pointing to the same page tables.

I can of course add a lock if you think it looks nicer, we are not in a
fast path after all

> > +	WRITE_ONCE(gmap->table, table);
> > +
> > +	return 0;
> > +}
> > +EXPORT_SYMBOL_GPL(s390_replace_asce);
> >   
>