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

Claudio Imbrenda
2021-10-12T08:33:55Z

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);
> >
>