Re: [PATCH v8 01/17] KVM: s390: pv: leak the topmost page table when destroy fails

Janis Schoetterl-Glausch
2022-03-03T14:40:53Z

On 3/2/22 19:11, Claudio Imbrenda wrote:
> Each secure guest must have a unique ASCE (address space control
> element); we must avoid that new guests use the same page for their
> ASCE, to avoid errors.
>
> Since the ASCE mostly consists of the address of the topmost page table
> (plus some 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 (and thus the leakage) 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: 29b40f105ec8d55 ("KVM: s390: protvirt: Add initial vm and cpu lifecycle handling")
> ---
> arch/s390/include/asm/gmap.h | 2 +
> arch/s390/kvm/pv.c | 9 +++--
> arch/s390/mm/gmap.c | 71 ++++++++++++++++++++++++++++++++++++
> 3 files changed, 79 insertions(+), 3 deletions(-)
>
[...]

> +/**
> + * s390_replace_asce - Try to replace the current ASCE of a gmap with
> + * another equivalent one.
> + * @gmap the gmap
> + *
> + * If the allocation of the new top level page table fails, the ASCE is not
> + * replaced.
> + * In any case, the old ASCE is always 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));

Is concurrent modification of *gmap->table possible during the copy?

> +
> + /*
> + * The caller has to deal with the old ASCE, but here we make sure
> + * the new one is properly added to the list of page tables, so that
> + * it will be freed when the VM is torn down.
> + */
> + spin_lock(&gmap->guest_table_lock);
> + list_add(&page->lru, &gmap->crst_list);
> + spin_unlock(&gmap->guest_table_lock);
> +
> + /* Set new table origin while preserving existing ASCE control bits */
> + asce = (gmap->asce & _ASCE_ORIGIN) | __pa(table);
> + WRITE_ONCE(gmap->asce, asce);
> + WRITE_ONCE(gmap->mm->context.gmap_asce, asce);
> + WRITE_ONCE(gmap->table, table);
> +
> + return 0;
> +}
> +EXPORT_SYMBOL_GPL(s390_replace_asce);