linux-kernel - Re: [PATCH v10 01/19] KVM: s390: pv: leak the topmost page table when destroy fails

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Message-ID: <cc057c0a-58ee-1012-34e4-575b053230db@linux.ibm.com>
Date:   Thu, 14 Apr 2022 13:30:33 +0200
From:   Janosch Frank <frankja@...ux.ibm.com>
To:     Claudio Imbrenda <imbrenda@...ux.ibm.com>, kvm@...r.kernel.org
Cc:     borntraeger@...ibm.com, thuth@...hat.com, pasic@...ux.ibm.com,
        david@...hat.com, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, scgl@...ux.ibm.com,
        mimu@...ux.ibm.com, nrb@...ux.ibm.com
Subject: Re: [PATCH v10 01/19] KVM: s390: pv: leak the topmost page table when
 destroy fails

On 4/14/22 10:02, 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.
> 

Reviewed-by: Janosch Frank <frankja@...ux.ibm.com>

> +	/*
> +	 * In case the ASCE needs to be "removed" multiple times, for example
> +	 * if the VM is rebooted into secure mode several times
> +	 * concurrently, or if s390_replace_asce fails after calling
> +	 * s390_remove_old_asce and is attempted again later. In that case
> +	 * the old asce has been removed from the list, and therefore it
> +	 * will not be freed when the VM terminates, but the ASCE is still
> +	 * in use and still pointed to.
> +	 * A subsequent call to replace_asce will follow the pointer and try
> +	 * to remove the same page from the list again.
> +	 * Therefore it's necessary that the page of the ASCE has valid
> +	 * pointers, so list_del can work (and do nothing) without
> +	 * dereferencing stale or invalid pointers.
> +	 */
> +	INIT_LIST_HEAD(&old->lru);
> +	spin_unlock(&gmap->guest_table_lock);
> +}
> +EXPORT_SYMBOL_GPL(s390_remove_old_asce);
> +
> +/**
> + * s390_replace_asce - Try to replace the current ASCE of a gmap with
> + * another equivalent one.

with a copy?

> + * @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

removed from the gmap crst list

> + * 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));
> +
> +	/*
> +	 * 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);