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Date:   Thu, 14 Apr 2022 14:19:08 +0200
From:   Claudio Imbrenda <imbrenda@linux.ibm.com>
To:     Janosch Frank <frankja@linux.ibm.com>
Cc:     kvm@vger.kernel.org, borntraeger@de.ibm.com, thuth@redhat.com,
        pasic@linux.ibm.com, david@redhat.com, linux-s390@vger.kernel.org,
        linux-kernel@vger.kernel.org, scgl@linux.ibm.com,
        mimu@linux.ibm.com, nrb@linux.ibm.com
Subject: Re: [PATCH v10 01/19] KVM: s390: pv: leak the topmost page table
 when destroy fails
Message-ID: <20220414141908.1ba04a38@p-imbrenda>
In-Reply-To: <cc057c0a-58ee-1012-34e4-575b053230db@linux.ibm.com>
References: <20220414080311.1084834-1-imbrenda@linux.ibm.com>
        <20220414080311.1084834-2-imbrenda@linux.ibm.com>
        <cc057c0a-58ee-1012-34e4-575b053230db@linux.ibm.com>
Organization: IBM
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On Thu, 14 Apr 2022 13:30:33 +0200
Janosch Frank <frankja@linux.ibm.com> wrote:

> 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@linux.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?

will fix

> 
> > + * @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

will fix

> 
> > + * 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);  
>