Re: [for-4.15][PATCH v3 1/3] xen/iommu: x86: Clear the root page-table before freeing the page-tables

[Julien Grall <julien@xxxxxxx>]

Hi Jan,

On 17/02/2021 14:54, Jan Beulich wrote:
> On 17.02.2021 15:24, Julien Grall wrote:
> > --- a/xen/drivers/passthrough/x86/iommu.c
> > +++ b/xen/drivers/passthrough/x86/iommu.c
> > @@ -267,6 +267,15 @@ int iommu_free_pgtables(struct domain *d)
> >       struct page_info *pg;
> >       unsigned int done = 0;
> >   
> > +    if ( !is_iommu_enabled(d) )
> > +        return 0;
> > +
> > +    /*
> > +     * Pages will be moved to the free list below. So we want to
> > +     * clear the root page-table to avoid any potential use after-free.
> > +     */
> > +    hd->platform_ops->clear_root_pgtable(d);
> Taking amd_iommu_alloc_root() as example, is this really correct
> prior to what is now patch 2? 
Yes, there are no more use-after-free...
        
> What guarantees a new root table
> won't get allocated subsequently?
It doesn't prevent root table allocation. I view the two as distincts 
issues, hence the two patches.
Cheers,

--
Julien Grall