Re: [for-4.15][PATCH v3 2/3] xen/x86: iommu: Ignore IOMMU mapping requests when a domain is dying

[Julien Grall <julien@xxxxxxx>]

Hi Jan,

On 17/02/2021 15:01, Jan Beulich wrote:
> On 17.02.2021 15:24, Julien Grall wrote:
> > From: Julien Grall <jgrall@xxxxxxxxxx>
> >
> > The new x86 IOMMU page-tables allocator will release the pages when
> > relinquishing the domain resources. However, this is not sufficient
> > when the domain is dying because nothing prevents page-table to be
> > allocated.
> >
> > Currently page-table allocations can only happen from iommu_map(). As
> > the domain is dying, there is no good reason to continue to modify the
> > IOMMU page-tables.
> While I agree this to be the case right now, I'm not sure it is a
> good idea to build on it (in that you leave the unmap paths
> untouched).
I don't build on that assumption. See next patch.

> Imo there's a fair chance this would be overlooked at
> the point super page mappings get introduced (which has been long
> overdue), and I thought prior discussion had lead to a possible
> approach without risking use-after-free due to squashed unmap
> requests.
I know you suggested to zap the root page-tables... However, I don't 
think this is 4.15 material and you agree with this (you were the one 
pointed out that out).
> > --- a/xen/drivers/passthrough/x86/iommu.c
> > +++ b/xen/drivers/passthrough/x86/iommu.c
> > @@ -273,6 +273,9 @@ int iommu_free_pgtables(struct domain *d)
> >       /*
> >        * 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.
> > +     *
> > +     * After this call, no more IOMMU mapping can happen.
> > +     *
> >        */
> >       hd->platform_ops->clear_root_pgtable(d);
> I.e. you utilize the call in place of spin_barrier(). Maybe worth
> saying in the comment?
Sure.

> Also, nit: Stray blank comment line.
Cheers,

--
Julien Grall