Re: [PATCH v3 10/10] iommu/vt-d: Disallow nesting on domains with read-only mappings

[Baolu Lu]

On 5/24/23 3:44 PM, Tian, Kevin wrote:
> > From: Liu, Yi L <yi.l.liu@xxxxxxxxx>
> > Sent: Thursday, May 11, 2023 10:51 PM
> >
> > From: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> >
> > When remapping hardware is configured by system software in scalable
> > mode
> > as Nested (PGTT=011b) and with PWSNP field Set in the PASID-table-entry,
> > it may Set Accessed bit and Dirty bit (and Extended Access bit if enabled)
> > in first-stage page-table entries even when second-stage mappings indicate
> > that corresponding first-stage page-table is Read-Only.
> >
> > As the result, contents of pages designated by VMM as Read-Only can be
> > modified by IOMMU via PML5E (PML4E for 4-level tables) access as part of
> > address translation process due to DMAs issued by Guest.
> >
> > Disallow the nested translation when there are read-only pages in the
> > corresponding second-stage mappings. And, no read-only pages are allowed
> > to be configured in the second-stage table of a nested translation.
> > For the latter, an alternative is to disallow read-only mappings in
> > any stage-2 domain as long as it's ever been used as a parent. In this
> > way, we can simply replace the user counter with a flag.
> >
> > In concept if the user understands this errata and does expect to
> > enable nested translation it should never install any RO mapping
> > in stage-2 in the entire VM life cycle."
>
> IMHO the alternative is reasonable and simpler. If the user decides to
> enable nesting it should keep the nesting-friendly configuration static
> since whether nesting is enabled on a device is according to viommu
> configuration (i.e. whether the guest attaches the device to identity
> domain or non-identity domain) and it's not good to break the nesting
> setup just because the host inadvertently adds a RO mapping to s2 in
> the middle between guest is detached/put back to identity domain
> and then re-attach to an unmanaged domain.

Fair enough.

> > +	if (!(prot & DMA_PTE_WRITE) && !domain->read_only_mapped) {
> > +		spin_lock_irqsave(&domain->lock, flags);
> > +		if (domain->nested_users > 0) {
> > +			spin_unlock_irqrestore(&domain->lock, flags);
> > +			return -EINVAL;
> > +		}
> > +
>
> this is worth a one-off warning. Same in the other path.

Sure.

Best regards,
baolu