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

Tian, Kevin
2023-05-24T07:45:12Z

> From: Liu, Yi L <yi.l.liu@intel.com>
> Sent: Thursday, May 11, 2023 10:51 PM
>
> From: Lu Baolu <baolu.lu@linux.intel.com>
>
> 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.
>
> + 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.