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

[Baolu Lu <baolu.lu@linux.intel.com>]

On 5/24/23 3:44 PM, Tian, Kevin wrote:
>> 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.

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