Re: [PATCH v6 01/10] iommu: Introduce domain attachment handle

Baolu Lu
2024-06-13T03:07:24Z

On 6/12/24 9:10 PM, Jason Gunthorpe wrote:
> On Thu, Jun 06, 2024 at 01:33:29PM +0800, Baolu Lu wrote:
>
>>> But if certain path (other than iopf) in the iommu core needs to know
>>> the exact domain pointer then this change breaks it.
>>
>> The iommu core should not fetch the domain pointer in paths other than
>> attach/detach/replace. There is currently no reference counter for an
>> iommu domain, hence fetching the domain for other purposes will
>> potentially lead to a use-after-free issue.
>
> If you are doing this then we should get rid of
> iommu_get_domain_for_dev_pasid, and always return the handle
> there. Making it clear that all those flows require handles to work.

I removed iommu_get_domain_for_dev_pasid() in this series. The iopf
handling is the only path that requires fetching domain, where we
requires attach handle to work.

>
> But is this really OK? What happened to the patch fixing the error
> unwind in attach_pasid? Doesn't it always need the domain to undo a
> failed attach?

attach_pasid switches from being unassigned to being assigned with a
real domain, so the unwind operation simply involves calling
iommu_ops->remove_dev_pasid.

In the future, probably we will introduce a replace interface for pasid.
In that scenario, the caller should explicitly pass both the old and new
domains. If the replace operation fails, the interface will revert to
the old domain.

In my mind, the iommu core should only manage the default domain for
kernel DMA. All domains that are allocated by domain_alloc interfaces
should be managed by the callers and passed through the attach/detach
/replace interfaces.

>
> Jason
>

Best regards,
baolu