Re: [PATCH] iommu: iommu_group_claim_dma_owner() must always assign a domain

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

Hi Jason,

On 2022/5/4 08:11, Jason Gunthorpe wrote:
> Once the group enters 'owned' mode it can never be assigned back to the
> default_domain or to a NULL domain. It must always be actively assigned to
> a current domain. If the caller hasn't provided a domain then the core
> must provide an explicit DMA blocking domain that has no DMA map.
> 
> Lazily create a group-global blocking DMA domain when
> iommu_group_claim_dma_owner is first called and immediately assign the
> group to it. This ensures that DMA is immediately fully isolated on all
> IOMMU drivers.
> 
> If the user attaches/detaches while owned then detach will set the group
> back to the blocking domain.
> 
> Slightly reorganize the call chains so that
> __iommu_group_attach_core_domain() is the function that removes any caller
> configured domain and sets the domains back a core owned domain with an
> appropriate lifetime.
> 
> __iommu_group_attach_domain() is the worker function that can change the
> domain assigned to a group to any target domain, including NULL.
> 
> Add comments clarifying how the NULL vs detach_dev vs default_domain works
> based on Robin's remarks.
> 
> This fixes an oops with VFIO and SMMUv3 because VFIO will call
> iommu_detach_group() and then immediately iommu_domain_free(), but
> SMMUv3 has no way to know that the domain it is holding a pointer to
> has been freed. Now the iommu_detach_group() will assign the blocking
> domain and SMMUv3 will no longer hold a stale domain reference.
> 
> Fixes: 1ea2a07a532b ("iommu: Add DMA ownership management interfaces")
> Reported-by: Qian Cai <quic_qiancai@quicinc.com>
> Signed-off-by: Robin Murphy <robin.murphy@arm.com>
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> ---
>   drivers/iommu/iommu.c | 112 +++++++++++++++++++++++++++++++-----------
>   1 file changed, 82 insertions(+), 30 deletions(-)
> 
> This is based on Robins draft here:
> 
> https://lore.kernel.org/linux-iommu/18831161-473f-e04f-4a81-1c7062ad192d@arm.com/
> 
> With some rework. I re-organized the call chains instead of introducing
> iommu_group_user_attached(), fixed a recursive locking for
> iommu_group_get_purgatory(), and made a proper commit message.
> 
> Still only compile tested, so RFCish.
> 
> Nicolin/Lu? What do you think, can you check it?

Thank you for the patch.

With below additional changes, this patch works on my Intel test
machine.

diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
index 513da82f2ed1..7c415e9b6906 100644
--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c
@@ -2063,7 +2063,8 @@ static int __iommu_attach_group(struct 
iommu_domain *domain,
  {
         int ret;

-       if (group->domain && group->domain != group->default_domain)
+       if (group->domain && group->domain != group->default_domain &&
+           group->domain != group->blocking_domain)
                 return -EBUSY;

         ret = __iommu_group_for_each_dev(group, domain,
@@ -2125,7 +2126,7 @@ static int __iommu_group_attach_domain(struct 
iommu_group *group,
          * Note that this is called in error unwind paths, attaching to a
          * domain that has already been attached cannot fail.
          */
-       ret = __iommu_group_for_each_dev(group, group->default_domain,
+       ret = __iommu_group_for_each_dev(group, new_domain,
                                          iommu_group_do_attach_device);
         if (ret)
                 return ret;
@@ -3180,7 +3181,9 @@ int iommu_group_claim_dma_owner(struct iommu_group 
*group, void *owner)
                 ret = -EPERM;
                 goto unlock_out;
         } else {
-               if (group->domain && group->domain != 
group->default_domain) {
+               if (group->domain &&
+                   group->domain != group->default_domain &&
+                   group->domain != group->blocking_domain) {
                         ret = -EBUSY;
                         goto unlock_out;

Best regards,
baolu
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