Re: [PATCH v3] iommu: iommu_group_claim_dma_owner() must always assign a domain

[Eric Farman <farman@linux.ibm.com>]

On Mon, 2022-05-09 at 13:19 -0300, Jason Gunthorpe via iommu 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_set_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_set_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>
> Tested-by: Baolu Lu <baolu.lu@linux.intel.com>
> Tested-by: Nicolin Chen <nicolinc@nvidia.com>
> Co-developed-by: Robin Murphy <robin.murphy@arm.com>
> Signed-off-by: Robin Murphy <robin.murphy@arm.com>
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> --
> 
> Just minor polishing as discussed
> 
> v3:
>  - Change names to __iommu_group_set_domain() /
>    __iommu_group_set_core_domain()
>  - Clarify comments
>  - Call __iommu_group_set_domain() directly in
>    iommu_group_release_dma_owner() since we know it is always
> selecting
>    the default_domain
> v2: 
> https://lore.kernel.org/r/0-v2-f62259511ac0+6-iommu_dma_block_jgg@nvidia.com
>  - Remove redundant detach_dev ops check in __iommu_detach_device and
>    make the added WARN_ON fail instead
>  - Check for blocking_domain in __iommu_attach_group() so VFIO can
>    actually attach a new group
>  - Update comments and spelling
>  - Fix missed change to new_domain in iommu_group_do_detach_device()
> v1: 
> https://lore.kernel.org/r/0-v1-6e9d2d0a759d+11b-iommu_dma_block_jgg@nvidia.com
> 
> ---
>  drivers/iommu/iommu.c | 127 ++++++++++++++++++++++++++++++--------
> ----
>  1 file changed, 91 insertions(+), 36 deletions(-)
> 
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index 0c42ece2585406..0b22e51e90f416 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -44,6 +44,7 @@ struct iommu_group {
>  	char *name;
>  	int id;
>  	struct iommu_domain *default_domain;
> +	struct iommu_domain *blocking_domain;
>  	struct iommu_domain *domain;
>  	struct list_head entry;
>  	unsigned int owner_cnt;
> @@ -82,8 +83,8 @@ static int __iommu_attach_device(struct
> iommu_domain *domain,
>  				 struct device *dev);
>  static int __iommu_attach_group(struct iommu_domain *domain,
>  				struct iommu_group *group);
> -static void __iommu_detach_group(struct iommu_domain *domain,
> -				 struct iommu_group *group);
> +static int __iommu_group_set_domain(struct iommu_group *group,
> +				    struct iommu_domain *new_domain);
>  static int iommu_create_device_direct_mappings(struct iommu_group
> *group,
>  					       struct device *dev);
>  static struct iommu_group *iommu_group_get_for_dev(struct device
> *dev);
> @@ -596,6 +597,8 @@ static void iommu_group_release(struct kobject
> *kobj)
>  
>  	if (group->default_domain)
>  		iommu_domain_free(group->default_domain);
> +	if (group->blocking_domain)
> +		iommu_domain_free(group->blocking_domain);
>  
>  	kfree(group->name);
>  	kfree(group);
> @@ -1907,6 +1910,24 @@ void iommu_domain_free(struct iommu_domain
> *domain)
>  }
>  EXPORT_SYMBOL_GPL(iommu_domain_free);
>  
> +/*
> + * Put the group's domain back to the appropriate core-owned domain
> - either the
> + * standard kernel-mode DMA configuration or an all-DMA-blocked
> domain.
> + */
> +static void __iommu_group_set_core_domain(struct iommu_group *group)
> +{
> +	struct iommu_domain *new_domain;
> +	int ret;
> +
> +	if (group->owner)
> +		new_domain = group->blocking_domain;
> +	else
> +		new_domain = group->default_domain;
> +
> +	ret = __iommu_group_set_domain(group, new_domain);
> +	WARN(ret, "iommu driver failed to attach the default/blocking
> domain");
> +}
> +
>  static int __iommu_attach_device(struct iommu_domain *domain,
>  				 struct device *dev)
>  {
> @@ -1963,9 +1984,6 @@ static void __iommu_detach_device(struct
> iommu_domain *domain,
>  	if (iommu_is_attach_deferred(dev))
>  		return;
>  
> -	if (unlikely(domain->ops->detach_dev == NULL))
> -		return;
> -
>  	domain->ops->detach_dev(domain, dev);
>  	trace_detach_device_from_domain(dev);
>  }
> @@ -1979,12 +1997,10 @@ void iommu_detach_device(struct iommu_domain
> *domain, struct device *dev)
>  		return;
>  
>  	mutex_lock(&group->mutex);
> -	if (iommu_group_device_count(group) != 1) {
> -		WARN_ON(1);
> +	if (WARN_ON(domain != group->domain) ||
> +	    WARN_ON(iommu_group_device_count(group) != 1))
>  		goto out_unlock;
> -	}
> -
> -	__iommu_detach_group(domain, group);
> +	__iommu_group_set_core_domain(group);
>  
>  out_unlock:
>  	mutex_unlock(&group->mutex);
> @@ -2040,7 +2056,8 @@ 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,
> @@ -2072,38 +2089,49 @@ static int
> iommu_group_do_detach_device(struct device *dev, void *data)
>  	return 0;
>  }
>  
> -static void __iommu_detach_group(struct iommu_domain *domain,
> -				 struct iommu_group *group)
> +static int __iommu_group_set_domain(struct iommu_group *group,
> +				    struct iommu_domain *new_domain)
>  {
>  	int ret;
>  
> +	if (group->domain == new_domain)
> +		return 0;
> +
>  	/*
> -	 * If the group has been claimed already, do not re-attach the
> default
> -	 * domain.
> +	 * New drivers should support default domains and so the
> detach_dev() op
> +	 * will never be called. Otherwise the NULL domain represents
> some
> +	 * platform specific behavior.
>  	 */
> -	if (!group->default_domain || group->owner) {
> -		__iommu_group_for_each_dev(group, domain,
> +	if (!new_domain) {
> +		if (WARN_ON(!group->domain->ops->detach_dev))
> +			return -EINVAL;
> +		__iommu_group_for_each_dev(group, group->domain,
>  					   iommu_group_do_detach_device
> );
>  		group->domain = NULL;
> -		return;
> +		return 0;
>  	}
>  
> -	if (group->domain == group->default_domain)
> -		return;
> -
> -	/* Detach by re-attaching to the default domain */
> -	ret = __iommu_group_for_each_dev(group, group->default_domain,
> +	/*
> +	 * Changing the domain is done by calling attach_dev() on the
> new
> +	 * domain. This switch does not have to be atomic and DMA can
> be
> +	 * discarded during the transition. DMA must only be able to
> access
> +	 * either new_domain or group->domain, never something else.
> +	 *
> +	 * 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, new_domain,
>  					 iommu_group_do_attach_device);
> -	if (ret != 0)
> -		WARN_ON(1);
> -	else
> -		group->domain = group->default_domain;
> +	if (ret)
> +		return ret;
> +	group->domain = new_domain;
> +	return 0;
>  }
>  
>  void iommu_detach_group(struct iommu_domain *domain, struct
> iommu_group *group)
>  {
>  	mutex_lock(&group->mutex);
> -	__iommu_detach_group(domain, group);
> +	__iommu_group_set_core_domain(group);
>  	mutex_unlock(&group->mutex);
>  }
>  EXPORT_SYMBOL_GPL(iommu_detach_group);
> @@ -3088,6 +3116,29 @@ void iommu_device_unuse_default_domain(struct
> device *dev)
>  	iommu_group_put(group);
>  }
>  
> +static int __iommu_group_alloc_blocking_domain(struct iommu_group
> *group)
> +{
> +	struct group_device *dev =
> +		list_first_entry(&group->devices, struct group_device,
> list);
> +
> +	if (group->blocking_domain)
> +		return 0;
> +
> +	group->blocking_domain =
> +		__iommu_domain_alloc(dev->dev->bus,
> IOMMU_DOMAIN_BLOCKED);
> +	if (!group->blocking_domain) {
> +		/*
> +		 * For drivers that do not yet understand
> IOMMU_DOMAIN_BLOCKED
> +		 * create an empty domain instead.
> +		 */
> +		group->blocking_domain = __iommu_domain_alloc(
> +			dev->dev->bus, IOMMU_DOMAIN_UNMANAGED);
> +		if (!group->blocking_domain)
> +			return -EINVAL;

Hi Jason,

I got a heads up from Matt about the s390 KVM vfio- variants failing on
linux-next.

For vfio-ap and vfio-ccw, they fail on the above error. Both calls to
__iommu_domain_alloc fail because while dev->dev->bus is non-NULL (it
points to the mdev bus_type registered in mdev_init()), the bus-
>iommu_ops pointer is NULL. Which makes sense; the iommu_group is vfio-
noiommu, via vfio_register_emulated_iommu_dev(), and mdev didn't
establish an iommu_ops for its bus.

The caller of this, iommu_group_claim_dma_owner(), was added to
vfio_group_set_container() by commit 70693f470848 ("vfio: Set DMA
ownership for VFIO devices") [1] ... But that's as far as I got without
making some probably incorrect decisions. Do you have any thoughts
here?

Thanks,
Eric

[1] 
https://lore.kernel.org/r/20220418005000.897664-8-baolu.lu@linux.intel.com

> +	}
> +	return 0;
> +}
> +
>  /**
>   * iommu_group_claim_dma_owner() - Set DMA ownership of a group
>   * @group: The group.
> @@ -3111,9 +3162,14 @@ int iommu_group_claim_dma_owner(struct
> iommu_group *group, void *owner)
>  			goto unlock_out;
>  		}
>  
> +		ret = __iommu_group_alloc_blocking_domain(group);
> +		if (ret)
> +			goto unlock_out;
> +
> +		ret = __iommu_group_set_domain(group, group-
> >blocking_domain);
> +		if (ret)
> +			goto unlock_out;
>  		group->owner = owner;
> -		if (group->domain)
> -			__iommu_detach_group(group->domain, group);
>  	}
>  
>  	group->owner_cnt++;
> @@ -3132,18 +3188,17 @@
> EXPORT_SYMBOL_GPL(iommu_group_claim_dma_owner);
>   */
>  void iommu_group_release_dma_owner(struct iommu_group *group)
>  {
> +	int ret;
> +
>  	mutex_lock(&group->mutex);
>  	if (WARN_ON(!group->owner_cnt || !group->owner))
>  		goto unlock_out;
>  
>  	group->owner_cnt = 0;
> -	/*
> -	 * The UNMANAGED domain should be detached before all USER
> -	 * owners have been released.
> -	 */
> -	if (!WARN_ON(group->domain) && group->default_domain)
> -		__iommu_attach_group(group->default_domain, group);
>  	group->owner = NULL;
> +	ret = __iommu_group_set_domain(group, group->default_domain);
> +	WARN(ret, "iommu driver failed to attach the default domain");
> +
>  unlock_out:
>  	mutex_unlock(&group->mutex);
>  }
> 
> base-commit: da844db4722bdd333142b40f0e414e2aedc2a4c0

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