Re: [PATCH v2 4/8] iommu: Factor out some helpers

[Jason Gunthorpe]

On Thu, Jan 26, 2023 at 06:26:19PM +0000, Robin Murphy wrote:
> The pattern for picking the first device out of the group list is
> repeated a few times now, so it's clearly worth factoring out to hide
> the group_device detail from places that don't need to know. Similarly,
> the safety check for dev_iommu_ops() at public interfaces starts looking
> a bit repetitive, and might not be completely obvious at first glance,
> so let's factor that out for clarity as well, in preparation for more
> uses of both.
> 
> Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx>
> ---
> 
> v2: - Add dev_iommu_ops_valid() helper
>     - Add lockdep assertion [Jason]
> 
>  drivers/iommu/iommu.c | 39 ++++++++++++++++++++++-----------------
>  1 file changed, 22 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index 77f076030995..440bb3b7bded 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -284,6 +284,12 @@ static void dev_iommu_free(struct device *dev)
>  	kfree(param);
>  }
>  
> +/* Only needed in public APIs which allow unchecked devices for convenience */
> +static bool dev_iommu_ops_valid(struct device *dev)
> +{
> +	return dev->iommu && dev->iommu->iommu_dev;
> +}

I did an audit and more than half the callers need this test, and a
few places are missing it already.

We've kind of made a systematic error that assumes being in a group is
sufficient to prove there are non-NULL ops.

So I suggest that we make dev_iommu_ops() return NULL in all cases
where there is no driver and have a new API to get the ops in cases
where the call chain knows that it is safe, and there are only 5 of
those cases.

diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
index de91dd88705bd3..4b04ad50de8d6b 100644
--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c
@@ -98,7 +98,8 @@ 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);
+static struct iommu_group *iommu_group_get_for_dev(struct device *dev,
+						   const struct iommu_ops *ops);
 static ssize_t iommu_group_store_type(struct iommu_group *group,
 				      const char *buf, size_t count);
 
@@ -130,6 +131,19 @@ static struct bus_type * const iommu_buses[] = {
 #endif
 };
 
+/*
+ * This may only be called if it is already clear from the calling context that
+ * the device has an ops. Seeing the device is part of an iommu_group is not
+ * enough as VFIO and POWER put devices in iommu_groups and do not attach
+ * drivers. Thus the only places that are safe have either already called
+ * dev_iommu_ops() on their call chain, or were responsible for assigning ops in
+ * the first place.
+ */
+static inline const struct iommu_ops *dev_iommu_ops_safe(struct device *dev)
+{
+	return dev->iommu->iommu_dev->ops;
+}
+
 /*
  * Use a function instead of an array here because the domain-type is a
  * bit-field, so an array would waste memory.
@@ -338,7 +352,7 @@ static int __iommu_probe_device(struct device *dev, struct list_head *group_list
 	dev->iommu->iommu_dev = iommu_dev;
 	dev->iommu->max_pasids = dev_iommu_get_max_pasids(dev);
 
-	group = iommu_group_get_for_dev(dev);
+	group = iommu_group_get_for_dev(dev, ops);
 	if (IS_ERR(group)) {
 		ret = PTR_ERR(group);
 		goto out_release;
@@ -414,7 +428,8 @@ int iommu_probe_device(struct device *dev)
 	mutex_unlock(&group->mutex);
 	iommu_group_put(group);
 
-	ops = dev_iommu_ops(dev);
+	/* __iommu_probe_device() set the ops */
+	ops = dev_iommu_ops_safe(dev);
 	if (ops->probe_finalize)
 		ops->probe_finalize(dev);
 
@@ -430,14 +445,13 @@ int iommu_probe_device(struct device *dev)
 
 void iommu_release_device(struct device *dev)
 {
-	const struct iommu_ops *ops;
+	const struct iommu_ops *ops = dev_iommu_ops(dev);
 
-	if (!dev->iommu)
+	if (!ops)
 		return;
 
 	iommu_device_unlink(dev->iommu->iommu_dev, dev);
 
-	ops = dev_iommu_ops(dev);
 	if (ops->release_device)
 		ops->release_device(dev);
 
@@ -614,13 +628,6 @@ int iommu_get_group_resv_regions(struct iommu_group *group,
 	list_for_each_entry(device, &group->devices, list) {
 		struct list_head dev_resv_regions;
 
-		/*
-		 * Non-API groups still expose reserved_regions in sysfs,
-		 * so filter out calls that get here that way.
-		 */
-		if (!device->dev->iommu)
-			break;
-
 		INIT_LIST_HEAD(&dev_resv_regions);
 		iommu_get_resv_regions(device->dev, &dev_resv_regions);
 		ret = iommu_insert_device_resv_regions(&dev_resv_regions, head);
@@ -1332,7 +1339,8 @@ int iommu_page_response(struct device *dev,
 	struct iommu_fault_event *evt;
 	struct iommu_fault_page_request *prm;
 	struct dev_iommu *param = dev->iommu;
-	const struct iommu_ops *ops = dev_iommu_ops(dev);
+	/* This API should only be called from an IOMMU driver */
+	const struct iommu_ops *ops = dev_iommu_ops_safe(dev);
 	bool has_pasid = msg->flags & IOMMU_PAGE_RESP_PASID_VALID;
 
 	if (!ops->page_response)
@@ -1601,7 +1609,8 @@ EXPORT_SYMBOL_GPL(fsl_mc_device_group);
 
 static int iommu_get_def_domain_type(struct device *dev)
 {
-	const struct iommu_ops *ops = dev_iommu_ops(dev);
+	/* It is not locked but all callers know there is an ops */
+	const struct iommu_ops *ops = dev_iommu_ops_safe(dev);
 
 	if (dev_is_pci(dev) && to_pci_dev(dev)->untrusted)
 		return IOMMU_DOMAIN_DMA;
@@ -1658,9 +1667,9 @@ static int iommu_alloc_default_domain(struct iommu_group *group,
  * to the returned IOMMU group, which will already include the provided
  * device.  The reference should be released with iommu_group_put().
  */
-static struct iommu_group *iommu_group_get_for_dev(struct device *dev)
+static struct iommu_group *iommu_group_get_for_dev(struct device *dev,
+						   const struct iommu_ops *ops)
 {
-	const struct iommu_ops *ops = dev_iommu_ops(dev);
 	struct iommu_group *group;
 	int ret;
 
@@ -1795,7 +1804,8 @@ static int __iommu_group_dma_attach(struct iommu_group *group)
 
 static int iommu_group_do_probe_finalize(struct device *dev, void *data)
 {
-	const struct iommu_ops *ops = dev_iommu_ops(dev);
+	/* It is unlocked but all callers know there is an ops */
+	const struct iommu_ops *ops = dev_iommu_ops_safe(dev);
 
 	if (ops->probe_finalize)
 		ops->probe_finalize(dev);
@@ -1884,13 +1894,9 @@ EXPORT_SYMBOL_GPL(iommu_present);
  */
 bool device_iommu_capable(struct device *dev, enum iommu_cap cap)
 {
-	const struct iommu_ops *ops;
-
-	if (!dev->iommu || !dev->iommu->iommu_dev)
-		return false;
+	const struct iommu_ops *ops = dev_iommu_ops(dev);
 
-	ops = dev_iommu_ops(dev);
-	if (!ops->capable)
+	if (!ops || !ops->capable)
 		return false;
 
 	return ops->capable(dev, cap);
@@ -2620,7 +2626,11 @@ void iommu_get_resv_regions(struct device *dev, struct list_head *list)
 {
 	const struct iommu_ops *ops = dev_iommu_ops(dev);
 
-	if (ops->get_resv_regions)
+	/*
+	 * Non-API groups still expose reserved_regions in sysfs, so filter out
+	 * calls that get here that way.
+	 */
+	if (ops && ops->get_resv_regions)
 		ops->get_resv_regions(dev, list);
 }
 
@@ -2979,6 +2989,11 @@ static ssize_t iommu_group_store_type(struct iommu_group *group,
 	/* Since group has only one device */
 	grp_dev = list_first_entry(&group->devices, struct group_device, list);
 	dev = grp_dev->dev;
+	if (!dev_iommu_ops(dev)) {
+		/* The group doesn't have an iommu driver attached */
+		mutex_unlock(&group->mutex);
+		return -EINVAL;
+	}
 	get_device(dev);
 
 	/*
@@ -3301,7 +3316,7 @@ static void __iommu_remove_group_pasid(struct iommu_group *group,
 	const struct iommu_ops *ops;
 
 	list_for_each_entry(device, &group->devices, list) {
-		ops = dev_iommu_ops(device->dev);
+		ops = dev_iommu_ops_safe(device->dev);
 		ops->remove_dev_pasid(device->dev, pasid);
 	}
 }
@@ -3413,6 +3428,9 @@ struct iommu_domain *iommu_sva_domain_alloc(struct device *dev,
 	const struct iommu_ops *ops = dev_iommu_ops(dev);
 	struct iommu_domain *domain;
 
+	if (!ops)
+		return NULL;
+
 	domain = ops->domain_alloc(IOMMU_DOMAIN_SVA);
 	if (!domain)
 		return NULL;
diff --git a/include/linux/iommu.h b/include/linux/iommu.h
index 46e1347bfa2286..60e84f8c7c46e9 100644
--- a/include/linux/iommu.h
+++ b/include/linux/iommu.h
@@ -441,14 +441,11 @@ static inline void iommu_iotlb_gather_init(struct iommu_iotlb_gather *gather)
 	};
 }
 
+/* May return NULL if the device has no iommu driver */
 static inline const struct iommu_ops *dev_iommu_ops(struct device *dev)
 {
-	/*
-	 * Assume that valid ops must be installed if iommu_probe_device()
-	 * has succeeded. The device ops are essentially for internal use
-	 * within the IOMMU subsystem itself, so we should be able to trust
-	 * ourselves not to misuse the helper.
-	 */
+	if (!dev->iommu)
+		return NULL;
 	return dev->iommu->iommu_dev->ops;
 }