iommu: Use refcount for fault data access

The per-device fault data structure stores information about faults
occurring on a device. Its lifetime spans from IOPF enablement to
disablement. Multiple paths, including IOPF reporting, handling, and
responding, may access it concurrently.

Previously, a mutex protected the fault data from use after free. But
this is not performance friendly due to the critical nature of IOPF
handling paths.

Refine this with a refcount-based approach. The fault data pointer is
obtained within an RCU read region with a refcount. The fault data
pointer is returned for usage only when the pointer is valid and a
refcount is successfully obtained. The fault data is freed with
kfree_rcu(), ensuring data is only freed after all RCU critical regions
complete.

An iopf handling work starts once an iopf group is created. The handling
work continues until iommu_page_response() is called to respond to the
iopf and the iopf group is freed. During this time, the device fault
parameter should always be available. Add a pointer to the device fault
parameter in the iopf_group structure and hold the reference until the
iopf_group is freed.

Make iommu_page_response() static as it is only used in io-pgfault.c.

Co-developed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Tested-by: Yan Zhao <yan.y.zhao@intel.com>
Link: https://lore.kernel.org/r/20240212012227.119381-13-baolu.lu@linux.intel.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/io-pgfault.c

@@ -13,6 +13,32 @@
 
 #include "iommu-priv.h"
 
+/*
+ * Return the fault parameter of a device if it exists. Otherwise, return NULL.
+ * On a successful return, the caller takes a reference of this parameter and
+ * should put it after use by calling iopf_put_dev_fault_param().
+ */
+static struct iommu_fault_param *iopf_get_dev_fault_param(struct device *dev)
+{
+	struct dev_iommu *param = dev->iommu;
+	struct iommu_fault_param *fault_param;
+
+	rcu_read_lock();
+	fault_param = rcu_dereference(param->fault_param);
+	if (fault_param && !refcount_inc_not_zero(&fault_param->users))
+		fault_param = NULL;
+	rcu_read_unlock();
+
+	return fault_param;
+}
+
+/* Caller must hold a reference of the fault parameter. */
+static void iopf_put_dev_fault_param(struct iommu_fault_param *fault_param)
+{
+	if (refcount_dec_and_test(&fault_param->users))
+		kfree_rcu(fault_param, rcu);
+}
+
 void iopf_free_group(struct iopf_group *group)
 {
 	struct iopf_fault *iopf, *next;
@@ -22,6 +48,8 @@ void iopf_free_group(struct iopf_group *group)
 			kfree(iopf);
 	}
 
+	/* Pair with iommu_report_device_fault(). */
+	iopf_put_dev_fault_param(group->fault_param);
 	kfree(group);
 }
 EXPORT_SYMBOL_GPL(iopf_free_group);
@@ -135,7 +163,7 @@ static int iommu_handle_iopf(struct iommu_fault *fault,
 		goto cleanup_partial;
 	}
 
-	group->dev = dev;
+	group->fault_param = iopf_param;
 	group->last_fault.fault = *fault;
 	INIT_LIST_HEAD(&group->faults);
 	group->domain = domain;
@@ -178,64 +206,61 @@ cleanup_partial:
  */
 int iommu_report_device_fault(struct device *dev, struct iopf_fault *evt)
 {
+	bool last_prq = evt->fault.type == IOMMU_FAULT_PAGE_REQ &&
+		(evt->fault.prm.flags & IOMMU_FAULT_PAGE_REQUEST_LAST_PAGE);
 	struct iommu_fault_param *fault_param;
-	struct iopf_fault *evt_pending = NULL;
-	struct dev_iommu *param = dev->iommu;
-	int ret = 0;
+	struct iopf_fault *evt_pending;
+	int ret;
 
-	mutex_lock(&param->lock);
-	fault_param = param->fault_param;
-	if (!fault_param) {
-		mutex_unlock(&param->lock);
+	fault_param = iopf_get_dev_fault_param(dev);
+	if (!fault_param)
 		return -EINVAL;
-	}
 
 	mutex_lock(&fault_param->lock);
-	if (evt->fault.type == IOMMU_FAULT_PAGE_REQ &&
-	    (evt->fault.prm.flags & IOMMU_FAULT_PAGE_REQUEST_LAST_PAGE)) {
+	if (last_prq) {
 		evt_pending = kmemdup(evt, sizeof(struct iopf_fault),
 				      GFP_KERNEL);
 		if (!evt_pending) {
 			ret = -ENOMEM;
-			goto done_unlock;
+			goto err_unlock;
 		}
 		list_add_tail(&evt_pending->list, &fault_param->faults);
 	}
 
 	ret = iommu_handle_iopf(&evt->fault, fault_param);
-	if (ret && evt_pending) {
+	if (ret)
+		goto err_free;
+
+	mutex_unlock(&fault_param->lock);
+	/* The reference count of fault_param is now held by iopf_group. */
+	if (!last_prq)
+		iopf_put_dev_fault_param(fault_param);
+
+	return 0;
+err_free:
+	if (last_prq) {
 		list_del(&evt_pending->list);
 		kfree(evt_pending);
 	}
-done_unlock:
+err_unlock:
 	mutex_unlock(&fault_param->lock);
-	mutex_unlock(&param->lock);
+	iopf_put_dev_fault_param(fault_param);
 
 	return ret;
 }
 EXPORT_SYMBOL_GPL(iommu_report_device_fault);
 
-int iommu_page_response(struct device *dev,
-			struct iommu_page_response *msg)
+static int iommu_page_response(struct iopf_group *group,
+			       struct iommu_page_response *msg)
 {
 	bool needs_pasid;
 	int ret = -EINVAL;
 	struct iopf_fault *evt;
 	struct iommu_fault_page_request *prm;
-	struct dev_iommu *param = dev->iommu;
-	struct iommu_fault_param *fault_param;
+	struct device *dev = group->fault_param->dev;
 	const struct iommu_ops *ops = dev_iommu_ops(dev);
 	bool has_pasid = msg->flags & IOMMU_PAGE_RESP_PASID_VALID;
-
-	if (!ops->page_response)
-		return -ENODEV;
-
-	mutex_lock(&param->lock);
-	fault_param = param->fault_param;
-	if (!fault_param) {
-		mutex_unlock(&param->lock);
-		return -EINVAL;
-	}
+	struct iommu_fault_param *fault_param = group->fault_param;
 
 	/* Only send response if there is a fault report pending */
 	mutex_lock(&fault_param->lock);
@@ -276,10 +301,9 @@ int iommu_page_response(struct device *dev,
 
 done_unlock:
 	mutex_unlock(&fault_param->lock);
-	mutex_unlock(&param->lock);
+
 	return ret;
 }
-EXPORT_SYMBOL_GPL(iommu_page_response);
 
 /**
  * iopf_queue_flush_dev - Ensure that all queued faults have been processed
@@ -295,22 +319,20 @@ EXPORT_SYMBOL_GPL(iommu_page_response);
  */
 int iopf_queue_flush_dev(struct device *dev)
 {
-	int ret = 0;
 	struct iommu_fault_param *iopf_param;
-	struct dev_iommu *param = dev->iommu;
 
-	if (!param)
+	/*
+	 * It's a driver bug to be here after iopf_queue_remove_device().
+	 * Therefore, it's safe to dereference the fault parameter without
+	 * holding the lock.
+	 */
+	iopf_param = rcu_dereference_check(dev->iommu->fault_param, true);
+	if (WARN_ON(!iopf_param))
 		return -ENODEV;
 
-	mutex_lock(&param->lock);
-	iopf_param = param->fault_param;
-	if (iopf_param)
-		flush_workqueue(iopf_param->queue->wq);
-	else
-		ret = -ENODEV;
-	mutex_unlock(&param->lock);
+	flush_workqueue(iopf_param->queue->wq);
 
-	return ret;
+	return 0;
 }
 EXPORT_SYMBOL_GPL(iopf_queue_flush_dev);
 
@@ -335,7 +357,7 @@ int iopf_group_response(struct iopf_group *group,
 	    (iopf->fault.prm.flags & IOMMU_FAULT_PAGE_RESPONSE_NEEDS_PASID))
 		resp.flags = IOMMU_PAGE_RESP_PASID_VALID;
 
-	return iommu_page_response(group->dev, &resp);
+	return iommu_page_response(group, &resp);
 }
 EXPORT_SYMBOL_GPL(iopf_group_response);
 
@@ -384,10 +406,15 @@ int iopf_queue_add_device(struct iopf_queue *queue, struct device *dev)
 	int ret = 0;
 	struct dev_iommu *param = dev->iommu;
 	struct iommu_fault_param *fault_param;
+	const struct iommu_ops *ops = dev_iommu_ops(dev);
+
+	if (!ops->page_response)
+		return -ENODEV;
 
 	mutex_lock(&queue->lock);
 	mutex_lock(&param->lock);
-	if (param->fault_param) {
+	if (rcu_dereference_check(param->fault_param,
+				  lockdep_is_held(&param->lock))) {
 		ret = -EBUSY;
 		goto done_unlock;
 	}
@@ -402,10 +429,11 @@ int iopf_queue_add_device(struct iopf_queue *queue, struct device *dev)
 	INIT_LIST_HEAD(&fault_param->faults);
 	INIT_LIST_HEAD(&fault_param->partial);
 	fault_param->dev = dev;
+	refcount_set(&fault_param->users, 1);
 	list_add(&fault_param->queue_list, &queue->devices);
 	fault_param->queue = queue;
 
-	param->fault_param = fault_param;
+	rcu_assign_pointer(param->fault_param, fault_param);
 
 done_unlock:
 	mutex_unlock(&param->lock);
@@ -429,10 +457,12 @@ int iopf_queue_remove_device(struct iopf_queue *queue, struct device *dev)
 	int ret = 0;
 	struct iopf_fault *iopf, *next;
 	struct dev_iommu *param = dev->iommu;
-	struct iommu_fault_param *fault_param = param->fault_param;
+	struct iommu_fault_param *fault_param;
 
 	mutex_lock(&queue->lock);
 	mutex_lock(&param->lock);
+	fault_param = rcu_dereference_check(param->fault_param,
+					    lockdep_is_held(&param->lock));
 	if (!fault_param) {
 		ret = -ENODEV;
 		goto unlock;
@@ -454,8 +484,9 @@ int iopf_queue_remove_device(struct iopf_queue *queue, struct device *dev)
 	list_for_each_entry_safe(iopf, next, &fault_param->partial, list)
 		kfree(iopf);
 
-	param->fault_param = NULL;
-	kfree(fault_param);
+	/* dec the ref owned by iopf_queue_add_device() */
+	rcu_assign_pointer(param->fault_param, NULL);
+	iopf_put_dev_fault_param(fault_param);
 unlock:
 	mutex_unlock(&param->lock);
 	mutex_unlock(&queue->lock);

drivers/iommu/iommu-sva.c

@@ -251,7 +251,7 @@ static void iommu_sva_handle_iopf(struct work_struct *work)
 
 static int iommu_sva_iopf_handler(struct iopf_group *group)
 {
-	struct iommu_fault_param *fault_param = group->dev->iommu->fault_param;
+	struct iommu_fault_param *fault_param = group->fault_param;
 
 	INIT_WORK(&group->work, iommu_sva_handle_iopf);
 	if (!queue_work(fault_param->queue->wq, &group->work))

include/linux/iommu.h

@@ -41,6 +41,7 @@ struct iommu_dirty_ops;
 struct notifier_block;
 struct iommu_sva;
 struct iommu_dma_cookie;
+struct iommu_fault_param;
 
 #define IOMMU_FAULT_PERM_READ	(1 << 0) /* read */
 #define IOMMU_FAULT_PERM_WRITE	(1 << 1) /* write */
@@ -129,8 +130,9 @@ struct iopf_group {
 	struct iopf_fault last_fault;
 	struct list_head faults;
 	struct work_struct work;
-	struct device *dev;
 	struct iommu_domain *domain;
+	/* The device's fault data parameter. */
+	struct iommu_fault_param *fault_param;
 };
 
 /**
@@ -679,6 +681,8 @@ struct iommu_device {
 /**
  * struct iommu_fault_param - per-device IOMMU fault data
  * @lock: protect pending faults list
+ * @users: user counter to manage the lifetime of the data
+ * @rcu: rcu head for kfree_rcu()
  * @dev: the device that owns this param
  * @queue: IOPF queue
  * @queue_list: index into queue->devices
@@ -688,6 +692,8 @@ struct iommu_device {
  */
 struct iommu_fault_param {
 	struct mutex lock;
+	refcount_t users;
+	struct rcu_head rcu;
 
 	struct device *dev;
 	struct iopf_queue *queue;
@@ -715,7 +721,7 @@ struct iommu_fault_param {
  */
 struct dev_iommu {
 	struct mutex lock;
-	struct iommu_fault_param	*fault_param;
+	struct iommu_fault_param __rcu	*fault_param;
 	struct iommu_fwspec		*fwspec;
 	struct iommu_device		*iommu_dev;
 	void				*priv;
@@ -1543,7 +1549,6 @@ void iopf_queue_free(struct iopf_queue *queue);
 int iopf_queue_discard_partial(struct iopf_queue *queue);
 void iopf_free_group(struct iopf_group *group);
 int iommu_report_device_fault(struct device *dev, struct iopf_fault *evt);
-int iommu_page_response(struct device *dev, struct iommu_page_response *msg);
 int iopf_group_response(struct iopf_group *group,
 			enum iommu_page_response_code status);
 #else
@@ -1588,12 +1593,6 @@ iommu_report_device_fault(struct device *dev, struct iopf_fault *evt)
 	return -ENODEV;
 }
 
-static inline int
-iommu_page_response(struct device *dev, struct iommu_page_response *msg)
-{
-	return -ENODEV;
-}
-
 static inline int iopf_group_response(struct iopf_group *group,
 				      enum iommu_page_response_code status)
 {