[PATCH v10 12/16] iommu: Use refcount for fault data access

From: Lu Baolu
Date: Mon Jan 22 2024 - 00:53:38 EST

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@xxxxxxxxxx>
Signed-off-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
Signed-off-by: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
Tested-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
---
include/linux/iommu.h | 17 +++--
drivers/iommu/io-pgfault.c | 127 +++++++++++++++++++++++--------------
drivers/iommu/iommu-sva.c | 2 +-
3 files changed, 88 insertions(+), 58 deletions(-)

diff --git a/include/linux/iommu.h b/include/linux/iommu.h
index fc912aed7886..396d7b0d88b2 100644
--- a/include/linux/iommu.h
+++ b/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
+ * @ruc: 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)
{
diff --git a/drivers/iommu/io-pgfault.c b/drivers/iommu/io-pgfault.c
index 5aea8402be47..ce7058892b59 100644
--- a/drivers/iommu/io-pgfault.c
+++ b/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 @@ static int iommu_handle_iopf(struct iommu_fault *fault,
*/
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);
diff --git a/drivers/iommu/iommu-sva.c b/drivers/iommu/iommu-sva.c
index 9de878e40413..b51995b4fe90 100644
--- a/drivers/iommu/iommu-sva.c
+++ b/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))
--
2.34.1