Re: [PATCH v8 12/12] iommu: Use refcount for fault data access

Jason Gunthorpe
2023-12-11T15:12:37Z

On Thu, Dec 07, 2023 at 02:43:08PM +0800, Lu Baolu wrote:
> +/*
> + * 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;
> +
> + if (!param)
> + return NULL;

Is it actually possible to call this function on a device that does
not have an iommu driver probed? I'd be surprised by that, maybe this
should be WARN_ONE

> +
> + rcu_read_lock();
> + fault_param = param->fault_param;

The RCU stuff is not right, like this:

diff --git a/drivers/iommu/io-pgfault.c b/drivers/iommu/io-pgfault.c
index 2ace32c6d13bf3..0258f79c8ddf98 100644
--- a/drivers/iommu/io-pgfault.c
+++ b/drivers/iommu/io-pgfault.c
@@ -40,7 +40,7 @@ static struct iommu_fault_param *iopf_get_dev_fault_param(struct device *dev)
return NULL;

rcu_read_lock();
- fault_param = param->fault_param;
+ fault_param = rcu_dereference(param->fault_param);
if (fault_param && !refcount_inc_not_zero(&fault_param->users))
fault_param = NULL;
rcu_read_unlock();
@@ -51,17 +51,8 @@ static struct iommu_fault_param *iopf_get_dev_fault_param(struct device *dev)
/* Caller must hold a reference of the fault parameter. */
static void iopf_put_dev_fault_param(struct iommu_fault_param *fault_param)
{
- struct dev_iommu *param = fault_param->dev->iommu;
-
- rcu_read_lock();
- if (!refcount_dec_and_test(&fault_param->users)) {
- rcu_read_unlock();
- return;
- }
- rcu_read_unlock();
-
- param->fault_param = NULL;
- kfree_rcu(fault_param, rcu);
+ if (refcount_dec_and_test(&fault_param->users))
+ kfree_rcu(fault_param, rcu);
}

/**
@@ -174,7 +165,7 @@ static int iommu_handle_iopf(struct iommu_fault *fault,
}

mutex_unlock(&iopf_param->lock);
- ret = domain->iopf_handler(group);
+ ret = domain->iopf_handler(iopf_param, group);
mutex_lock(&iopf_param->lock);
if (ret)
iopf_free_group(group);
@@ -398,7 +389,8 @@ int iopf_queue_add_device(struct iopf_queue *queue, struct device *dev)

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;
}
@@ -418,7 +410,7 @@ int iopf_queue_add_device(struct iopf_queue *queue, struct device *dev)
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);
@@ -442,10 +434,12 @@ 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;
@@ -467,7 +461,10 @@ 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);

- iopf_put_dev_fault_param(fault_param);
+ /* dec the ref owned by iopf_queue_add_device() */
+ rcu_assign_pointer(param->fault_param, NULL);
+ if (refcount_dec_and_test(&fault_param->users))
+ kfree_rcu(fault_param, rcu);
unlock:
mutex_unlock(&param->lock);
mutex_unlock(&queue->lock);
diff --git a/drivers/iommu/iommu-sva.c b/drivers/iommu/iommu-sva.c
index 325d1810e133a1..63c1a233a7e91f 100644
--- a/drivers/iommu/iommu-sva.c
+++ b/drivers/iommu/iommu-sva.c
@@ -232,10 +232,9 @@ static void iommu_sva_handle_iopf(struct work_struct *work)
iopf_free_group(group);
}

-static int iommu_sva_iopf_handler(struct iopf_group *group)
+static int iommu_sva_iopf_handler(struct iommu_fault_param *fault_param,
+ struct iopf_group *group)
{
- struct iommu_fault_param *fault_param = group->dev->iommu->fault_param;
-
INIT_WORK(&group->work, iommu_sva_handle_iopf);
if (!queue_work(fault_param->queue->wq, &group->work))
return -EBUSY;
diff --git a/include/linux/iommu.h b/include/linux/iommu.h
index 8020bb44a64ab1..e16fa9811d5023 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 */
@@ -210,7 +211,8 @@ struct iommu_domain {
unsigned long pgsize_bitmap; /* Bitmap of page sizes in use */
struct iommu_domain_geometry geometry;
struct iommu_dma_cookie *iova_cookie;
- int (*iopf_handler)(struct iopf_group *group);
+ int (*iopf_handler)(struct iommu_fault_param *fault_param,
+ struct iopf_group *group);
void *fault_data;
union {
struct {
@@ -637,7 +639,7 @@ 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;