Re: [PATCH v2 05/11] iommu/sva: Assign a PASID to mm on PASID allocation and free it on mm exit

[Thomas Gleixner]

On Tue, Jan 25 2022 at 07:18, Fenghua Yu wrote:
> On Mon, Jan 24, 2022 at 09:55:56PM +0100, Thomas Gleixner wrote:
>  /**
>   * ioasid_put - Release a reference to an ioasid
>   * @ioasid: the ID to remove

which in turn makes ioasid_put() a misnomer and the whole refcounting of
the ioasid a pointless exercise.

While looking at ioasid_put() usage I tripped over the following UAF
issue:

--- a/drivers/iommu/intel/iommu.c
+++ b/drivers/iommu/intel/iommu.c
@@ -4817,8 +4817,10 @@ static int aux_domain_add_dev(struct dma
 	auxiliary_unlink_device(domain, dev);
 link_failed:
 	spin_unlock_irqrestore(&device_domain_lock, flags);
-	if (list_empty(&domain->subdevices) && domain->default_pasid > 0)
+	if (list_empty(&domain->subdevices) && domain->default_pasid > 0) {
 		ioasid_put(domain->default_pasid);
+		domain->default_pasid = INVALID_IOASID;
+	}
 
 	return ret;
 }
@@ -4847,8 +4849,10 @@ static void aux_domain_remove_dev(struct
 
 	spin_unlock_irqrestore(&device_domain_lock, flags);
 
-	if (list_empty(&domain->subdevices) && domain->default_pasid > 0)
+	if (list_empty(&domain->subdevices) && domain->default_pasid > 0) {
 		ioasid_put(domain->default_pasid);
+		domain->default_pasid = INVALID_IOASID;
+	}
 }
 
 static int prepare_domain_attach_device(struct iommu_domain *domain,

Vs. ioasid_put() I think we should fold the following:

--- a/drivers/iommu/intel/iommu.c
+++ b/drivers/iommu/intel/iommu.c
@@ -4818,7 +4818,7 @@ static int aux_domain_add_dev(struct dma
 link_failed:
 	spin_unlock_irqrestore(&device_domain_lock, flags);
 	if (list_empty(&domain->subdevices) && domain->default_pasid > 0) {
-		ioasid_put(domain->default_pasid);
+		ioasid_free(domain->default_pasid);
 		domain->default_pasid = INVALID_IOASID;
 	}
 
@@ -4850,7 +4850,7 @@ static void aux_domain_remove_dev(struct
 	spin_unlock_irqrestore(&device_domain_lock, flags);
 
 	if (list_empty(&domain->subdevices) && domain->default_pasid > 0) {
-		ioasid_put(domain->default_pasid);
+		ioasid_free(domain->default_pasid);
 		domain->default_pasid = INVALID_IOASID;
 	}
 }
--- a/drivers/iommu/ioasid.c
+++ b/drivers/iommu/ioasid.c
@@ -2,7 +2,7 @@
 /*
  * I/O Address Space ID allocator. There is one global IOASID space, split into
  * subsets. Users create a subset with DECLARE_IOASID_SET, then allocate and
- * free IOASIDs with ioasid_alloc and ioasid_put.
+ * free IOASIDs with ioasid_alloc() and ioasid_free().
  */
 #include <linux/ioasid.h>
 #include <linux/module.h>
@@ -15,7 +15,6 @@ struct ioasid_data {
 	struct ioasid_set *set;
 	void *private;
 	struct rcu_head rcu;
-	refcount_t refs;
 };
 
 /*
@@ -315,7 +314,6 @@ ioasid_t ioasid_alloc(struct ioasid_set
 
 	data->set = set;
 	data->private = private;
-	refcount_set(&data->refs, 1);
 
 	/*
 	 * Custom allocator needs allocator data to perform platform specific
@@ -348,17 +346,11 @@ ioasid_t ioasid_alloc(struct ioasid_set
 EXPORT_SYMBOL_GPL(ioasid_alloc);
 
 /**
- * ioasid_put - Release a reference to an ioasid
+ * ioasid_free - Free an ioasid
  * @ioasid: the ID to remove
- *
- * Put a reference to the IOASID, free it when the number of references drops to
- * zero.
- *
- * Return: %true if the IOASID was freed, %false otherwise.
  */
-bool ioasid_put(ioasid_t ioasid)
+void ioasid_free(ioasid_t ioasid)
 {
-	bool free = false;
 	struct ioasid_data *ioasid_data;
 
 	spin_lock(&ioasid_allocator_lock);
@@ -368,10 +360,6 @@ bool ioasid_put(ioasid_t ioasid)
 		goto exit_unlock;
 	}
 
-	free = refcount_dec_and_test(&ioasid_data->refs);
-	if (!free)
-		goto exit_unlock;
-
 	active_allocator->ops->free(ioasid, active_allocator->ops->pdata);
 	/* Custom allocator needs additional steps to free the xa element */
 	if (active_allocator->flags & IOASID_ALLOCATOR_CUSTOM) {
@@ -381,9 +369,8 @@ bool ioasid_put(ioasid_t ioasid)
 
 exit_unlock:
 	spin_unlock(&ioasid_allocator_lock);
-	return free;
 }
-EXPORT_SYMBOL_GPL(ioasid_put);
+EXPORT_SYMBOL_GPL(ioasid_free);
 
 /**
  * ioasid_find - Find IOASID data
--- a/include/linux/ioasid.h
+++ b/include/linux/ioasid.h
@@ -34,7 +34,7 @@ struct ioasid_allocator_ops {
 #if IS_ENABLED(CONFIG_IOASID)
 ioasid_t ioasid_alloc(struct ioasid_set *set, ioasid_t min, ioasid_t max,
 		      void *private);
-bool ioasid_put(ioasid_t ioasid);
+void ioasid_free(ioasid_t ioasid);
 void *ioasid_find(struct ioasid_set *set, ioasid_t ioasid,
 		  bool (*getter)(void *));
 int ioasid_register_allocator(struct ioasid_allocator_ops *allocator);
@@ -52,10 +52,7 @@ static inline ioasid_t ioasid_alloc(stru
 	return INVALID_IOASID;
 }
 
-static inline bool ioasid_put(ioasid_t ioasid)
-{
-	return false;
-}
+static inline void ioasid_free(ioasid_t ioasid) { }
 
 static inline void *ioasid_find(struct ioasid_set *set, ioasid_t ioasid,
 				bool (*getter)(void *))
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -423,7 +423,7 @@ static inline void mm_pasid_set(struct m
 static inline void mm_pasid_drop(struct mm_struct *mm)
 {
 	if (pasid_valid(mm->pasid)) {
-		ioasid_put(mm->pasid);
+		ioasid_free(mm->pasid);
 		mm->pasid = INVALID_IOASID;
 	}
 }