drivers, net, mlx4: convert mlx4_srq.refcount from atomic_t to refcount_t

atomic_t variables are currently used to implement reference
counters with the following properties:
 - counter is initialized to 1 using atomic_set()
 - a resource is freed upon counter reaching zero
 - once counter reaches zero, its further
   increments aren't allowed
 - counter schema uses basic atomic operations
   (set, inc, inc_not_zero, dec_and_test, etc.)

Such atomic variables should be converted to a newly provided
refcount_t type and API that prevents accidental counter overflows
and underflows. This is important since overflows and underflows
can lead to use-after-free situation and be exploitable.

The variable mlx4_srq.refcount is used as pure reference counter.
Convert it to refcount_t and fix up the operations.
Suggested-by: Kees Cook <keescook@chromium.org>
Reviewed-by: David Windsor <dwindsor@gmail.com>
Reviewed-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mellanox/mlx4/srq.c

@@ -49,7 +49,7 @@ void mlx4_srq_event(struct mlx4_dev *dev, u32 srqn, int event_type)
 	srq = radix_tree_lookup(&srq_table->tree, srqn & (dev->caps.num_srqs - 1));
 	rcu_read_unlock();
 	if (srq)
-		atomic_inc(&srq->refcount);
+		refcount_inc(&srq->refcount);
 	else {
 		mlx4_warn(dev, "Async event for bogus SRQ %08x\n", srqn);
 		return;
@@ -57,7 +57,7 @@ void mlx4_srq_event(struct mlx4_dev *dev, u32 srqn, int event_type)
 
 	srq->event(srq, event_type);
 
-	if (atomic_dec_and_test(&srq->refcount))
+	if (refcount_dec_and_test(&srq->refcount))
 		complete(&srq->free);
 }
 
@@ -203,7 +203,7 @@ int mlx4_srq_alloc(struct mlx4_dev *dev, u32 pdn, u32 cqn, u16 xrcd,
 	if (err)
 		goto err_radix;
 
-	atomic_set(&srq->refcount, 1);
+	refcount_set(&srq->refcount, 1);
 	init_completion(&srq->free);
 
 	return 0;
@@ -232,7 +232,7 @@ void mlx4_srq_free(struct mlx4_dev *dev, struct mlx4_srq *srq)
 	radix_tree_delete(&srq_table->tree, srq->srqn);
 	spin_unlock_irq(&srq_table->lock);
 
-	if (atomic_dec_and_test(&srq->refcount))
+	if (refcount_dec_and_test(&srq->refcount))
 		complete(&srq->free);
 	wait_for_completion(&srq->free);
 

include/linux/mlx4/device.h

@@ -781,7 +781,7 @@ struct mlx4_srq {
 	int			max_gs;
 	int			wqe_shift;
 
-	atomic_t		refcount;
+	refcount_t		refcount;
 	struct completion	free;
 };