drivers, net, mlx4: convert mlx4_cq.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_cq.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/cq.c

@@ -69,7 +69,7 @@ void mlx4_cq_tasklet_cb(unsigned long data)
 	list_for_each_entry_safe(mcq, temp, &ctx->process_list, tasklet_ctx.list) {
 		list_del_init(&mcq->tasklet_ctx.list);
 		mcq->tasklet_ctx.comp(mcq);
-		if (atomic_dec_and_test(&mcq->refcount))
+		if (refcount_dec_and_test(&mcq->refcount))
 			complete(&mcq->free);
 		if (time_after(jiffies, end))
 			break;
@@ -92,7 +92,7 @@ static void mlx4_add_cq_to_tasklet(struct mlx4_cq *cq)
 	 * still arrive.
 	 */
 	if (list_empty_careful(&cq->tasklet_ctx.list)) {
-		atomic_inc(&cq->refcount);
+		refcount_inc(&cq->refcount);
 		kick = list_empty(&tasklet_ctx->list);
 		list_add_tail(&cq->tasklet_ctx.list, &tasklet_ctx->list);
 		if (kick)
@@ -344,7 +344,7 @@ int mlx4_cq_alloc(struct mlx4_dev *dev, int nent,
 	cq->cons_index = 0;
 	cq->arm_sn     = 1;
 	cq->uar        = uar;
-	atomic_set(&cq->refcount, 1);
+	refcount_set(&cq->refcount, 1);
 	init_completion(&cq->free);
 	cq->comp = mlx4_add_cq_to_tasklet;
 	cq->tasklet_ctx.priv =
@@ -386,7 +386,7 @@ void mlx4_cq_free(struct mlx4_dev *dev, struct mlx4_cq *cq)
 	    priv->eq_table.eq[MLX4_EQ_ASYNC].irq)
 		synchronize_irq(priv->eq_table.eq[MLX4_EQ_ASYNC].irq);
 
-	if (atomic_dec_and_test(&cq->refcount))
+	if (refcount_dec_and_test(&cq->refcount))
 		complete(&cq->free);
 	wait_for_completion(&cq->free);
 

include/linux/mlx4/device.h

@@ -40,7 +40,7 @@
 #include <linux/cpu_rmap.h>
 #include <linux/crash_dump.h>
 
-#include <linux/atomic.h>
+#include <linux/refcount.h>
 
 #include <linux/timecounter.h>
 
@@ -751,7 +751,7 @@ struct mlx4_cq {
 	int			cqn;
 	unsigned		vector;
 
-	atomic_t		refcount;
+	refcount_t		refcount;
 	struct completion	free;
 	struct {
 		struct list_head list;