net, sunrpc: convert gss_cl_ctx.count from atomic_t to refcount_t

refcount_t type and corresponding API should be
used instead of atomic_t when the variable is used as
a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free
situations.

Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David Windsor <dwindsor@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/sunrpc/auth_gss.h

@@ -13,6 +13,7 @@
 #define _LINUX_SUNRPC_AUTH_GSS_H
 
 #ifdef __KERNEL__
+#include <linux/refcount.h>
 #include <linux/sunrpc/auth.h>
 #include <linux/sunrpc/svc.h>
 #include <linux/sunrpc/gss_api.h>
@@ -65,7 +66,7 @@ struct rpc_gss_init_res {
  * the wire when communicating with a server. */
 
 struct gss_cl_ctx {
-	atomic_t		count;
+	refcount_t		count;
 	enum rpc_gss_proc	gc_proc;
 	u32			gc_seq;
 	spinlock_t		gc_seq_lock;

net/sunrpc/auth_gss/auth_gss.c

@@ -117,14 +117,14 @@ static const struct rpc_pipe_ops gss_upcall_ops_v1;
 static inline struct gss_cl_ctx *
 gss_get_ctx(struct gss_cl_ctx *ctx)
 {
-	atomic_inc(&ctx->count);
+	refcount_inc(&ctx->count);
 	return ctx;
 }
 
 static inline void
 gss_put_ctx(struct gss_cl_ctx *ctx)
 {
-	if (atomic_dec_and_test(&ctx->count))
+	if (refcount_dec_and_test(&ctx->count))
 		gss_free_ctx(ctx);
 }
 
@@ -200,7 +200,7 @@ gss_alloc_context(void)
 		ctx->gc_proc = RPC_GSS_PROC_DATA;
 		ctx->gc_seq = 1;	/* NetApp 6.4R1 doesn't accept seq. no. 0 */
 		spin_lock_init(&ctx->gc_seq_lock);
-		atomic_set(&ctx->count,1);
+		refcount_set(&ctx->count,1);
 	}
 	return ctx;
 }