net, sctp: convert sctp_chunk.refcnt 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/net/sctp/structs.h

@@ -524,7 +524,7 @@ int sctp_chunk_abandoned(struct sctp_chunk *);
 struct sctp_chunk {
 	struct list_head list;
 
-	atomic_t refcnt;
+	refcount_t refcnt;
 
 	/* How many times this chunk have been sent, for prsctp RTX policy */
 	int sent_count;

net/sctp/sm_make_chunk.c

@@ -1345,7 +1345,7 @@ struct sctp_chunk *sctp_chunkify(struct sk_buff *skb,
 	INIT_LIST_HEAD(&retval->transmitted_list);
 	INIT_LIST_HEAD(&retval->frag_list);
 	SCTP_DBG_OBJCNT_INC(chunk);
-	atomic_set(&retval->refcnt, 1);
+	refcount_set(&retval->refcnt, 1);
 
 nodata:
 	return retval;
@@ -1458,13 +1458,13 @@ void sctp_chunk_free(struct sctp_chunk *chunk)
 /* Grab a reference to the chunk. */
 void sctp_chunk_hold(struct sctp_chunk *ch)
 {
-	atomic_inc(&ch->refcnt);
+	refcount_inc(&ch->refcnt);
 }
 
 /* Release a reference to the chunk. */
 void sctp_chunk_put(struct sctp_chunk *ch)
 {
-	if (atomic_dec_and_test(&ch->refcnt))
+	if (refcount_dec_and_test(&ch->refcnt))
 		sctp_chunk_destroy(ch);
 }