net: sctp: fix skb leak in sctp_inq_free()

In case of GSO, 'chunk->skb' pointer may point to an entry from
fraglist created in 'sctp_packet_gso_append()'. To avoid freeing
random fraglist entry (and so undefined behavior and/or memory
leak), introduce 'sctp_inq_chunk_free()' helper to ensure that
'chunk->skb' is set to 'chunk->head_skb' (i.e. fraglist head)
before calling 'sctp_chunk_free()', and use the aforementioned
helper in 'sctp_inq_pop()' as well.

Reported-by: [e-mail]
Closes: https://syzkaller.appspot.com/bug?id=0d8351bbe54fd04a492c2daab0164138db008042
Fixes: 90017accff61 ("sctp: Add GSO support")
Suggested-by: Xin Long <[e-mail]>
Signed-off-by: Dmitry Antipov <[e-mail]>
Acked-by: Xin Long <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/sctp/inqueue.c

···
 	INIT_WORK(&queue->immediate, NULL);
 }
 
+/* Properly release the chunk which is being worked on. */
+static inline void sctp_inq_chunk_free(struct sctp_chunk *chunk)
+{
+	if (chunk->head_skb)
+		chunk->skb = chunk->head_skb;
+	sctp_chunk_free(chunk);
+}
+
 /* Release the memory associated with an SCTP inqueue.  */
 void sctp_inq_free(struct sctp_inq *queue)
 {
···
 	 * free it as well.
 	 */
 	if (queue->in_progress) {
-		sctp_chunk_free(queue->in_progress);
+		sctp_inq_chunk_free(queue->in_progress);
 		queue->in_progress = NULL;
 	}
 }
···
 				goto new_skb;
 			}
 
-			if (chunk->head_skb)
-				chunk->skb = chunk->head_skb;
-			sctp_chunk_free(chunk);
+			sctp_inq_chunk_free(chunk);
 			chunk = queue->in_progress = NULL;
 		} else {
 			/* Nothing to do. Next chunk in the packet, please. */