sctp: fix ICMP processing if skb is non-linear

[ Upstream commit 804ec7ebe8ea003999ca8d1bfc499edc6a9e07df ]

sometimes ICMP replies to INIT chunks are ignored by the client, even if
the encapsulated SCTP headers match an open socket. This happens when the
ICMP packet is carried by a paged skb: use skb_header_pointer() to read
packet contents beyond the SCTP header, so that chunk header and initiate
tag are validated correctly.

v2:
- don't use skb_header_pointer() to read the transport header, since
  icmp_socket_deliver() already puts these 8 bytes in the linear area.
- change commit message to make specific reference to INIT chunks.

Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Acked-by: Vlad Yasevich <vyasevich@gmail.com>
Reviewed-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: dcaratti

net/sctp/input.c

@@ -472,15 +472,14 @@ struct sock *sctp_err_lookup(struct net *net, int family, struct sk_buff *skb,
 			     struct sctp_association **app,
 			     struct sctp_transport **tpp)
 {
+	struct sctp_init_chunk *chunkhdr, _chunkhdr;
 	union sctp_addr saddr;
 	union sctp_addr daddr;
 	struct sctp_af *af;
 	struct sock *sk = NULL;
 	struct sctp_association *asoc;
 	struct sctp_transport *transport = NULL;
-	struct sctp_init_chunk *chunkhdr;
 	__u32 vtag = ntohl(sctphdr->vtag);
-	int len = skb->len - ((void *)sctphdr - (void *)skb->data);
 
 	*app = NULL; *tpp = NULL;
 
@@ -515,13 +514,16 @@ struct sock *sctp_err_lookup(struct net *net, int family, struct sk_buff *skb,
 	 * discard the packet.
 	 */
 	if (vtag == 0) {
-		chunkhdr = (void *)sctphdr + sizeof(struct sctphdr);
-		if (len < sizeof(struct sctphdr) + sizeof(sctp_chunkhdr_t)
-			  + sizeof(__be32) ||
+		/* chunk header + first 4 octects of init header */
+		chunkhdr = skb_header_pointer(skb, skb_transport_offset(skb) +
+					      sizeof(struct sctphdr),
+					      sizeof(struct sctp_chunkhdr) +
+					      sizeof(__be32), &_chunkhdr);
+		if (!chunkhdr ||
 		    chunkhdr->chunk_hdr.type != SCTP_CID_INIT ||
-		    ntohl(chunkhdr->init_hdr.init_tag) != asoc->c.my_vtag) {
+		    ntohl(chunkhdr->init_hdr.init_tag) != asoc->c.my_vtag)
 			goto out;
-		}
+
 	} else if (vtag != asoc->c.peer_vtag) {
 		goto out;
 	}