net: sctp: fix and consolidate SCTP checksumming code

This fixes an outstanding bug found through IPVS, where SCTP packets
with skb->data_len > 0 (non-linearized) and empty frag_list, but data
accumulated in frags[] member, are forwarded with incorrect checksum
letting SCTP initial handshake fail on some systems. Linearizing each
SCTP skb in IPVS to prevent that would not be a good solution as
this leads to an additional and unnecessary performance penalty on
the load-balancer itself for no good reason (as we actually only want
to update the checksum, and can do that in a different/better way
presented here).

The actual problem is elsewhere, namely, that SCTP's checksumming
in sctp_compute_cksum() does not take frags[] into account like
skb_checksum() does. So while we are fixing this up, we better reuse
the existing code that we have anyway in __skb_checksum() and use it
for walking through the data doing checksumming. This will not only
fix this issue, but also consolidates some SCTP code with core
sk_buff code, bringing it closer together and removing respectively
avoiding reimplementation of skb_checksum() for no good reason.

As crc32c() can use hardware implementation within the crypto layer,
we leave that intact (it wraps around / falls back to e.g. slice-by-8
algorithm in __crc32c_le() otherwise); plus use the __crc32c_le_combine()
combinator for crc32c blocks.

Also, we remove all other SCTP checksumming code, so that we only
have to use sctp_compute_cksum() from now on; for doing that, we need
to transform SCTP checkumming in output path slightly, and can leave
the rest intact.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 1 insertions, 8 deletions

diff --git a/net/sctp/output.c b/net/sctp/output.c
index 319137340d1..e650978daf2 100644
--- a/net/sctp/output.c
+++ b/net/sctp/output.c
@@ -390,7 +390,6 @@ int sctp_packet_transmit(struct sctp_packet *packet)
 	__u8 has_data = 0;
 	struct dst_entry *dst = tp->dst;
 	unsigned char *auth = NULL;	/* pointer to auth in skb data */
-	__u32 cksum_buf_len = sizeof(struct sctphdr);
 
 	pr_debug("%s: packet:%p\n", __func__, packet);
 
@@ -493,7 +492,6 @@ int sctp_packet_transmit(struct sctp_packet *packet)
 		if (chunk == packet->auth)
 			auth = skb_tail_pointer(nskb);
 
-		cksum_buf_len += chunk->skb->len;
 		memcpy(skb_put(nskb, chunk->skb->len),
 			       chunk->skb->data, chunk->skb->len);
 
@@ -538,12 +536,7 @@ int sctp_packet_transmit(struct sctp_packet *packet)
 	if (!sctp_checksum_disable) {
 		if (!(dst->dev->features & NETIF_F_SCTP_CSUM) ||
 		    (dst_xfrm(dst) != NULL) || packet->ipfragok) {
-			__u32 crc32 = sctp_start_cksum((__u8 *)sh, cksum_buf_len);
-
-			/* 3) Put the resultant value into the checksum field in the
-			 *    common header, and leave the rest of the bits unchanged.
-			 */
-			sh->checksum = sctp_end_cksum(crc32);
+			sh->checksum = sctp_compute_cksum(nskb, 0);
 		} else {
 			/* no need to seed pseudo checksum for SCTP */
 			nskb->ip_summed = CHECKSUM_PARTIAL;