net: sctp: fix checksum marking for outgoing packets

Packets to devices without NETIF_F_SCTP_CSUM (including NETIF_F_NO_CSUM)
should be properly checksummed because the packets can be diverted or
rerouted after construction. This still leaves packets diverted from
NETIF_F_SCTP_CSUM-enabled devices with broken checksums. Fixing this
needs implementing software offload fallback in networking core.

For users of sctp_checksum_disable, skb->ip_summed should be left as
CHECKSUM_NONE and not CHECKSUM_UNNECESSARY as per include/linux/skbuff.h.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 8 insertions, 11 deletions

diff --git a/net/sctp/output.c b/net/sctp/output.c
index b4f3cf06d8da..08b3cead6503 100644
--- a/net/sctp/output.c
+++ b/net/sctp/output.c
@@ -500,23 +500,20 @@ int sctp_packet_transmit(struct sctp_packet *packet)
          * Note: Adler-32 is no longer applicable, as has been replaced
          * by CRC32-C as described in <draft-ietf-tsvwg-sctpcsum-02.txt>.
          */
-        if (!sctp_checksum_disable &&
-            !(dst->dev->features & (NETIF_F_NO_CSUM | NETIF_F_SCTP_CSUM))) {
-                __u32 crc32 = sctp_start_cksum((__u8 *)sh, cksum_buf_len);
+        if (!sctp_checksum_disable) {
+                if (!(dst->dev->features & NETIF_F_SCTP_CSUM)) {
+                        __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);
-        } else {
-                if (dst->dev->features & NETIF_F_SCTP_CSUM) {
+                        /* 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);
+                } else {
                         /* no need to seed pseudo checksum for SCTP */
                         nskb->ip_summed = CHECKSUM_PARTIAL;
                         nskb->csum_start = (skb_transport_header(nskb) -
                                             nskb->head);
                         nskb->csum_offset = offsetof(struct sctphdr, checksum);
-                } else {
-                        nskb->ip_summed = CHECKSUM_UNNECESSARY;
                 }
         }