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>

2 files changed, 20 insertions, 45 deletions

diff --git a/include/net/sctp/checksum.h b/include/net/sctp/checksum.h
index 259924d63ba6..6bd44fe94c26 100644
--- a/include/net/sctp/checksum.h
+++ b/include/net/sctp/checksum.h
@@ -42,56 +42,38 @@
 #include <linux/types.h>
 #include <net/sctp/sctp.h>
 #include <linux/crc32c.h>
+#include <linux/crc32.h>
 
-static inline __u32 sctp_crc32c(__u32 crc, u8 *buffer, u16 length)
+static inline __wsum sctp_csum_update(const void *buff, int len, __wsum sum)
 {
-        return crc32c(crc, buffer, length);
-}
-
-static inline __u32 sctp_start_cksum(__u8 *buffer, __u16 length)
-{
-        __u32 crc = ~(__u32)0;
-        __u8  zero[sizeof(__u32)] = {0};
-
-        /* Optimize this routine to be SCTP specific, knowing how
-         * to skip the checksum field of the SCTP header.
+        /* This uses the crypto implementation of crc32c, which is either
+         * implemented w/ hardware support or resolves to __crc32c_le().
          */
-
-        /* Calculate CRC up to the checksum. */
-        crc = sctp_crc32c(crc, buffer, sizeof(struct sctphdr) - sizeof(__u32));
-
-        /* Skip checksum field of the header. */
-        crc = sctp_crc32c(crc, zero, sizeof(__u32));
-
-        /* Calculate the rest of the CRC. */
-        crc = sctp_crc32c(crc, &buffer[sizeof(struct sctphdr)],
-                            length - sizeof(struct sctphdr));
-        return crc;
-}
-
-static inline __u32 sctp_update_cksum(__u8 *buffer, __u16 length, __u32 crc32)
-{
-        return sctp_crc32c(crc32, buffer, length);
+        return crc32c(sum, buff, len);
 }
 
-static inline __le32 sctp_end_cksum(__u32 crc32)
+static inline __wsum sctp_csum_combine(__wsum csum, __wsum csum2,
+                                       int offset, int len)
 {
-        return cpu_to_le32(~crc32);
+        return __crc32c_le_combine(csum, csum2, len);
 }
 
-/* Calculate the CRC32C checksum of an SCTP packet.  */
 static inline __le32 sctp_compute_cksum(const struct sk_buff *skb,
                                         unsigned int offset)
 {
-        const struct sk_buff *iter;
+        struct sctphdr *sh = sctp_hdr(skb);
+        __le32 ret, old = sh->checksum;
+        const struct skb_checksum_ops ops = {
+                .update  = sctp_csum_update,
+                .combine = sctp_csum_combine,
+        };
 
-        __u32 crc32 = sctp_start_cksum(skb->data + offset,
-                                       skb_headlen(skb) - offset);
-        skb_walk_frags(skb, iter)
-                crc32 = sctp_update_cksum((__u8 *) iter->data,
-                                          skb_headlen(iter), crc32);
+        sh->checksum = 0;
+        ret = cpu_to_le32(~__skb_checksum(skb, offset, skb->len - offset,
+                                          ~(__u32)0, &ops));
+        sh->checksum = old;
 
-        return sctp_end_cksum(crc32);
+        return ret;
 }
 
 #endif /* __sctp_checksum_h__ */
diff --git a/net/sctp/output.c b/net/sctp/output.c
index 319137340d15..e650978daf27 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;