[DCCP]: Support for partial checksums (RFC 4340, sec. 9.2)

This patch does the following:
  a) introduces variable-length checksums as specified in [RFC 4340, sec. 9.2]
  b) provides necessary socket options and documentation as to how to use them
  c) basic support and infrastructure for the Minimum Checksum Coverage feature
     [RFC 4340, sec. 9.2.1]: acceptability tests, user notification and user
     interface

In addition, it

 (1) fixes two bugs in the DCCPv4 checksum computation:
 	* pseudo-header used checksum_len instead of skb->len
	* incorrect checksum coverage calculation based on dccph_x
 (2) removes dccp_v4_verify_checksum() since it reduplicates code of the
     checksum computation; code calling this function is updated accordingly.
 (3) now uses skb_checksum(), which is safer than checksum_partial() if the
     sk_buff has is a non-linear buffer (has pages attached to it).
 (4) fixes an outstanding TODO item:
        * If P.CsCov is too large for the packet size, drop packet and return.

The code has been tested with applications, the latest version of tcpdump now
comes with support for partial DCCP checksums.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>

1 files changed, 47 insertions, 27 deletions

diff --git a/net/dccp/ipv6.c b/net/dccp/ipv6.c
index 201801e1532d..193b946fd039 100644
--- a/net/dccp/ipv6.c
+++ b/net/dccp/ipv6.c
@@ -58,12 +58,22 @@ static void dccp_v6_hash(struct sock *sk)
         }
 }
 
-static inline u16 dccp_v6_check(struct dccp_hdr *dh, int len,
-                                struct in6_addr *saddr,
-                                struct in6_addr *daddr,
-                                unsigned long base)
+/* add pseudo-header to DCCP checksum stored in skb->csum */
+static inline u16 dccp_v6_csum_finish(struct sk_buff *skb,
+                                      struct in6_addr *saddr,
+                                      struct in6_addr *daddr)
 {
-        return csum_ipv6_magic(saddr, daddr, len, IPPROTO_DCCP, base);
+        return csum_ipv6_magic(saddr, daddr, skb->len, IPPROTO_DCCP, skb->csum);
+}
+
+static inline void dccp_v6_send_check(struct sock *sk, int unused_value,
+                                      struct sk_buff *skb)
+{
+        struct ipv6_pinfo *np = inet6_sk(sk);
+        struct dccp_hdr *dh = dccp_hdr(skb);
+
+        dccp_csum_outgoing(skb);
+        dh->dccph_checksum = dccp_v6_csum_finish(skb, &np->saddr, &np->daddr);
 }
 
 static __u32 dccp_v6_init_sequence(struct sock *sk, struct sk_buff *skb)
@@ -280,12 +290,9 @@ static int dccp_v6_send_response(struct sock *sk, struct request_sock *req,
         if (skb != NULL) {
                 struct dccp_hdr *dh = dccp_hdr(skb);
 
-                dh->dccph_checksum = dccp_v6_check(dh, skb->len,
-                                                   &ireq6->loc_addr,
-                                                   &ireq6->rmt_addr,
-                                                   csum_partial((char *)dh,
-                                                                skb->len,
-                                                                skb->csum));
+                dh->dccph_checksum = dccp_v6_csum_finish(skb,
+                                                         &ireq6->loc_addr,
+                                                         &ireq6->rmt_addr);
                 ipv6_addr_copy(&fl.fl6_dst, &ireq6->rmt_addr);
                 err = ip6_xmit(sk, skb, &fl, opt, 0);
                 if (err == NET_XMIT_CN)
@@ -305,18 +312,6 @@ static void dccp_v6_reqsk_destructor(struct request_sock *req)
                 kfree_skb(inet6_rsk(req)->pktopts);
 }
 
-static void dccp_v6_send_check(struct sock *sk, int len, struct sk_buff *skb)
-{
-        struct ipv6_pinfo *np = inet6_sk(sk);
-        struct dccp_hdr *dh = dccp_hdr(skb);
-
-        dh->dccph_checksum = csum_ipv6_magic(&np->saddr, &np->daddr,
-                                             len, IPPROTO_DCCP,
-                                             csum_partial((char *)dh,
-                                                          dh->dccph_doff << 2,
-                                                          skb->csum));
-}
-
 static void dccp_v6_ctl_send_reset(struct sk_buff *rxskb)
 {
         struct dccp_hdr *rxdh = dccp_hdr(rxskb), *dh;
@@ -360,12 +355,14 @@ static void dccp_v6_ctl_send_reset(struct sk_buff *rxskb)
         dccp_hdr_set_ack(dccp_hdr_ack_bits(skb),
                          DCCP_SKB_CB(rxskb)->dccpd_seq);
 
+        dccp_csum_outgoing(skb);
+        dh->dccph_checksum = dccp_v6_csum_finish(skb, &rxskb->nh.ipv6h->saddr,
+                                                      &rxskb->nh.ipv6h->daddr);
+
         memset(&fl, 0, sizeof(fl));
         ipv6_addr_copy(&fl.fl6_dst, &rxskb->nh.ipv6h->saddr);
         ipv6_addr_copy(&fl.fl6_src, &rxskb->nh.ipv6h->daddr);
-        dh->dccph_checksum = csum_ipv6_magic(&fl.fl6_src, &fl.fl6_dst,
-                                             sizeof(*dh), IPPROTO_DCCP,
-                                             skb->csum);
+
         fl.proto = IPPROTO_DCCP;
         fl.oif = inet6_iif(rxskb);
         fl.fl_ip_dport = dh->dccph_dport;
@@ -825,12 +822,22 @@ static int dccp_v6_rcv(struct sk_buff **pskb)
         const struct dccp_hdr *dh;
         struct sk_buff *skb = *pskb;
         struct sock *sk;
+        int min_cov;
 
-        /* Step 1: Check header basics: */
+        /* Step 1: Check header basics */
 
         if (dccp_invalid_packet(skb))
                 goto discard_it;
 
+        /* Step 1: If header checksum is incorrect, drop packet and return. */
+        if (dccp_v6_csum_finish(skb, &skb->nh.ipv6h->saddr,
+                                     &skb->nh.ipv6h->daddr)) {
+                LIMIT_NETDEBUG(KERN_WARNING
+                               "%s: dropped packet with invalid checksum\n",
+                               __FUNCTION__);
+                goto discard_it;
+        }
+
         dh = dccp_hdr(skb);
 
         DCCP_SKB_CB(skb)->dccpd_seq  = dccp_hdr_seq(skb);
@@ -869,6 +876,19 @@ static int dccp_v6_rcv(struct sk_buff **pskb)
                 goto no_dccp_socket;
         }
 
+        /*
+         * RFC 4340, sec. 9.2.1: Minimum Checksum Coverage
+         *      o if MinCsCov = 0, only packets with CsCov = 0 are accepted
+         *      o if MinCsCov > 0, also accept packets with CsCov >= MinCsCov
+         */
+        min_cov = dccp_sk(sk)->dccps_pcrlen;
+        if (dh->dccph_cscov  &&  (min_cov == 0 || dh->dccph_cscov < min_cov))  {
+                dccp_pr_debug("Packet CsCov %d does not satisfy MinCsCov %d\n",
+                              dh->dccph_cscov, min_cov);
+                /* FIXME: send Data Dropped option (see also dccp_v4_rcv) */
+                goto discard_and_relse;
+        }
+
         if (!xfrm6_policy_check(sk, XFRM_POLICY_IN, skb))
                 goto discard_and_relse;