TCPCT part 1f: Initiator Cookie => Responder

Calculate and format <SYN> TCP_COOKIE option.

This is a significantly revised implementation of an earlier (year-old)
patch that no longer applies cleanly, with permission of the original
author (Adam Langley):

    http://thread.gmane.org/gmane.linux.network/102586

Requires:
   TCPCT part 1c: sysctl_tcp_cookie_size, socket option TCP_COOKIE_TRANSACTIONS
   TCPCT part 1d: define TCP cookie option, extend existing struct's

Signed-off-by: William.Allen.Simpson@gmail.com
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 163 insertions, 30 deletions

diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 307f318fe931..35dd983a8a99 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -365,15 +365,45 @@ static inline int tcp_urg_mode(const struct tcp_sock *tp)
 #define OPTION_TS               (1 << 1)
 #define OPTION_MD5              (1 << 2)
 #define OPTION_WSCALE           (1 << 3)
+#define OPTION_COOKIE_EXTENSION (1 << 4)
 
 struct tcp_out_options {
         u8 options;             /* bit field of OPTION_* */
         u8 ws;                  /* window scale, 0 to disable */
         u8 num_sack_blocks;     /* number of SACK blocks to include */
+        u8 hash_size;           /* bytes in hash_location */
         u16 mss;                /* 0 to disable */
         __u32 tsval, tsecr;     /* need to include OPTION_TS */
+        __u8 *hash_location;    /* temporary pointer, overloaded */
 };
 
+/* The sysctl int routines are generic, so check consistency here.
+ */
+static u8 tcp_cookie_size_check(u8 desired)
+{
+        if (desired > 0) {
+                /* previously specified */
+                return desired;
+        }
+        if (sysctl_tcp_cookie_size <= 0) {
+                /* no default specified */
+                return 0;
+        }
+        if (sysctl_tcp_cookie_size <= TCP_COOKIE_MIN) {
+                /* value too small, specify minimum */
+                return TCP_COOKIE_MIN;
+        }
+        if (sysctl_tcp_cookie_size >= TCP_COOKIE_MAX) {
+                /* value too large, specify maximum */
+                return TCP_COOKIE_MAX;
+        }
+        if (0x1 & sysctl_tcp_cookie_size) {
+                /* 8-bit multiple, illegal, fix it */
+                return (u8)(sysctl_tcp_cookie_size + 0x1);
+        }
+        return (u8)sysctl_tcp_cookie_size;
+}
+
 /* Write previously computed TCP options to the packet.
  *
  * Beware: Something in the Internet is very sensitive to the ordering of
@@ -388,17 +418,34 @@ struct tcp_out_options {
  * (but it may well be that other scenarios fail similarly).
  */
 static void tcp_options_write(__be32 *ptr, struct tcp_sock *tp,
-                              const struct tcp_out_options *opts,
-                              __u8 **md5_hash) {
-        if (unlikely(OPTION_MD5 & opts->options)) {
-                *ptr++ = htonl((TCPOPT_NOP << 24) |
-                               (TCPOPT_NOP << 16) |
-                               (TCPOPT_MD5SIG << 8) |
-                               TCPOLEN_MD5SIG);
-                *md5_hash = (__u8 *)ptr;
+                              struct tcp_out_options *opts)
+{
+        u8 options = opts->options;     /* mungable copy */
+
+        /* Having both authentication and cookies for security is redundant,
+         * and there's certainly not enough room.  Instead, the cookie-less
+         * extension variant is proposed.
+         *
+         * Consider the pessimal case with authentication.  The options
+         * could look like:
+         *   COOKIE|MD5(20) + MSS(4) + SACK|TS(12) + WSCALE(4) == 40
+         */
+        if (unlikely(OPTION_MD5 & options)) {
+                if (unlikely(OPTION_COOKIE_EXTENSION & options)) {
+                        *ptr++ = htonl((TCPOPT_COOKIE << 24) |
+                                       (TCPOLEN_COOKIE_BASE << 16) |
+                                       (TCPOPT_MD5SIG << 8) |
+                                       TCPOLEN_MD5SIG);
+                } else {
+                        *ptr++ = htonl((TCPOPT_NOP << 24) |
+                                       (TCPOPT_NOP << 16) |
+                                       (TCPOPT_MD5SIG << 8) |
+                                       TCPOLEN_MD5SIG);
+                }
+                options &= ~OPTION_COOKIE_EXTENSION;
+                /* overload cookie hash location */
+                opts->hash_location = (__u8 *)ptr;
                 ptr += 4;
-        } else {
-                *md5_hash = NULL;
         }
 
         if (unlikely(opts->mss)) {
@@ -407,12 +454,13 @@ static void tcp_options_write(__be32 *ptr, struct tcp_sock *tp,
                                opts->mss);
         }
 
-        if (likely(OPTION_TS & opts->options)) {
-                if (unlikely(OPTION_SACK_ADVERTISE & opts->options)) {
+        if (likely(OPTION_TS & options)) {
+                if (unlikely(OPTION_SACK_ADVERTISE & options)) {
                         *ptr++ = htonl((TCPOPT_SACK_PERM << 24) |
                                        (TCPOLEN_SACK_PERM << 16) |
                                        (TCPOPT_TIMESTAMP << 8) |
                                        TCPOLEN_TIMESTAMP);
+                        options &= ~OPTION_SACK_ADVERTISE;
                 } else {
                         *ptr++ = htonl((TCPOPT_NOP << 24) |
                                        (TCPOPT_NOP << 16) |
@@ -423,15 +471,52 @@ static void tcp_options_write(__be32 *ptr, struct tcp_sock *tp,
                 *ptr++ = htonl(opts->tsecr);
         }
 
-        if (unlikely(OPTION_SACK_ADVERTISE & opts->options &&
-                     !(OPTION_TS & opts->options))) {
+        /* Specification requires after timestamp, so do it now.
+         *
+         * Consider the pessimal case without authentication.  The options
+         * could look like:
+         *   MSS(4) + SACK|TS(12) + COOKIE(20) + WSCALE(4) == 40
+         */
+        if (unlikely(OPTION_COOKIE_EXTENSION & options)) {
+                __u8 *cookie_copy = opts->hash_location;
+                u8 cookie_size = opts->hash_size;
+
+                /* 8-bit multiple handled in tcp_cookie_size_check() above,
+                 * and elsewhere.
+                 */
+                if (0x2 & cookie_size) {
+                        __u8 *p = (__u8 *)ptr;
+
+                        /* 16-bit multiple */
+                        *p++ = TCPOPT_COOKIE;
+                        *p++ = TCPOLEN_COOKIE_BASE + cookie_size;
+                        *p++ = *cookie_copy++;
+                        *p++ = *cookie_copy++;
+                        ptr++;
+                        cookie_size -= 2;
+                } else {
+                        /* 32-bit multiple */
+                        *ptr++ = htonl(((TCPOPT_NOP << 24) |
+                                        (TCPOPT_NOP << 16) |
+                                        (TCPOPT_COOKIE << 8) |
+                                        TCPOLEN_COOKIE_BASE) +
+                                       cookie_size);
+                }
+
+                if (cookie_size > 0) {
+                        memcpy(ptr, cookie_copy, cookie_size);
+                        ptr += (cookie_size / 4);
+                }
+        }
+
+        if (unlikely(OPTION_SACK_ADVERTISE & options)) {
                 *ptr++ = htonl((TCPOPT_NOP << 24) |
                                (TCPOPT_NOP << 16) |
                                (TCPOPT_SACK_PERM << 8) |
                                TCPOLEN_SACK_PERM);
         }
 
-        if (unlikely(OPTION_WSCALE & opts->options)) {
+        if (unlikely(OPTION_WSCALE & options)) {
                 *ptr++ = htonl((TCPOPT_NOP << 24) |
                                (TCPOPT_WINDOW << 16) |
                                (TCPOLEN_WINDOW << 8) |
@@ -466,14 +551,18 @@ static unsigned tcp_syn_options(struct sock *sk, struct sk_buff *skb,
                                 struct tcp_out_options *opts,
                                 struct tcp_md5sig_key **md5) {
         struct tcp_sock *tp = tcp_sk(sk);
-        unsigned size = 0;
+        struct tcp_cookie_values *cvp = tp->cookie_values;
         struct dst_entry *dst = __sk_dst_get(sk);
+        unsigned remaining = MAX_TCP_OPTION_SPACE;
+        u8 cookie_size = (!tp->rx_opt.cookie_out_never && cvp != NULL) ?
+                         tcp_cookie_size_check(cvp->cookie_desired) :
+                         0;
 
 #ifdef CONFIG_TCP_MD5SIG
         *md5 = tp->af_specific->md5_lookup(sk, sk);
         if (*md5) {
                 opts->options |= OPTION_MD5;
-                size += TCPOLEN_MD5SIG_ALIGNED;
+                remaining -= TCPOLEN_MD5SIG_ALIGNED;
         }
 #else
         *md5 = NULL;
@@ -489,7 +578,7 @@ static unsigned tcp_syn_options(struct sock *sk, struct sk_buff *skb,
          * SACKs don't matter, we never delay an ACK when we have any of those
          * going out.  */
         opts->mss = tcp_advertise_mss(sk);
-        size += TCPOLEN_MSS_ALIGNED;
+        remaining -= TCPOLEN_MSS_ALIGNED;
 
         if (likely(sysctl_tcp_timestamps &&
                    !dst_feature(dst, RTAX_FEATURE_NO_TSTAMP) &&
@@ -497,22 +586,68 @@ static unsigned tcp_syn_options(struct sock *sk, struct sk_buff *skb,
                 opts->options |= OPTION_TS;
                 opts->tsval = TCP_SKB_CB(skb)->when;
                 opts->tsecr = tp->rx_opt.ts_recent;
-                size += TCPOLEN_TSTAMP_ALIGNED;
+                remaining -= TCPOLEN_TSTAMP_ALIGNED;
         }
         if (likely(sysctl_tcp_window_scaling &&
                    !dst_feature(dst, RTAX_FEATURE_NO_WSCALE))) {
                 opts->ws = tp->rx_opt.rcv_wscale;
                 opts->options |= OPTION_WSCALE;
-                size += TCPOLEN_WSCALE_ALIGNED;
+                remaining -= TCPOLEN_WSCALE_ALIGNED;
         }
         if (likely(sysctl_tcp_sack &&
                    !dst_feature(dst, RTAX_FEATURE_NO_SACK))) {
                 opts->options |= OPTION_SACK_ADVERTISE;
                 if (unlikely(!(OPTION_TS & opts->options)))
-                        size += TCPOLEN_SACKPERM_ALIGNED;
+                        remaining -= TCPOLEN_SACKPERM_ALIGNED;
         }
 
-        return size;
+        /* Note that timestamps are required by the specification.
+         *
+         * Odd numbers of bytes are prohibited by the specification, ensuring
+         * that the cookie is 16-bit aligned, and the resulting cookie pair is
+         * 32-bit aligned.
+         */
+        if (*md5 == NULL &&
+            (OPTION_TS & opts->options) &&
+            cookie_size > 0) {
+                int need = TCPOLEN_COOKIE_BASE + cookie_size;
+
+                if (0x2 & need) {
+                        /* 32-bit multiple */
+                        need += 2; /* NOPs */
+
+                        if (need > remaining) {
+                                /* try shrinking cookie to fit */
+                                cookie_size -= 2;
+                                need -= 4;
+                        }
+                }
+                while (need > remaining && TCP_COOKIE_MIN <= cookie_size) {
+                        cookie_size -= 4;
+                        need -= 4;
+                }
+                if (TCP_COOKIE_MIN <= cookie_size) {
+                        opts->options |= OPTION_COOKIE_EXTENSION;
+                        opts->hash_location = (__u8 *)&cvp->cookie_pair[0];
+                        opts->hash_size = cookie_size;
+
+                        /* Remember for future incarnations. */
+                        cvp->cookie_desired = cookie_size;
+
+                        if (cvp->cookie_desired != cvp->cookie_pair_size) {
+                                /* Currently use random bytes as a nonce,
+                                 * assuming these are completely unpredictable
+                                 * by hostile users of the same system.
+                                 */
+                                get_random_bytes(&cvp->cookie_pair[0],
+                                                 cookie_size);
+                                cvp->cookie_pair_size = cookie_size;
+                        }
+
+                        remaining -= need;
+                }
+        }
+        return MAX_TCP_OPTION_SPACE - remaining;
 }
 
 /* Set up TCP options for SYN-ACKs. */
@@ -627,7 +762,6 @@ static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
         struct tcp_out_options opts;
         unsigned tcp_options_size, tcp_header_size;
         struct tcp_md5sig_key *md5;
-        __u8 *md5_hash_location;
         struct tcphdr *th;
         int err;
 
@@ -698,7 +832,7 @@ static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
                 }
         }
 
-        tcp_options_write((__be32 *)(th + 1), tp, &opts, &md5_hash_location);
+        tcp_options_write((__be32 *)(th + 1), tp, &opts);
         if (likely((tcb->flags & TCPCB_FLAG_SYN) == 0))
                 TCP_ECN_send(sk, skb, tcp_header_size);
 
@@ -706,7 +840,7 @@ static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
         /* Calculate the MD5 hash, as we have all we need now */
         if (md5) {
                 sk->sk_route_caps &= ~NETIF_F_GSO_MASK;
-                tp->af_specific->calc_md5_hash(md5_hash_location,
+                tp->af_specific->calc_md5_hash(opts.hash_location,
                                                md5, sk, NULL, skb);
         }
 #endif
@@ -2230,14 +2364,13 @@ struct sk_buff *tcp_make_synack(struct sock *sk, struct dst_entry *dst,
                                 struct request_sock *req,
                                 struct request_values *rvp)
 {
+        struct tcp_out_options opts;
         struct inet_request_sock *ireq = inet_rsk(req);
         struct tcp_sock *tp = tcp_sk(sk);
         struct tcphdr *th;
-        int tcp_header_size;
-        struct tcp_out_options opts;
         struct sk_buff *skb;
         struct tcp_md5sig_key *md5;
-        __u8 *md5_hash_location;
+        int tcp_header_size;
         int mss;
 
         skb = sock_wmalloc(sk, MAX_TCP_HEADER + 15, 1, GFP_ATOMIC);
@@ -2298,14 +2431,14 @@ struct sk_buff *tcp_make_synack(struct sock *sk, struct dst_entry *dst,
 
         /* RFC1323: The window in SYN & SYN/ACK segments is never scaled. */
         th->window = htons(min(req->rcv_wnd, 65535U));
-        tcp_options_write((__be32 *)(th + 1), tp, &opts, &md5_hash_location);
+        tcp_options_write((__be32 *)(th + 1), tp, &opts);
         th->doff = (tcp_header_size >> 2);
         TCP_INC_STATS(sock_net(sk), TCP_MIB_OUTSEGS);
 
 #ifdef CONFIG_TCP_MD5SIG
         /* Okay, we have all we need - do the md5 hash if needed */
         if (md5) {
-                tcp_rsk(req)->af_specific->calc_md5_hash(md5_hash_location,
+                tcp_rsk(req)->af_specific->calc_md5_hash(opts.hash_location,
                                                md5, NULL, req, skb);
         }
 #endif