syncookies: avoid magic values and document which-bit-is-what-option

Was a bit more difficult to read than needed due to magic shifts;
add defines and document the used encoding scheme.

Joint work with Daniel Borkmann.

Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 35 insertions, 15 deletions

diff --git a/net/ipv4/syncookies.c b/net/ipv4/syncookies.c
index 4ac7bcaf2f46..c3792c0557dd 100644
--- a/net/ipv4/syncookies.c
+++ b/net/ipv4/syncookies.c
@@ -19,10 +19,6 @@
 #include <net/tcp.h>
 #include <net/route.h>
 
-/* Timestamps: lowest bits store TCP options */
-#define TSBITS 6
-#define TSMASK (((__u32)1 << TSBITS) - 1)
-
 extern int sysctl_tcp_syncookies;
 
 static u32 syncookie_secret[2][16-4+SHA_DIGEST_WORDS] __read_mostly;
@@ -30,6 +26,30 @@ static u32 syncookie_secret[2][16-4+SHA_DIGEST_WORDS] __read_mostly;
 #define COOKIEBITS 24   /* Upper bits store count */
 #define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1)
 
+/* TCP Timestamp: 6 lowest bits of timestamp sent in the cookie SYN-ACK
+ * stores TCP options:
+ *
+ * MSB                               LSB
+ * | 31 ...   6 |  5  |  4   | 3 2 1 0 |
+ * |  Timestamp | ECN | SACK | WScale  |
+ *
+ * When we receive a valid cookie-ACK, we look at the echoed tsval (if
+ * any) to figure out which TCP options we should use for the rebuilt
+ * connection.
+ *
+ * A WScale setting of '0xf' (which is an invalid scaling value)
+ * means that original syn did not include the TCP window scaling option.
+ */
+#define TS_OPT_WSCALE_MASK      0xf
+#define TS_OPT_SACK             BIT(4)
+#define TS_OPT_ECN              BIT(5)
+/* There is no TS_OPT_TIMESTAMP:
+ * if ACK contains timestamp option, we already know it was
+ * requested/supported by the syn/synack exchange.
+ */
+#define TSBITS  6
+#define TSMASK  (((__u32)1 << TSBITS) - 1)
+
 static DEFINE_PER_CPU(__u32 [16 + 5 + SHA_WORKSPACE_WORDS],
                       ipv4_cookie_scratch);
 
@@ -67,9 +87,11 @@ __u32 cookie_init_timestamp(struct request_sock *req)
 
         ireq = inet_rsk(req);
 
-        options = ireq->wscale_ok ? ireq->snd_wscale : 0xf;
-        options |= ireq->sack_ok << 4;
-        options |= ireq->ecn_ok << 5;
+        options = ireq->wscale_ok ? ireq->snd_wscale : TS_OPT_WSCALE_MASK;
+        if (ireq->sack_ok)
+                options |= TS_OPT_SACK;
+        if (ireq->ecn_ok)
+                options |= TS_OPT_ECN;
 
         ts = ts_now & ~TSMASK;
         ts |= options;
@@ -219,16 +241,13 @@ static inline struct sock *get_cookie_sock(struct sock *sk, struct sk_buff *skb,
  * additional tcp options in the timestamp.
  * This extracts these options from the timestamp echo.
  *
- * The lowest 4 bits store snd_wscale.
- * next 2 bits indicate SACK and ECN support.
- *
  * return false if we decode an option that should not be.
  */
 bool cookie_check_timestamp(struct tcp_options_received *tcp_opt,
                         struct net *net, bool *ecn_ok)
 {
         /* echoed timestamp, lowest bits contain options */
-        u32 options = tcp_opt->rcv_tsecr & TSMASK;
+        u32 options = tcp_opt->rcv_tsecr;
 
         if (!tcp_opt->saw_tstamp)  {
                 tcp_clear_options(tcp_opt);
@@ -238,19 +257,20 @@ bool cookie_check_timestamp(struct tcp_options_received *tcp_opt,
         if (!sysctl_tcp_timestamps)
                 return false;
 
-        tcp_opt->sack_ok = (options & (1 << 4)) ? TCP_SACK_SEEN : 0;
-        *ecn_ok = (options >> 5) & 1;
+        tcp_opt->sack_ok = (options & TS_OPT_SACK) ? TCP_SACK_SEEN : 0;
+        *ecn_ok = options & TS_OPT_ECN;
         if (*ecn_ok && !net->ipv4.sysctl_tcp_ecn)
                 return false;
 
         if (tcp_opt->sack_ok && !sysctl_tcp_sack)
                 return false;
 
-        if ((options & 0xf) == 0xf)
+        if ((options & TS_OPT_WSCALE_MASK) == TS_OPT_WSCALE_MASK)
                 return true; /* no window scaling */
 
         tcp_opt->wscale_ok = 1;
-        tcp_opt->snd_wscale = options & 0xf;
+        tcp_opt->snd_wscale = options & TS_OPT_WSCALE_MASK;
+
         return sysctl_tcp_window_scaling != 0;
 }
 EXPORT_SYMBOL(cookie_check_timestamp);