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authorFlorian Westphal <fw@strlen.de>2014-11-03 19:35:01 +0300
committerDavid S. Miller <davem@davemloft.net>2014-11-05 00:06:08 +0300
commit274e2da0ecb4d28e3d4e9f029b096e0e8c401a66 (patch)
tree641b8c19c7f26efddc74839c5a0d0ed15e34de1b /net/ipv4/syncookies.c
parent436f7c206860729d543a457aca5887e52039a5f4 (diff)
downloadlinux-274e2da0ecb4d28e3d4e9f029b096e0e8c401a66.tar.xz
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>
Diffstat (limited to 'net/ipv4/syncookies.c')
-rw-r--r--net/ipv4/syncookies.c50
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);