[PATCH 3.16 141/148] tcp: syncookies: extend validity range

Ben Hutchings
2020-02-08T18:31:34Z

From: Eric Dumazet <edumazet@google.com>

commit 264ea103a7473f51aced838e68ed384ea2c759f5 upstream.

Now we allow storing more request socks per listener, we might
hit syncookie mode less often and hit following bug in our stack :

When we send a burst of syncookies, then exit this mode,
tcp_synq_no_recent_overflow() can return false if the ACK packets coming
from clients are coming three seconds after the end of syncookie
episode.

This is a way too strong requirement and conflicts with rest of
syncookie code which allows ACK to be aged up to 2 minutes.

Perfectly valid ACK packets are dropped just because clients might be
in a crowded wifi environment or on another planet.

So let's fix this, and also change tcp_synq_overflow() to not
dirty a cache line for every syncookie we send, as we are under attack.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Florian Westphal <fw@strlen.de>
Acked-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
include/net/tcp.h | 38 ++++++++++++++++++++++++--------------
1 file changed, 24 insertions(+), 14 deletions(-)

--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -326,18 +326,6 @@ static inline bool tcp_too_many_orphans(

bool tcp_check_oom(struct sock *sk, int shift);

-/* syncookies: remember time of last synqueue overflow */
-static inline void tcp_synq_overflow(struct sock *sk)
-{
- tcp_sk(sk)->rx_opt.ts_recent_stamp = jiffies;
-}
-
-/* syncookies: no recent synqueue overflow on this listening socket? */
-static inline bool tcp_synq_no_recent_overflow(const struct sock *sk)
-{
- unsigned long last_overflow = tcp_sk(sk)->rx_opt.ts_recent_stamp;
- return time_after(jiffies, last_overflow + TCP_TIMEOUT_FALLBACK);
-}

extern struct proto tcp_prot;

@@ -485,13 +473,35 @@ struct sock *cookie_v4_check(struct sock
* i.e. a sent cookie is valid only at most for 2*60 seconds (or less if
* the counter advances immediately after a cookie is generated).
*/
-#define MAX_SYNCOOKIE_AGE 2
+#define MAX_SYNCOOKIE_AGE 2
+#define TCP_SYNCOOKIE_PERIOD (60 * HZ)
+#define TCP_SYNCOOKIE_VALID (MAX_SYNCOOKIE_AGE * TCP_SYNCOOKIE_PERIOD)
+
+/* syncookies: remember time of last synqueue overflow
+ * But do not dirty this field too often (once per second is enough)
+ */
+static inline void tcp_synq_overflow(struct sock *sk)
+{
+ unsigned long last_overflow = tcp_sk(sk)->rx_opt.ts_recent_stamp;
+ unsigned long now = jiffies;
+
+ if (time_after(now, last_overflow + HZ))
+ tcp_sk(sk)->rx_opt.ts_recent_stamp = now;
+}
+
+/* syncookies: no recent synqueue overflow on this listening socket? */
+static inline bool tcp_synq_no_recent_overflow(const struct sock *sk)
+{
+ unsigned long last_overflow = tcp_sk(sk)->rx_opt.ts_recent_stamp;
+
+ return time_after(jiffies, last_overflow + TCP_SYNCOOKIE_VALID);
+}

static inline u32 tcp_cookie_time(void)
{
u64 val = get_jiffies_64();

- do_div(val, 60 * HZ);
+ do_div(val, TCP_SYNCOOKIE_PERIOD);
return val;
}