tcp: do not create inetpeer on SYNACK message

Another problem on SYNFLOOD/DDOS attack is the inetpeer cache getting
larger and larger, using lots of memory and cpu time.

tcp_v4_send_synack()
->inet_csk_route_req()
 ->ip_route_output_flow()
  ->rt_set_nexthop()
   ->rt_init_metrics()
    ->inet_getpeer( create = true)

This is a side effect of commit a4daad6b (net: Pre-COW metrics for
TCP) added in 2.6.39

Possible solution :

Instruct inet_csk_route_req() to remove FLOWI_FLAG_PRECOW_METRICS

Before patch :

# grep peer /proc/slabinfo
inet_peer_cache   4175430 4175430    192   42    2 : tunables    0    0    0 : slabdata  99415  99415      0

Samples: 41K of event 'cycles', Event count (approx.): 30716565122
+  20,24%      ksoftirqd/0  [kernel.kallsyms]           [k] inet_getpeer
+   8,19%      ksoftirqd/0  [kernel.kallsyms]           [k] peer_avl_rebalance.isra.1
+   4,81%      ksoftirqd/0  [kernel.kallsyms]           [k] sha_transform
+   3,64%      ksoftirqd/0  [kernel.kallsyms]           [k] fib_table_lookup
+   2,36%      ksoftirqd/0  [ixgbe]                     [k] ixgbe_poll
+   2,16%      ksoftirqd/0  [kernel.kallsyms]           [k] __ip_route_output_key
+   2,11%      ksoftirqd/0  [kernel.kallsyms]           [k] kernel_map_pages
+   2,11%      ksoftirqd/0  [kernel.kallsyms]           [k] ip_route_input_common
+   2,01%      ksoftirqd/0  [kernel.kallsyms]           [k] __inet_lookup_established
+   1,83%      ksoftirqd/0  [kernel.kallsyms]           [k] md5_transform
+   1,75%      ksoftirqd/0  [kernel.kallsyms]           [k] check_leaf.isra.9
+   1,49%      ksoftirqd/0  [kernel.kallsyms]           [k] ipt_do_table
+   1,46%      ksoftirqd/0  [kernel.kallsyms]           [k] hrtimer_interrupt
+   1,45%      ksoftirqd/0  [kernel.kallsyms]           [k] kmem_cache_alloc
+   1,29%      ksoftirqd/0  [kernel.kallsyms]           [k] inet_csk_search_req
+   1,29%      ksoftirqd/0  [kernel.kallsyms]           [k] __netif_receive_skb
+   1,16%      ksoftirqd/0  [kernel.kallsyms]           [k] copy_user_generic_string
+   1,15%      ksoftirqd/0  [kernel.kallsyms]           [k] kmem_cache_free
+   1,02%      ksoftirqd/0  [kernel.kallsyms]           [k] tcp_make_synack
+   0,93%      ksoftirqd/0  [kernel.kallsyms]           [k] _raw_spin_lock_bh
+   0,87%      ksoftirqd/0  [kernel.kallsyms]           [k] __call_rcu
+   0,84%      ksoftirqd/0  [kernel.kallsyms]           [k] rt_garbage_collect
+   0,84%      ksoftirqd/0  [kernel.kallsyms]           [k] fib_rules_lookup
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Hans Schillstrom <hans.schillstrom@ericsson.com>
Cc: Jesper Dangaard Brouer <brouer@redhat.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/inet_connection_sock.c

@@ -377,7 +377,8 @@ struct dst_entry *inet_csk_route_req(struct sock *sk,
 
 	flowi4_init_output(fl4, sk->sk_bound_dev_if, sk->sk_mark,
 			   RT_CONN_FLAGS(sk), RT_SCOPE_UNIVERSE,
-			   sk->sk_protocol, inet_sk_flowi_flags(sk),
+			   sk->sk_protocol,
+			   inet_sk_flowi_flags(sk) & ~FLOWI_FLAG_PRECOW_METRICS,
 			   (opt && opt->opt.srr) ? opt->opt.faddr : ireq->rmt_addr,
 			   ireq->loc_addr, ireq->rmt_port, inet_sk(sk)->inet_sport);
 	security_req_classify_flow(req, flowi4_to_flowi(fl4));