tcp: make challenge acks less predictable

Yue Cao claims that current host rate limiting of challenge ACKS
(RFC 5961) could leak enough information to allow a patient attacker
to hijack TCP sessions. He will soon provide details in an academic
paper.

This patch increases the default limit from 100 to 1000, and adds
some randomization so that the attacker can no longer hijack
sessions without spending a considerable amount of probes.

Based on initial analysis and patch from Linus.

Note that we also have per socket rate limiting, so it is tempting
to remove the host limit in the future.

v2: randomize the count of challenge acks per second, not the period.

Fixes: 282f23c6ee34 ("tcp: implement RFC 5961 3.2")
Reported-by: Yue Cao <ycao009@ucr.edu>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
static bool tcp_should_expand_sndbuf(const struct sock *sk)
{
	const struct tcp_sock *tp = tcp_sk(sk);

	/* If the user specified a specific send buffer setting, do
	 * not modify it.
	 */
	if (sk->sk_userlocks & SOCK_SNDBUF_LOCK)
		return false;

	/* If we are under global TCP memory pressure, do not expand.  */
	if (tcp_under_memory_pressure(sk))
		return false;

	/* If we are under soft global TCP memory pressure, do not expand.  */
	if (sk_memory_allocated(sk) >= sk_prot_mem_limits(sk, 0))
		return false;

	/* If we filled the congestion window, do not expand.  */
	if (tcp_packets_in_flight(tp) >= tp->snd_cwnd)
		return false;

	return true;
}

static bool tcp_should_expand_sndbuf(const struct sock *sk)
{
	const struct tcp_sock *tp = tcp_sk(sk);

	/* If the user specified a specific send buffer setting, do
	 * not modify it.
	 */
	if (sk->sk_userlocks & SOCK_SNDBUF_LOCK)
		return false;

	/* If we are under global TCP memory pressure, do not expand.  */
	if (tcp_under_memory_pressure(sk))
		return false;

	/* If we are under soft global TCP memory pressure, do not expand.  */
	if (sk_memory_allocated(sk) >= sk_prot_mem_limits(sk, 0))
		return false;

	/* If we filled the congestion window, do not expand.  */
	if (tcp_packets_in_flight(tp) >= tp->snd_cwnd)
		return false;

	return true;
}
