Merge branch 'tcp-tcp-challenge-ack-fixes'

Eric Dumazet says:

====================
tcp: tcp challenge ack fixes

syzbot found a typical data-race addressed in the first patch.

While we are at it, second patch makes the global rate limit
per net-ns and disabled by default.
====================

Link: https://lore.kernel.org/r/20220830185656.268523-1-eric.dumazet@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/networking/ip-sysctl.rst

@@ -1035,7 +1035,10 @@ tcp_limit_output_bytes - INTEGER
 tcp_challenge_ack_limit - INTEGER
 	Limits number of Challenge ACK sent per second, as recommended
 	in RFC 5961 (Improving TCP's Robustness to Blind In-Window Attacks)
-	Default: 1000
+	Note that this per netns rate limit can allow some side channel
+	attacks and probably should not be enabled.
+	TCP stack implements per TCP socket limits anyway.
+	Default: INT_MAX (unlimited)
 
 UDP variables
 =============

include/net/netns/ipv4.h

@@ -179,6 +179,8 @@ struct netns_ipv4 {
 	unsigned int sysctl_tcp_fastopen_blackhole_timeout;
 	atomic_t tfo_active_disable_times;
 	unsigned long tfo_active_disable_stamp;
+	u32 tcp_challenge_timestamp;
+	u32 tcp_challenge_count;
 
 	int sysctl_udp_wmem_min;
 	int sysctl_udp_rmem_min;

net/ipv4/tcp_input.c

@@ -3614,12 +3614,9 @@ bool tcp_oow_rate_limited(struct net *net, const struct sk_buff *skb,
 /* RFC 5961 7 [ACK Throttling] */
 static void tcp_send_challenge_ack(struct sock *sk)
 {
-	/* unprotected vars, we dont care of overwrites */
-	static u32 challenge_timestamp;
-	static unsigned int challenge_count;
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct net *net = sock_net(sk);
-	u32 count, now;
+	u32 count, now, ack_limit;
 
 	/* First check our per-socket dupack rate limit. */
 	if (__tcp_oow_rate_limited(net,
@@ -3627,18 +3624,22 @@ static void tcp_send_challenge_ack(struct sock *sk)
 				   &tp->last_oow_ack_time))
 		return;
 
+	ack_limit = READ_ONCE(net->ipv4.sysctl_tcp_challenge_ack_limit);
+	if (ack_limit == INT_MAX)
+		goto send_ack;
+
 	/* Then check host-wide RFC 5961 rate limit. */
 	now = jiffies / HZ;
-	if (now != challenge_timestamp) {
-		u32 ack_limit = READ_ONCE(net->ipv4.sysctl_tcp_challenge_ack_limit);
+	if (now != READ_ONCE(net->ipv4.tcp_challenge_timestamp)) {
 		u32 half = (ack_limit + 1) >> 1;
 
-		challenge_timestamp = now;
-		WRITE_ONCE(challenge_count, half + prandom_u32_max(ack_limit));
+		WRITE_ONCE(net->ipv4.tcp_challenge_timestamp, now);
+		WRITE_ONCE(net->ipv4.tcp_challenge_count, half + prandom_u32_max(ack_limit));
 	}
-	count = READ_ONCE(challenge_count);
+	count = READ_ONCE(net->ipv4.tcp_challenge_count);
 	if (count > 0) {
-		WRITE_ONCE(challenge_count, count - 1);
+		WRITE_ONCE(net->ipv4.tcp_challenge_count, count - 1);
+send_ack:
 		NET_INC_STATS(net, LINUX_MIB_TCPCHALLENGEACK);
 		tcp_send_ack(sk);
 	}

net/ipv4/tcp_ipv4.c

@@ -3139,8 +3139,10 @@ static int __net_init tcp_sk_init(struct net *net)
 	net->ipv4.sysctl_tcp_tso_win_divisor = 3;
 	/* Default TSQ limit of 16 TSO segments */
 	net->ipv4.sysctl_tcp_limit_output_bytes = 16 * 65536;
-	/* rfc5961 challenge ack rate limiting */
-	net->ipv4.sysctl_tcp_challenge_ack_limit = 1000;
+
+	/* rfc5961 challenge ack rate limiting, per net-ns, disabled by default. */
+	net->ipv4.sysctl_tcp_challenge_ack_limit = INT_MAX;
+
 	net->ipv4.sysctl_tcp_min_tso_segs = 2;
 	net->ipv4.sysctl_tcp_tso_rtt_log = 9;  /* 2^9 = 512 usec */
 	net->ipv4.sysctl_tcp_min_rtt_wlen = 300;