tcp: fix stale per-CPU tcp_tw_isn leak enabling ISN prediction

Blamed commit moved the TIME_WAIT-derived ISN from the skb control
block to a per-CPU variable, assuming the value would always be consumed
by tcp_conn_request() for the same packet that wrote it. That assumption
is violated by multiple drop paths between the producer
(__this_cpu_write(tcp_tw_isn, isn) in tcp_v{4,6}_rcv()) and the consumer
(tcp_conn_request()):

 - min_ttl / min_hopcount check
 - xfrm policy check
 - tcp_inbound_hash() MD5/AO mismatch
 - tcp_filter() eBPF/SO_ATTACH_FILTER drop
 - th->syn && th->fin discard in tcp_rcv_state_process() TCP_LISTEN
 - psp_sk_rx_policy_check() in tcp_v{4,6}_do_rcv()
 - tcp_checksum_complete() in tcp_v{4,6}_do_rcv()
 - tcp_v{4,6}_cookie_check() returning NULL

When a packet is dropped on any of these paths, tcp_tw_isn is left set.

The next SYN processed on the same CPU then consumes the non zero value in
tcp_conn_request(), receiving a potentially predictable ISN.

This patch moves back tcp_tw_isn to skb->cb[], getting rid of the per-cpu
variable.

Note that tcp_v{4,6}_fill_cb() do not set it.

Very litle impact on overall code size/complexity:

$ scripts/bloat-o-meter -t vmlinux.old vmlinux.new
add/remove: 0/0 grow/shrink: 2/1 up/down: 8/-15 (-7)
Function                                     old     new   delta
tcp_v6_rcv                                  3038    3042      +4
tcp_v4_rcv                                  3035    3039      +4
tcp_conn_request                            2938    2923     -15
Total: Before=24436060, After=24436053, chg -0.00%

Fixes: 41eecbd712b7 ("tcp: replace TCP_SKB_CB(skb)->tcp_tw_isn with a per-cpu field")
Reported-by: Chris Mason <clm@meta.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260519084611.2485277-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

3 files changed, 8 insertions, 13 deletions

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 432fa28e47d4..389a7cc17110 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -299,9 +299,6 @@ enum {
 DEFINE_PER_CPU(unsigned int, tcp_orphan_count);
 EXPORT_PER_CPU_SYMBOL_GPL(tcp_orphan_count);
 
-DEFINE_PER_CPU(u32, tcp_tw_isn);
-EXPORT_PER_CPU_SYMBOL_GPL(tcp_tw_isn);
-
 long sysctl_tcp_mem[3] __read_mostly;
 
 DEFINE_PER_CPU(int, tcp_memory_per_cpu_fw_alloc);
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index d5c9e65d9760..de9f68a9c0cf 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -7589,6 +7589,7 @@ int tcp_conn_request(struct request_sock_ops *rsk_ops,
 		     struct sock *sk, struct sk_buff *skb)
 {
 	struct tcp_fastopen_cookie foc = { .len = -1 };
+	u32 isn = TCP_SKB_CB(skb)->tcp_tw_isn;
 	struct tcp_options_received tmp_opt;
 	const struct tcp_sock *tp = tcp_sk(sk);
 	struct net *net = sock_net(sk);
@@ -7599,20 +7600,16 @@ int tcp_conn_request(struct request_sock_ops *rsk_ops,
 	struct dst_entry *dst;
 	struct flowi fl;
 	u8 syncookies;
-	u32 isn;
 
 #ifdef CONFIG_TCP_AO
 	const struct tcp_ao_hdr *aoh;
 #endif
 
-	isn = __this_cpu_read(tcp_tw_isn);
-	if (isn) {
-		/* TW buckets are converted to open requests without
-		 * limitations, they conserve resources and peer is
-		 * evidently real one.
-		 */
-		__this_cpu_write(tcp_tw_isn, 0);
-	} else {
+	/* If isn is non-zero, this SYN originally matched a TIME_WAIT socket.
+	 * TW sockets are converted to open requests without limitations,
+	 * we skip the queue limits and syncookie checks in the block below.
+	 */
+	if (!isn) {
 		syncookies = READ_ONCE(net->ipv4.sysctl_tcp_syncookies);
 
 		if (syncookies == 2 || inet_csk_reqsk_queue_is_full(sk)) {
diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index c0526cc03980..fdc81150ff6c 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -2198,6 +2198,7 @@ lookup:
 		}
 	}
 
+	isn = 0;
 process:
 	if (static_branch_unlikely(&ip4_min_ttl)) {
 		/* min_ttl can be changed concurrently from do_ip_setsockopt() */
@@ -2227,6 +2228,7 @@ process:
 	th = (const struct tcphdr *)skb->data;
 	iph = ip_hdr(skb);
 	tcp_v4_fill_cb(skb, iph, th);
+	TCP_SKB_CB(skb)->tcp_tw_isn = isn;
 
 	skb->dev = NULL;
 
@@ -2313,7 +2315,6 @@ do_time_wait:
 			sk = sk2;
 			tcp_v4_restore_cb(skb);
 			refcounted = false;
-			__this_cpu_write(tcp_tw_isn, isn);
 			goto process;
 		}