[PATCH nf-next v2] netfilter: nft_synproxy: only handle pure SYN and ACK packets

[Nguyen Ngoc Thang <ngocthang2710.1999@gmail.com>]

nft_synproxy treats any segment with SYN set as a client's initial SYN
and any segment with ACK set as the client's final ACK. A SYN-ACK thus
gets answered with a fresh SYN-ACK cookie.

If that reply is routed back to the same host, e.g. to a peer address
covered by an address on lo, it re-enters the input hook, is answered
again, and never stops. Every such SYN starts its own endless loop over
the loopback backlog, so NET_RX softirq keeps the CPU busy and memory
fills with skbs and rtable entries until workqueues stall and the
machine OOMs.

Use the same flag checks as ip(6)t_SYNPROXY: SYN without ACK, FIN or
RST is an initial SYN; ACK without SYN, FIN or RST is the client ACK.
Anything else falls through to the next expression.

Reported-by: syzbot+d7795c8487ca4e20ed88@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=d7795c8487ca4e20ed88
Reported-by: syzbot+d0d2f1a65f45b319d25d@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=d0d2f1a65f45b319d25d
Signed-off-by: Nguyen Ngoc Thang <ngocthang2710.1999@gmail.com>
---
v2:
- Retarget to nf-next (Pablo).
- Drop the Fixes: tag accordingly.

v1: https://lore.kernel.org/all/20261004160615.142456-1-ngocthang2710.1999@gmail.com/

Notes (not for the changelog):

Reproduced in QEMU (x86_64, 2 vCPUs, KASAN+lockdep) with syzbot's C
repro for d7795c8487ca4e20ed88. Minimised: only the nft batch and the
injected SYN matter, which is exactly the d0d2f1a65f45b319d25d program.

The rule is an inet table, base chain on input, with an unconditional
"synproxy" expression. The SYN goes from 172.20.20.187 to .170 via tun;
syzkaller puts 172.20.20.10/24 on lo, so the whole /24 is local and the
SYN-ACK cookie comes back through lo. Instrumented trace:

  synproxy tx .170->.187 in=syz_tun out=lo syn=1 ack=1
  synproxy tx .187->.170 in=lo      out=lo syn=1 ack=1
  synproxy tx .170->.187 in=lo      out=lo syn=1 ack=1
  ...

Before (3fd2ff60f6d6, full C repro):
  - synproxy syn_received ~400k per CPU per netns after 15s
  - rtable 80k -> 295k, skbuff_head_cache >1M, softirq time explodes
  - "BUG: workqueue lockup", then OOM / hung task, VM wedges

After (2 runs x 300s, full C repro):
  - no lockup, hung task or OOM
  - rtable flat (~2.2k), skbuff_head_cache flat (~80k)
  - syn_received grows slowly (~9.5k/300s): real SYNs still answered

ACK|FIN and ACK|RST no longer hit the cookie check (and its NF_DROP);
they continue, as with ip(6)t_SYNPROXY's XT_CONTINUE. The usual
"ct state invalid drop" rule still catches them.

 net/netfilter/nft_synproxy.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/net/netfilter/nft_synproxy.c b/net/netfilter/nft_synproxy.c
index 9ed288c9d168..a580cb7324da 100644
--- a/net/netfilter/nft_synproxy.c
+++ b/net/netfilter/nft_synproxy.c
@@ -53,13 +53,13 @@ static void nft_synproxy_eval_v4(const struct nft_synproxy *priv,
 	struct synproxy_net *snet = synproxy_pernet(net);
 	struct sk_buff *skb = pkt->skb;
 
-	if (tcp->syn) {
+	if (tcp->syn && !(tcp->ack || tcp->fin || tcp->rst)) {
 		/* Initial SYN from client */
 		nft_synproxy_tcp_options(opts, tcp, snet, &info);
 		synproxy_send_client_synack(net, skb, tcp, opts);
 		consume_skb(skb);
 		regs->verdict.code = NF_STOLEN;
-	} else if (tcp->ack) {
+	} else if (tcp->ack && !(tcp->fin || tcp->rst || tcp->syn)) {
 		/* ACK from client */
 		if (synproxy_recv_client_ack(net, skb, tcp, opts,
 					     ntohl(tcp->seq))) {
@@ -84,13 +84,13 @@ static void nft_synproxy_eval_v6(const struct nft_synproxy *priv,
 	struct synproxy_net *snet = synproxy_pernet(net);
 	struct sk_buff *skb = pkt->skb;
 
-	if (tcp->syn) {
+	if (tcp->syn && !(tcp->ack || tcp->fin || tcp->rst)) {
 		/* Initial SYN from client */
 		nft_synproxy_tcp_options(opts, tcp, snet, &info);
 		synproxy_send_client_synack_ipv6(net, skb, tcp, opts);
 		consume_skb(skb);
 		regs->verdict.code = NF_STOLEN;
-	} else if (tcp->ack) {
+	} else if (tcp->ack && !(tcp->fin || tcp->rst || tcp->syn)) {
 		/* ACK from client */
 		if (synproxy_recv_client_ack_ipv6(net, skb, tcp, opts,
 						  ntohl(tcp->seq))) {
-- 
2.43.0