From 54d574d4afd9de820c8a017665c8095f0f5fdbaa Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Mon, 12 Oct 2026 12:10:29 +0200 Subject: [PATCH] tcp: use the echoed source route in cookie_v4_check() When the ACK carries a source route option, cookie_v4_check() routes the new socket to opt->faddr, where opt points to the options of the received packet. ip_options_compile() only fills faddr for options given by setsockopt() (when called without an skb), so on receive faddr is 0 and the lookup is done for daddr 0, which gives a local route. Use the options echoed by tcp_v4_save_options() instead, as inet_csk_route_req() and inet_csk_route_child_sock() do with ireq->ireq_opt. Read them after security_inet_conn_request(), since CIPSO can replace ireq->ireq_opt there. The route now goes to the first hop of the source route, so also refuse a strict source route through a gateway, as inet_csk_route_req() does. Found by sashiko while reviewing the sk_mc_loop() fix. Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") Signed-off-by: Eric Dumazet Reviewed-by: Kuniyuki Iwashima Link: https://patch.msgid.link/20261012101029.927296-3-edumazet@kernel.org Signed-off-by: Jakub Kicinski --- net/ipv4/syncookies.c | 12 ++++++++++-- 1 file changed, 10 insertions(+), 2 deletions(-) diff --git a/net/ipv4/syncookies.c b/net/ipv4/syncookies.c index 73e129768184..8c9691569e01 100644 --- a/net/ipv4/syncookies.c +++ b/net/ipv4/syncookies.c @@ -404,12 +404,12 @@ static struct request_sock *cookie_tcp_check(struct net *net, struct sock *sk, */ struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb) { - struct ip_options *opt = &TCP_SKB_CB(skb)->header.h4.opt; const struct tcphdr *th = tcp_hdr(skb); struct tcp_sock *tp = tcp_sk(sk); struct inet_request_sock *ireq; struct net *net = sock_net(sk); struct tcp_request_sock *treq; + struct ip_options_rcu *opt; struct request_sock *req; struct sock *ret = sk; struct flowi4 fl4; @@ -449,6 +449,8 @@ struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb) SKB_DR_SET(reason, SECURITY_HOOK); goto out_free; } + /* Load ireq->ireq_opt after the LSM hook, which can replace it. */ + opt = rcu_dereference_protected(ireq->ireq_opt, true); tcp_ao_syncookie(sk, skb, req, AF_INET); @@ -461,7 +463,7 @@ struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb) flowi4_init_output(&fl4, ireq->ir_iif, ireq->ir_mark, ip_sock_rt_tos(sk), ip_sock_rt_scope(sk), IPPROTO_TCP, inet_sk_flowi_flags(sk), - opt->srr ? opt->faddr : ireq->ir_rmt_addr, + (opt && opt->opt.srr) ? opt->opt.faddr : ireq->ir_rmt_addr, ireq->ir_loc_addr, th->source, th->dest, sk_uid(sk)); security_req_classify_flow(req, flowi4_to_flowi_common(&fl4)); @@ -470,6 +472,12 @@ struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb) SKB_DR_SET(reason, IP_OUTNOROUTES); goto out_free; } + /* Same check as in inet_csk_route_req(). */ + if (opt && opt->opt.is_strictroute && rt->rt_uses_gateway) { + ip_rt_put(rt); + SKB_DR_SET(reason, IP_OUTNOROUTES); + goto out_free; + } /* Try to redo what tcp_v4_send_synack did. */ req->rsk_window_clamp = READ_ONCE(tp->window_clamp) ? :