Subject: [PATCH net] tcp: fix race in tcp_v6_syn_recv_sock()

From: Eric Dumazet <[email-masked]>
Date: Thu, 6 Jun 2024 15:46:51 +0000

tcp_v6_syn_recv_sock() calls ip6_dst_store() before
inet_sk(newsk)->pinet6 has been set up.

This means ip6_dst_store() writes over the parent (listener)
np->dst_cookie.

This is racy because multiple threads could share the same
parent and their final np->dst_cookie could be wrong.

Move ip6_dst_store() call after inet_sk(newsk)->pinet6
has been changed and after the copy of parent ipv6_pinfo.

Fixes: e994b2f0fb92 ("tcp: do not lock listener to process SYN packets")
Signed-off-by: Eric Dumazet <[email-masked]>
---
 net/ipv6/tcp_ipv6.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index 8c577b651bfcd2f94b45e339ed4a2b47e93ff17a..729faf8bd366ad25d093a4ae931fb46ebd45b79c 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -1439,7 +1439,6 @@ static struct sock *tcp_v6_syn_recv_sock(const struct sock *sk, struct sk_buff *
 	 */
 
 	newsk->sk_gso_type = SKB_GSO_TCPV6;
-	ip6_dst_store(newsk, dst, NULL, NULL);
 	inet6_sk_rx_dst_set(newsk, skb);
 
 	inet_sk(newsk)->pinet6 = tcp_inet6_sk(newsk);
@@ -1450,6 +1449,8 @@ static struct sock *tcp_v6_syn_recv_sock(const struct sock *sk, struct sk_buff *
 
 	memcpy(newnp, np, sizeof(struct ipv6_pinfo));
 
+	ip6_dst_store(newsk, dst, NULL, NULL);
+
 	newsk->sk_v6_daddr = ireq->ir_v6_rmt_addr;
 	newnp->saddr = ireq->ir_v6_loc_addr;
 	newsk->sk_v6_rcv_saddr = ireq->ir_v6_loc_addr;
-- 
2.45.1.467.gbab1589fc0-goog