netdev - [PATCH net-next 5/9] ipv6: annotate data races around np->min

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Date:   Thu, 21 Oct 2021 09:22:49 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>
Subject: [PATCH net-next 5/9] ipv6: annotate data races around np->min_hopcount

From: Eric Dumazet <edumazet@...gle.com>

No report yet from KCSAN, yet worth documenting the races.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 net/ipv6/ipv6_sockglue.c | 5 ++++-
 net/ipv6/tcp_ipv6.c      | 6 ++++--
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/net/ipv6/ipv6_sockglue.c b/net/ipv6/ipv6_sockglue.c
index e4bdb09c558670f342f1abad5dfd8252f497aa68..9c3d28764b5c3a47a73491ea5d656867ece4fed2 100644
--- a/net/ipv6/ipv6_sockglue.c
+++ b/net/ipv6/ipv6_sockglue.c
@@ -950,7 +950,10 @@ static int do_ipv6_setsockopt(struct sock *sk, int level, int optname,
 			goto e_inval;
 		if (val < 0 || val > 255)
 			goto e_inval;
-		np->min_hopcount = val;
+		/* tcp_v6_err() and tcp_v6_rcv() might read min_hopcount
+		 * while we are changing it.
+		 */
+		WRITE_ONCE(np->min_hopcount, val);
 		retv = 0;
 		break;
 	case IPV6_DONTFRAG:
diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index 360c79c8e3099e54d125d454b7f5eb406678c91f..2247f525364b16e89afedbec8f4ec3367bf88aa8 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -414,7 +414,8 @@ static int tcp_v6_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
 	if (sk->sk_state == TCP_CLOSE)
 		goto out;
 
-	if (ipv6_hdr(skb)->hop_limit < tcp_inet6_sk(sk)->min_hopcount) {
+	/* min_hopcount can be changed concurrently from do_ipv6_setsockopt() */
+	if (ipv6_hdr(skb)->hop_limit < READ_ONCE(tcp_inet6_sk(sk)->min_hopcount)) {
 		__NET_INC_STATS(net, LINUX_MIB_TCPMINTTLDROP);
 		goto out;
 	}
@@ -1723,7 +1724,8 @@ INDIRECT_CALLABLE_SCOPE int tcp_v6_rcv(struct sk_buff *skb)
 			return 0;
 		}
 	}
-	if (hdr->hop_limit < tcp_inet6_sk(sk)->min_hopcount) {
+	/* min_hopcount can be changed concurrently from do_ipv6_setsockopt() */
+	if (hdr->hop_limit < READ_ONCE(tcp_inet6_sk(sk)->min_hopcount)) {
 		__NET_INC_STATS(net, LINUX_MIB_TCPMINTTLDROP);
 		goto discard_and_relse;
 	}
-- 
2.33.0.1079.g6e70778dc9-goog