From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: stable@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	patches@lists.linux.dev,
	Kuniyuki Iwashima <kuniyu@google.com>,
	Eric Dumazet <edumazet@google.com>,
	Jakub Kicinski <kuba@kernel.org>,
	Sasha Levin <sashal@kernel.org>
Subject: [PATCH 6.12 0574/1102] ipv6: mcast: Dont hold RTNL for IPV6_ADD_MEMBERSHIP and MCAST_JOIN_GROUP.
Date: Thu, 17 Sep 2026 16:08:37 +0100
Message-ID: <20260917151554.340117024@linuxfoundation.org>
In-Reply-To: <20260917151539.408551884@linuxfoundation.org>
References: <20260917151539.408551884@linuxfoundation.org>
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6.12-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Kuniyuki Iwashima <kuniyu@google.com>

[ Upstream commit 1767bb2d47b715a106287a8f963d9ec6cbab4e69 ]

In __ipv6_sock_mc_join(), per-socket mld data is protected by lock_sock(),
and only __dev_get_by_index() requires RTNL.

Let's use dev_get_by_index() and drop RTNL for IPV6_ADD_MEMBERSHIP and
MCAST_JOIN_GROUP.

Note that we must call rt6_lookup() and dev_hold() under RCU.

If rt6_lookup() returns an entry from the exception table, dst_dev_put()
could change rt->dev.dst to loopback concurrently, and the original device
could lose the refcount before dev_hold() and unblock device registration.

dst_dev_put() is called from NETDEV_UNREGISTER and synchronize_net() follows
it, so as long as rt6_lookup() and dev_hold() are called within the same
RCU critical section, the dev is alive.

Even if the race happens, they are synchronised by idev->dead and mcast
addresses are cleaned up.

For the racy access to rt->dst.dev, we use dst_dev().

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250702230210.3115355-7-kuni1840@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Stable-dep-of: c073d1b070f1 ("ipv6: mcast: use copy-on-write RCU updates in ip6_mc_source()")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 net/ipv6/ipv6_sockglue.c |  2 --
 net/ipv6/mcast.c         | 24 +++++++++++++-----------
 2 files changed, 13 insertions(+), 13 deletions(-)

diff --git a/net/ipv6/ipv6_sockglue.c b/net/ipv6/ipv6_sockglue.c
index 1e225e6489ea1..cb0dc885cbe40 100644
--- a/net/ipv6/ipv6_sockglue.c
+++ b/net/ipv6/ipv6_sockglue.c
@@ -121,11 +121,9 @@ static bool setsockopt_needs_rtnl(int optname)
 {
 	switch (optname) {
 	case IPV6_ADDRFORM:
-	case IPV6_ADD_MEMBERSHIP:
 	case IPV6_DROP_MEMBERSHIP:
 	case IPV6_JOIN_ANYCAST:
 	case IPV6_LEAVE_ANYCAST:
-	case MCAST_JOIN_GROUP:
 	case MCAST_LEAVE_GROUP:
 	case MCAST_JOIN_SOURCE_GROUP:
 	case MCAST_LEAVE_SOURCE_GROUP:
diff --git a/net/ipv6/mcast.c b/net/ipv6/mcast.c
index c060954c52757..98eef63a3cd6e 100644
--- a/net/ipv6/mcast.c
+++ b/net/ipv6/mcast.c
@@ -172,14 +172,12 @@ static int unsolicited_report_interval(struct inet6_dev *idev)
 static int __ipv6_sock_mc_join(struct sock *sk, int ifindex,
 			       const struct in6_addr *addr, unsigned int mode)
 {
-	struct net_device *dev = NULL;
-	struct ipv6_mc_socklist *mc_lst;
 	struct ipv6_pinfo *np = inet6_sk(sk);
+	struct ipv6_mc_socklist *mc_lst;
 	struct net *net = sock_net(sk);
+	struct net_device *dev = NULL;
 	int err;
 
-	ASSERT_RTNL();
-
 	if (!ipv6_addr_is_multicast(addr))
 		return -EINVAL;
 
@@ -199,13 +197,18 @@ static int __ipv6_sock_mc_join(struct sock *sk, int ifindex,
 
 	if (ifindex == 0) {
 		struct rt6_info *rt;
+
+		rcu_read_lock();
 		rt = rt6_lookup(net, addr, NULL, 0, NULL, 0);
 		if (rt) {
-			dev = rt->dst.dev;
+			dev = dst_dev(&rt->dst);
+			dev_hold(dev);
 			ip6_rt_put(rt);
 		}
-	} else
-		dev = __dev_get_by_index(net, ifindex);
+		rcu_read_unlock();
+	} else {
+		dev = dev_get_by_index(net, ifindex);
+	}
 
 	if (!dev) {
 		sock_kfree_s(sk, mc_lst, sizeof(*mc_lst));
@@ -216,12 +219,11 @@ static int __ipv6_sock_mc_join(struct sock *sk, int ifindex,
 	mc_lst->sfmode = mode;
 	RCU_INIT_POINTER(mc_lst->sflist, NULL);
 
-	/*
-	 *	now add/increase the group membership on the device
-	 */
-
+	/* now add/increase the group membership on the device */
 	err = __ipv6_dev_mc_inc(dev, addr, mode);
 
+	dev_put(dev);
+
 	if (err) {
 		sock_kfree_s(sk, mc_lst, sizeof(*mc_lst));
 		return err;
-- 
2.53.0