ipv6: mcast: Don't hold RTNL for IPV6_ADD_MEMBERSHIP and MCAST_JOIN_GROUP.

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>

diff --git a/net/ipv6/ipv6_sockglue.c b/net/ipv6/ipv6_sockglue.c
index 1e225e6489ea150c641ed0757083591def08dfc1..cb0dc885cbe404d051cc8ad797d015bb6c94d74e 100644 (file)
--- 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 b3f063b5ffd7d895f371a4ecac053f4f8a882b86..d55c1cb4189a4aee3e4c58a039c219dbac3720af 100644 (file)
--- a/net/ipv6/mcast.c
+++ b/net/ipv6/mcast.c
@@ -175,14 +175,12 @@ 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;
 
@@ -202,13 +200,18 @@ 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));
@@ -219,12 +222,11 @@ 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;