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

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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>
This commit is contained in:
Kuniyuki Iwashima 2025-07-02 16:01:23 -07:00 committed by Jakub Kicinski
parent e01b193e0b
commit 1767bb2d47
2 changed files with 13 additions and 13 deletions
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2
net/ipv6/ipv6_sockglue.c
View File

@ -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:

24
net/ipv6/mcast.c
View File

@ -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;