netdev - Re: [PATCH v2 net-next 06/15] ipv6: mcast: Don't hold RTNL for IPV6_ADD_MEMBERSHIP and MCAST_JOIN

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Message-ID: <CANn89i+aMsdJ+oQmo1y4sbjJM40NJpFNYn2YT5Gf1WrfMc1nOg@mail.gmail.com>
Date: Thu, 26 Jun 2025 07:36:58 -0700
From: Eric Dumazet <edumazet@...gle.com>
To: Kuniyuki Iwashima <kuni1840@...il.com>
Cc: "David S. Miller" <davem@...emloft.net>, David Ahern <dsahern@...nel.org>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, 
	Kuniyuki Iwashima <kuniyu@...gle.com>, netdev@...r.kernel.org
Subject: Re: [PATCH v2 net-next 06/15] ipv6: mcast: Don't hold RTNL for
 IPV6_ADD_MEMBERSHIP and MCAST_JOIN_GROUP.

On Tue, Jun 24, 2025 at 1:26 PM Kuniyuki Iwashima <kuni1840@...il.com> wrote:
>
> From: Kuniyuki Iwashima <kuniyu@...gle.com>
>
> 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.
>
> Signed-off-by: Kuniyuki Iwashima <kuniyu@...gle.com>
> ---
> v2: Hold rcu_read_lock() around rt6_lookup & dev_hold()
> ---
>  net/ipv6/ipv6_sockglue.c |  2 --
>  net/ipv6/mcast.c         | 22 ++++++++++++----------
>  2 files changed, 12 insertions(+), 12 deletions(-)
>
> diff --git a/net/ipv6/ipv6_sockglue.c b/net/ipv6/ipv6_sockglue.c
> index 1e225e6489ea..cb0dc885cbe4 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 b3f063b5ffd7..9fc7672926bf 100644
> --- 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;

We probably need safety here, READ_ONCE() at minimum.

This can probably be done in a separate series.

Reviewed-by: Eric Dumazet <edumazet@...gle.com>