netdev - Re: [RFC] net: add new socket option SO

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Date:   Thu, 2 Feb 2023 21:10:23 +0100
From:   Eric Dumazet <edumazet@...gle.com>
To:     Alok Tiagi <aloktiagi@...il.com>
Cc:     Hillf Danton <hdanton@...a.com>, ebiederm@...ssion.com,
        netdev@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC] net: add new socket option SO_SETNETNS

On Thu, Feb 2, 2023 at 8:55 PM Alok Tiagi <aloktiagi@...il.com> wrote:
>
> On Thu, Feb 02, 2023 at 09:48:10AM +0800, Hillf Danton wrote:
> > On Wed, 1 Feb 2023 19:22:57 +0000 aloktiagi <aloktiagi@...il.com>
> > > @@ -1535,6 +1535,52 @@ int sk_setsockopt(struct sock *sk, int level, int optname,
> > >             WRITE_ONCE(sk->sk_txrehash, (u8)val);
> > >             break;
> > >
> > > +   case SO_SETNETNS:
> > > +   {
> > > +           struct net *other_ns, *my_ns;
> > > +
> > > +           if (sk->sk_family != AF_INET && sk->sk_family != AF_INET6) {
> > > +                   ret = -EOPNOTSUPP;
> > > +                   break;
> > > +           }
> > > +
> > > +           if (sk->sk_type != SOCK_STREAM && sk->sk_type != SOCK_DGRAM) {
> > > +                   ret = -EOPNOTSUPP;
> > > +                   break;
> > > +           }
> > > +
> > > +           other_ns = get_net_ns_by_fd(val);
> > > +           if (IS_ERR(other_ns)) {
> > > +                   ret = PTR_ERR(other_ns);
> > > +                   break;
> > > +           }
> > > +
> > > +           if (!ns_capable(other_ns->user_ns, CAP_NET_ADMIN)) {
> > > +                   ret = -EPERM;
> > > +                   goto out_err;
> > > +           }
> > > +
> > > +           /* check that the socket has never been connected or recently disconnected */
> > > +           if (sk->sk_state != TCP_CLOSE || sk->sk_shutdown & SHUTDOWN_MASK) {
> > > +                   ret = -EOPNOTSUPP;
> > > +                   goto out_err;
> > > +           }
> > > +
> > > +           /* check that the socket is not bound to an interface*/
> > > +           if (sk->sk_bound_dev_if != 0) {
> > > +                   ret = -EOPNOTSUPP;
> > > +                   goto out_err;
> > > +           }
> > > +
> > > +           my_ns = sock_net(sk);
> > > +           sock_net_set(sk, other_ns);
> > > +           put_net(my_ns);
> > > +           break;
> >
> >               cpu 0                           cpu 2
> >               ---                             ---
> >                                               ns = sock_net(sk);
> >               my_ns = sock_net(sk);
> >               sock_net_set(sk, other_ns);
> >               put_net(my_ns);
> >                                               ns is invalid ?
>
> That is the reason we want the socket to be in an un-connected state. That
> should help us avoid this situation.

This is not enough....

Another thread might look at sock_net(sk), for example from inet_diag
or tcp timers
(which can be fired even in un-connected state)

Even UDP sockets can receive packets while being un-connected,
and they need to deref the net pointer.

Currently there is no protection about sock_net(sk) being changed on the fly,
and the struct net could disappear and be freed.

There are ~1500 uses of sock_net(sk) in the kernel, I do not think
you/we want to audit all
of them to check what could go wrong...