Re: [PATCH] net: bpf: handle return value of BPF_CGROUP_RUN_PROG_INET4_POST_BIND()

[Menglong Dong]

On Thu, Dec 30, 2021 at 5:09 AM Jakub Kicinski <kuba@xxxxxxxxxx> wrote:
>
> On Mon, 27 Dec 2021 14:20:35 +0800 menglong8.dong@xxxxxxxxx wrote:
> > From: Menglong Dong <imagedong@xxxxxxxxxxx>
> >
> > The return value of BPF_CGROUP_RUN_PROG_INET4_POST_BIND() in
> > __inet_bind() is not handled properly. While the return value
> > is non-zero, it will set inet_saddr and inet_rcv_saddr to 0 and
> > exit:
> >
> >       err = BPF_CGROUP_RUN_PROG_INET4_POST_BIND(sk);
> >       if (err) {
> >               inet->inet_saddr = inet->inet_rcv_saddr = 0;
> >               goto out_release_sock;
> >       }
> >
> > Let's take UDP for example and see what will happen. For UDP
> > socket, it will be added to 'udp_prot.h.udp_table->hash' and
> > 'udp_prot.h.udp_table->hash2' after the sk->sk_prot->get_port()
> > called success. If 'inet->inet_rcv_saddr' is specified here,
> > then 'sk' will be in the 'hslot2' of 'hash2' that it don't belong
> > to (because inet_saddr is changed to 0), and UDP packet received
> > will not be passed to this sock. If 'inet->inet_rcv_saddr' is not
> > specified here, the sock will work fine, as it can receive packet
> > properly, which is wired, as the 'bind()' is already failed.
> >
> > I'm not sure what should do here, maybe we should unhash the sock
> > for UDP? Therefor, user can try to bind another port?
>
> Enumarating the L4 unwind paths in L3 code seems like a fairly clear
> layering violation. A new callback to undo ->sk_prot->get_port() may
> be better.

Yeah, it seems there isn't an easier way to solve this problem, a new
callback is needed.

>
> Does IPv6 no need as similar change?
>

IPv6 nedd change too. This patch is just to get some suggestions :/

> You need to provide a selftest to validate the expected behavior.

I'll add it.

Thanks!
Menglong Dong

>
> > diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
> > index 04067b249bf3..9e5710f40a39 100644
> > --- a/net/ipv4/af_inet.c
> > +++ b/net/ipv4/af_inet.c
> > @@ -530,7 +530,14 @@ int __inet_bind(struct sock *sk, struct sockaddr *uaddr, int addr_len,
> >               if (!(flags & BIND_FROM_BPF)) {
> >                       err = BPF_CGROUP_RUN_PROG_INET4_POST_BIND(sk);
> >                       if (err) {
> > +                             if (sk->sk_prot == &udp_prot)
> > +                                     sk->sk_prot->unhash(sk);
> > +                             else if (sk->sk_prot == &tcp_prot)
> > +                                     inet_put_port(sk);
> > +
> >                               inet->inet_saddr = inet->inet_rcv_saddr = 0;
> > +                             err = -EPERM;
> > +
> >                               goto out_release_sock;
> >                       }
> >               }
>