Re: [PATCH bpf-next 1/2] bpf: allow rewriting to ports under ip_unprivileged_port_start

From: Stanislav Fomichev <sdf@google.com>
To: Andrey Ignatov <rdna@fb.com>
Cc: Martin KaFai Lau <kafai@fb.com>, Netdev <netdev@vger.kernel.org>,
	bpf <bpf@vger.kernel.org>, Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>
Subject: Re: [PATCH bpf-next 1/2] bpf: allow rewriting to ports under ip_unprivileged_port_start
Date: Fri, 22 Jan 2021 11:53:51 -0800	[thread overview]
Message-ID: <CAKH8qBumq7cHDeCpvA1T_rJyvY8+9uCUyb--YAhvcAx3p58faw@mail.gmail.com> (raw)
In-Reply-To: <YAspc5rk2sNWojDQ@rdna-mbp.dhcp.thefacebook.com>

On Fri, Jan 22, 2021 at 11:37 AM Andrey Ignatov <rdna@fb.com> wrote:
>
> Stanislav Fomichev <sdf@google.com> [Wed, 2021-01-20 18:09 -0800]:
> > At the moment, BPF_CGROUP_INET{4,6}_BIND hooks can rewrite user_port
> > to the privileged ones (< ip_unprivileged_port_start), but it will
> > be rejected later on in the __inet_bind or __inet6_bind.
> >
> > Let's export 'port_changed' event from the BPF program and bypass
> > ip_unprivileged_port_start range check when we've seen that
> > the program explicitly overrode the port. This is accomplished
> > by generating instructions to set ctx->port_changed along with
> > updating ctx->user_port.
> >
> > Signed-off-by: Stanislav Fomichev <sdf@google.com>
> > ---
> ...
> > @@ -244,17 +245,27 @@ int bpf_percpu_cgroup_storage_update(struct bpf_map *map, void *key,
> >       if (cgroup_bpf_enabled(type))   {                                      \
> >               lock_sock(sk);                                                 \
> >               __ret = __cgroup_bpf_run_filter_sock_addr(sk, uaddr, type,     \
> > -                                                       t_ctx);              \
> > +                                                       t_ctx, NULL);        \
> >               release_sock(sk);                                              \
> >       }                                                                      \
> >       __ret;                                                                 \
> >  })
> >
> > -#define BPF_CGROUP_RUN_PROG_INET4_BIND_LOCK(sk, uaddr)                              \
> > -     BPF_CGROUP_RUN_SA_PROG_LOCK(sk, uaddr, BPF_CGROUP_INET4_BIND, NULL)
> > -
> > -#define BPF_CGROUP_RUN_PROG_INET6_BIND_LOCK(sk, uaddr)                              \
> > -     BPF_CGROUP_RUN_SA_PROG_LOCK(sk, uaddr, BPF_CGROUP_INET6_BIND, NULL)
> > +#define BPF_CGROUP_RUN_PROG_INET_BIND_LOCK(sk, uaddr, type, flags)          \
> > +({                                                                          \
> > +     bool port_changed = false;                                             \
>
> I see the discussion with Martin in [0] on the program overriding the
> port but setting exactly same value as it already contains. Commenting
> on this patch since the code is here.
>
> From what I understand there is no use-case to support overriding the
> port w/o changing the value to just bypass the capability. In this case
> the code can be simplified.
>
> Here instead of introducing port_changed you can just remember the
> original ((struct sockaddr_in *)uaddr)->sin_port or
> ((struct sockaddr_in6 *)uaddr)->sin6_port (they have same offset/size so
> it can be simplified same way as in sock_addr_convert_ctx_access() for
> user_port) ...
>
> > +     int __ret = 0;                                                         \
> > +     if (cgroup_bpf_enabled(type))   {                                      \
> > +             lock_sock(sk);                                                 \
> > +             __ret = __cgroup_bpf_run_filter_sock_addr(sk, uaddr, type,     \
> > +                                                       NULL,                \
> > +                                                       &port_changed);      \
> > +             release_sock(sk);                                              \
> > +             if (port_changed)                                              \
>
> ... and then just compare the original and the new ports here.
>
> The benefits will be:
> * no need to introduce port_changed field in struct bpf_sock_addr_kern;
> * no need to do change program instructions;
> * no need to think about compiler optimizing out those instructions;
> * no need to think about multiple programs coordination, the flag will
>   be set only if port has actually changed what is easy to reason about
>   from user perspective.
>
> wdyt?
Martin mentioned in another email that we might want to do that when
we rewrite only the address portion of it.
I think it makes sense. Imagine doing 1.1.1.1:50 -> 2.2.2.2:50 it
seems like it should also work, right?
And in this case, we need to store and compare addresses as well and
it becomes messy :-/
It also seems like it would be nice to have this 'bypass
cap_net_bind_service" without changing the address while we are at it.