netdev - Re: [PATCH bpf-next v2 1/2] bpf: allow rewriting to ports under ip_unprivileged_port

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Date:   Mon, 25 Jan 2021 16:00:25 -0800
From:   Martin KaFai Lau <kafai@...com>
To:     Stanislav Fomichev <sdf@...gle.com>
CC:     Netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrey Ignatov <rdna@...com>
Subject: Re: [PATCH bpf-next v2 1/2] bpf: allow rewriting to ports under
 ip_unprivileged_port_start

On Mon, Jan 25, 2021 at 03:32:53PM -0800, Stanislav Fomichev wrote:
> On Mon, Jan 25, 2021 at 3:25 PM Martin KaFai Lau <kafai@...com> wrote:
> >
> > On Mon, Jan 25, 2021 at 09:26:40AM -0800, Stanislav Fomichev wrote:
> > > 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.
> > The description requires an update.
> Ah, sure, will update it.
> 
> > [ ... ]
> >
> > > diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
> > > index da649f20d6b2..cdf3c7e611d9 100644
> > > --- a/kernel/bpf/cgroup.c
> > > +++ b/kernel/bpf/cgroup.c
> > > @@ -1055,6 +1055,8 @@ EXPORT_SYMBOL(__cgroup_bpf_run_filter_sk);
> > >   * @uaddr: sockaddr struct provided by user
> > >   * @type: The type of program to be exectuted
> > >   * @t_ctx: Pointer to attach type specific context
> > > + * @flags: Pointer to u32 which contains higher bits of BPF program
> > > + *         return value (OR'ed together).
> > >   *
> > >   * socket is expected to be of type INET or INET6.
> > >   *
> > > @@ -1064,7 +1066,8 @@ EXPORT_SYMBOL(__cgroup_bpf_run_filter_sk);
> > >  int __cgroup_bpf_run_filter_sock_addr(struct sock *sk,
> > >                                     struct sockaddr *uaddr,
> > >                                     enum bpf_attach_type type,
> > > -                                   void *t_ctx)
> > > +                                   void *t_ctx,
> > > +                                   u32 *flags)
> > >  {
> > >       struct bpf_sock_addr_kern ctx = {
> > >               .sk = sk,
> > > @@ -1087,7 +1090,8 @@ int __cgroup_bpf_run_filter_sock_addr(struct sock *sk,
> > >       }
> > >
> > >       cgrp = sock_cgroup_ptr(&sk->sk_cgrp_data);
> > > -     ret = BPF_PROG_RUN_ARRAY(cgrp->bpf.effective[type], &ctx, BPF_PROG_RUN);
> > > +     ret = BPF_PROG_RUN_ARRAY_FLAGS(cgrp->bpf.effective[type], &ctx,
> > > +                                    BPF_PROG_RUN, flags);
> > >
> > >       return ret == 1 ? 0 : -EPERM;
> > >  }
> > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > > index d0eae51b31e4..ef7c3ca53214 100644
> > > --- a/kernel/bpf/verifier.c
> > > +++ b/kernel/bpf/verifier.c
> > > @@ -7986,6 +7986,11 @@ static int check_return_code(struct bpf_verifier_env *env)
> > >                   env->prog->expected_attach_type == BPF_CGROUP_INET4_GETSOCKNAME ||
> > >                   env->prog->expected_attach_type == BPF_CGROUP_INET6_GETSOCKNAME)
> > >                       range = tnum_range(1, 1);
> > > +             if (env->prog->expected_attach_type == BPF_CGROUP_INET4_BIND ||
> > > +                 env->prog->expected_attach_type == BPF_CGROUP_INET6_BIND) {
> > > +                     range = tnum_range(0, 3);
> > > +                     enforce_attach_type_range = tnum_range(0, 3);
> > It should be:
> >                         enforce_attach_type_range = tnum_range(2, 3);
> Hm, weren't we enforcing attach_type for bind progs from the beginning?
Ah, right.  Then there is no need to set enforce_attach_type_range at all.
"enforce_attach_type_range = tnum_range(0, 3);" can be removed.

> Also, looking at bpf_prog_attach_check_attach_type, it seems that we
> care only about BPF_PROG_TYPE_CGROUP_SKB for
> prog->enforce_expected_attach_type.
> Am I missing something?
It is because, from the very beginning, BPF_PROG_TYPE_CGROUP_SKB did not
enforce the attach_type in bpf_prog_attach_check_attach_type().