netdev - Re: [PATCH bpf-next 1/2] bpf/flow_dissector: add mode to enforce global BPF flow dissector

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Message-ID: <CAEf4BzZuEChOL828F91wLxUr3h2yfAkZvhsyoSx18uSFSxOtqw@mail.gmail.com>
Date:   Wed, 2 Oct 2019 16:29:39 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Stanislav Fomichev <sdf@...gle.com>
Cc:     Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Petar Penkov <ppenkov@...gle.com>
Subject: Re: [PATCH bpf-next 1/2] bpf/flow_dissector: add mode to enforce
 global BPF flow dissector

On Wed, Oct 2, 2019 at 10:35 AM Stanislav Fomichev <sdf@...gle.com> wrote:
>
> Always use init_net flow dissector BPF program if it's attached and fall
> back to the per-net namespace one. Also, deny installing new programs if
> there is already one attached to the root namespace.
> Users can still detach their BPF programs, but can't attach any
> new ones (-EPERM).
>
> Cc: Petar Penkov <ppenkov@...gle.com>
> Signed-off-by: Stanislav Fomichev <sdf@...gle.com>
> ---
>  Documentation/bpf/prog_flow_dissector.rst |  3 +++
>  net/core/flow_dissector.c                 | 11 ++++++++++-
>  2 files changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/bpf/prog_flow_dissector.rst b/Documentation/bpf/prog_flow_dissector.rst
> index a78bf036cadd..4d86780ab0f1 100644
> --- a/Documentation/bpf/prog_flow_dissector.rst
> +++ b/Documentation/bpf/prog_flow_dissector.rst
> @@ -142,3 +142,6 @@ BPF flow dissector doesn't support exporting all the metadata that in-kernel
>  C-based implementation can export. Notable example is single VLAN (802.1Q)
>  and double VLAN (802.1AD) tags. Please refer to the ``struct bpf_flow_keys``
>  for a set of information that's currently can be exported from the BPF context.
> +
> +When BPF flow dissector is attached to the root network namespace (machine-wide
> +policy), users can't override it in their child network namespaces.
> diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
> index 7c09d87d3269..494e2016fe84 100644
> --- a/net/core/flow_dissector.c
> +++ b/net/core/flow_dissector.c
> @@ -115,6 +115,11 @@ int skb_flow_dissector_bpf_prog_attach(const union bpf_attr *attr,
>         struct bpf_prog *attached;
>         struct net *net;
>
> +       if (rcu_access_pointer(init_net.flow_dissector_prog)) {
> +               /* Can't override root flow dissector program */
> +               return -EPERM;
> +       }

This is racy, shouldn't this be checked after grabbing a lock below?

> +
>         net = current->nsproxy->net_ns;
>         mutex_lock(&flow_dissector_mutex);
>         attached = rcu_dereference_protected(net->flow_dissector_prog,
> @@ -910,7 +915,11 @@ bool __skb_flow_dissect(const struct net *net,
>         WARN_ON_ONCE(!net);
>         if (net) {
>                 rcu_read_lock();
> -               attached = rcu_dereference(net->flow_dissector_prog);
> +               attached =
> +                       rcu_dereference(init_net.flow_dissector_prog);
> +
> +               if (!attached)
> +                       attached = rcu_dereference(net->flow_dissector_prog);
>
>                 if (attached) {
>                         struct bpf_flow_keys flow_keys;
> --
> 2.23.0.444.g18eeb5a265-goog
>