Re: [PATCH bpf] flow_dissector: Drop BPF flow dissector prog ref on netns cleanup

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Wed, May 20, 2020 at 10:24 AM Jakub Sitnicki <jakub@cloudflare.com> wrote:
>
> When attaching a flow dissector program to a network namespace with
> bpf(BPF_PROG_ATTACH, ...) we grab a reference to bpf_prog.
>
> If netns gets destroyed while a flow dissector is still attached, and there
> are no other references to the prog, we leak the reference and the program
> remains loaded.
>
> Leak can be reproduced by running flow dissector tests from selftests/bpf:
>
>   # bpftool prog list
>   # ./test_flow_dissector.sh
>   ...
>   selftests: test_flow_dissector [PASS]
>   # bpftool prog list
>   4: flow_dissector  name _dissect  tag e314084d332a5338  gpl
>           loaded_at 2020-05-20T18:50:53+0200  uid 0
>           xlated 552B  jited 355B  memlock 4096B  map_ids 3,4
>           btf_id 4
>   #
>
> Fix it by detaching the flow dissector program when netns is going away.
>
> Fixes: d58e468b1112 ("flow_dissector: implements flow dissector BPF hook")
> Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
> ---
>
> Discovered while working on bpf_link support for netns-attached progs.
> Looks like bpf tree material so pushing it out separately.
>
> -jkbs
>

[...]

>  /**
>   * __skb_flow_get_ports - extract the upper layer ports and return them
>   * @skb: sk_buff to extract the ports from
> @@ -1827,6 +1848,8 @@ EXPORT_SYMBOL(flow_keys_basic_dissector);
>
>  static int __init init_default_flow_dissectors(void)
>  {
> +       int err;
> +
>         skb_flow_dissector_init(&flow_keys_dissector,
>                                 flow_keys_dissector_keys,
>                                 ARRAY_SIZE(flow_keys_dissector_keys));
> @@ -1836,7 +1859,11 @@ static int __init init_default_flow_dissectors(void)
>         skb_flow_dissector_init(&flow_keys_basic_dissector,
>                                 flow_keys_basic_dissector_keys,
>                                 ARRAY_SIZE(flow_keys_basic_dissector_keys));
> -       return 0;
> +
> +       err = register_pernet_subsys(&flow_dissector_pernet_ops);
> +
> +       WARN_ON(err);

syzbot simulates memory allocation failures, which can bubble up here,
so this WARN_ON will probably trigger. I wonder if this could be
rewritten so that init fails, when registration fails? What are the
consequences?

> +       return err;
>  }
>
>  core_initcall(init_default_flow_dissectors);
> --
> 2.25.4
>