On Thu, 21 May 2020 17:53:14 -0700
Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote:
> On Thu, May 21, 2020 at 12:09 PM Andrii Nakryiko
> <andrii.nakryiko@xxxxxxxxx> wrote:
> >
> > On Wed, May 20, 2020 at 10:24 AM Jakub Sitnicki <jakub@xxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxx>
> > > ---
> > >
> > > 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?
>
> good catch. that warn is pointless.
> I removed it and force pushed the bpf tree.
Thanks for patching it up. I'll keep it in mind next time.
