On Wed, May 20, 2020 at 10:40:00AM -0700, sdf@xxxxxxxxxx wrote:
> On 05/20, Jakub Sitnicki 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.
> Oh, good catch!
Good catch indeed!
>
> > -jkbs
>
> > net/core/flow_dissector.c | 29 ++++++++++++++++++++++++++++-
> > 1 file changed, 28 insertions(+), 1 deletion(-)
>
> > diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
> > index 3eff84824c8b..b6179cd20158 100644
> > --- a/net/core/flow_dissector.c
> > +++ b/net/core/flow_dissector.c
> > @@ -179,6 +179,27 @@ int skb_flow_dissector_bpf_prog_detach(const union
> > bpf_attr *attr)
> > return 0;
> > }
>
> > +static void __net_exit flow_dissector_pernet_pre_exit(struct net *net)
> > +{
> > + struct bpf_prog *attached;
> > +
> > + /* We don't lock the update-side because there are no
> > + * references left to this netns when we get called. Hence
> > + * there can be no attach/detach in progress.
> > + */
> > + rcu_read_lock();
> > + attached = rcu_dereference(net->flow_dissector_prog);
> > + if (attached) {
> > + RCU_INIT_POINTER(net->flow_dissector_prog, NULL);
> > + bpf_prog_put(attached);
> > + }
> > + rcu_read_unlock();
> > +}
> I wonder, should we instead refactor existing
> skb_flow_dissector_bpf_prog_detach to accept netns (instead of attr)
> can call that here? Instead of reimplementing it (I don't think we
> care about mutex lock/unlock efficiency here?). Thoughts?
Agree. Would be good to share that bit of code.
