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

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Wed, May 20, 2020 at 10:40:00AM -0700, sdf@google.com 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@cloudflare.com>
> > ---
> 
> > 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.