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

[Jakub Sitnicki <jakub@xxxxxxxxxxxxxx>]

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

 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();
+}
+
+static struct pernet_operations flow_dissector_pernet_ops __net_initdata = {
+	.pre_exit = flow_dissector_pernet_pre_exit,
+};
+
 /**
  * __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);
+	return err;
 }
 
 core_initcall(init_default_flow_dissectors);
-- 
2.25.4