netdev - [PATCH bpf-next] flow_dissector: lookup netns by skb->sk if skb->dev is NULL

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Message-Id: <20180924204956.83718-1-willemdebruijn.kernel@gmail.com>
Date:   Mon, 24 Sep 2018 16:49:57 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     netdev@...r.kernel.org
Cc:     ast@...nel.org, daniel@...earbox.net, davem@...emloft.net,
        Willem de Bruijn <willemb@...gle.com>
Subject: [PATCH bpf-next] flow_dissector: lookup netns by skb->sk if skb->dev is NULL

From: Willem de Bruijn <willemb@...gle.com>

BPF flow dissectors are configured per network namespace.
__skb_flow_dissect looks up the netns through dev_net(skb->dev).

In some dissector paths skb->dev is NULL, such as for Unix sockets.
In these cases fall back to looking up the netns by socket.

Analyzing the codepaths leading to __skb_flow_dissect I did not find
a case where both skb->dev and skb->sk are NULL. Warn and fall back to
standard flow dissector if one is found.

Fixes: d58e468b1112 ("flow_dissector: implements flow dissector BPF hook")
Reported-by: Eric Dumazet <edumazet@...gle.com>
Signed-off-by: Willem de Bruijn <willemb@...gle.com>
---
 net/core/flow_dissector.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
index 5c5dd74b5b3b..738c7562e1e0 100644
--- a/net/core/flow_dissector.c
+++ b/net/core/flow_dissector.c
@@ -714,7 +714,7 @@ bool __skb_flow_dissect(const struct sk_buff *skb,
 	struct flow_dissector_key_vlan *key_vlan;
 	enum flow_dissect_ret fdret;
 	enum flow_dissector_key_id dissector_vlan = FLOW_DISSECTOR_KEY_MAX;
-	struct bpf_prog *attached;
+	struct bpf_prog *attached = NULL;
 	int num_hdrs = 0;
 	u8 ip_proto = 0;
 	bool ret;
@@ -755,8 +755,14 @@ bool __skb_flow_dissect(const struct sk_buff *skb,
 					      target_container);
 
 	rcu_read_lock();
-	attached = skb ? rcu_dereference(dev_net(skb->dev)->flow_dissector_prog)
-		       : NULL;
+	if (skb) {
+		if (skb->dev)
+			attached = rcu_dereference(dev_net(skb->dev)->flow_dissector_prog);
+		else if (skb->sk)
+			attached = rcu_dereference(sock_net(skb->sk)->flow_dissector_prog);
+		else
+			WARN_ON_ONCE(1);
+	}
 	if (attached) {
 		/* Note that even though the const qualifier is discarded
 		 * throughout the execution of the BPF program, all changes(the
-- 
2.19.0.444.g18242da7ef-goog