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

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

[Willem de Bruijn]

From: Willem de Bruijn <willemb@google.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@google.com>
Signed-off-by: Willem de Bruijn <willemb@google.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

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

[Song Liu]

On Mon, Sep 24, 2018 at 1:52 PM Willem de Bruijn
<willemdebruijn.kernel@gmail.com> wrote:
>
> From: Willem de Bruijn <willemb@google.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@google.com>
> Signed-off-by: Willem de Bruijn <willemb@google.com>

Acked-by: Song Liu <songliubraving@fb.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
>

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

[Daniel Borkmann]

On 09/24/2018 10:49 PM, Willem de Bruijn wrote:
> From: Willem de Bruijn <willemb@google.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@google.com>
> Signed-off-by: Willem de Bruijn <willemb@google.com>

Applied to bpf-next, thanks Willem!

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

[Eric Dumazet]

On 09/25/2018 08:38 AM, Daniel Borkmann wrote:
> On 09/24/2018 10:49 PM, Willem de Bruijn wrote:
>> From: Willem de Bruijn <willemb@google.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@google.com>
>> Signed-off-by: Willem de Bruijn <willemb@google.com>
> 
> Applied to bpf-next, thanks Willem!
> 

Sadly lib/test_bpf should still cause crashes, because populate_skb()
populates an skb attached to a fake device, for which net pointer is NULL.