linux-kernel - Re: [PATCH net] net: 802: reset skb->transport

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Message-ID: <62d899b3-dad0-4e61-b4da-4ce969dc7cf2@gmail.com>
Date: Thu, 2 Jan 2025 19:18:58 -0500
From: Antonio Pastor <antonio.pastor@...il.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: netdev@...r.kernel.org, pabeni@...hat.com, horms@...nel.org,
 kuba@...nel.org, "David S. Miller" <davem@...emloft.net>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] net: 802: reset skb->transport_header

> Sorry, this patch is wrong, it does not fix the potential issue yet.

No worries! Thanks for your patience with this. Much appreciated.

> Note how skb_transport_header(skb) is used in
> find_snap_client(skb_transport_header(skb));

I've spent so much time trying to figure out why the offset is wrong I 
lost sight that the core issue is that it is being used to begin with. 
Paolo Abeni hinted at that too.

> The proper way to fix the issue is to not rely on the transport header
> at all, only reset it after pulling the network header.
>
>
> diff --git a/net/802/psnap.c b/net/802/psnap.c
> index fca9d454905fe37d6b838f0f00b3a16767e44e74..389df460c8c4b92f9ec6198247db0ba15bfb8f2e
> 100644
> --- a/net/802/psnap.c
> +++ b/net/802/psnap.c
> @@ -55,11 +55,11 @@ static int snap_rcv(struct sk_buff *skb, struct
> net_device *dev,
>                  goto drop;
>
>          rcu_read_lock();
> -       proto = find_snap_client(skb_transport_header(skb));
> +       proto = find_snap_client(skb->data);
>          if (proto) {
>                  /* Pass the frame on. */
> -               skb->transport_header += 5;
>                  skb_pull_rcsum(skb, 5);
> +               skb_reset_transport_header(skb);
>                  rc = proto->rcvfunc(skb, dev, &snap_packet_type, orig_dev);
>          }
>          rcu_read_unlock();

Will send V2.