netdev - Re: [PATCH net-next] net: warn if transport header was not set

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Date:   Mon, 9 May 2022 10:20:59 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Paolo Abeni <pabeni@...hat.com>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net-next] net: warn if transport header was not set

On Mon,  9 May 2022 09:57:16 -0700 Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@...gle.com>
> 
> Make sure skb_transport_header() and skb_transport_offset() uses
> are not fooled if the transport header has not been set.
> 
> This change will likely expose existing bugs in linux networking stacks.
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> ---
>  include/linux/skbuff.h | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index d58669d6cb91aa30edc70d59a0a7e9d4e2298842..043c59fa0bd6d921f2d2e211348929681bfce186 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -2804,6 +2804,7 @@ static inline bool skb_transport_header_was_set(const struct sk_buff *skb)
>  
>  static inline unsigned char *skb_transport_header(const struct sk_buff *skb)
>  {
> +	WARN_ON_ONCE(!skb_transport_header_was_set(skb));
>  	return skb->head + skb->transport_header;
>  }
>  

This is for prod or for syzbot?

What are your feelings on putting this under a kconfig?

We already have a #ifdef DEBUG in skb_checksum_none_assert()
we could generalize that into some form of kconfig-gated
SKB_CHECK(). Kconfig-gated so that people don't feel self-conscious
about using it. I wrote such a patch a few times already but never 
sent it.