Re: [PATCH net 1/1] net: gso: limit recursive IP-in-IP segmentation

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Zihan Xi wrote:
> IPIP GSO/TSO support makes IP-in-IP GSO dispatch re-enter
> inet_gso_segment() or ipv6_gso_segment() for every nested IP header. The
> only state that tracks this nesting is encap_level, which records header
> bytes and has no recursion bound. A sufficiently deep chain can consume the
> kernel stack before a transport GSO callback is reached.
> 
> The unbounded callback nesting was introduced when inet_gso_segment() was
> made stackable by "ipv4: gso: make inet_gso_segment() stackable". GRE GSO
> support predated that change, and IP-in-IP GSO/TSO support later made the
> affected path reachable.
> 
> Track the number of IP GSO callbacks in skb_gso_cb and reject the 15th
> callback entry. Thus 14 callback entries are allowed to complete;
> GSO_RECURSION_LIMIT is the rejection threshold, not the number of
> successful callbacks. Initialize the counter for each top-level GSO
> operation and check it in both IPv4 and IPv6 handlers so mixed IP-in-IP
> nesting is bounded.
> 
> Fixes: 3347c9602955 ("ipv4: gso: make inet_gso_segment() stackable")
> Cc: stable@vger.kernel.org
> Reported-by: Vega <vega@nebusec.ai>
> Assisted-by: LLM
> Co-developed-by: Luxing Yin <root@tr0jan.top>
> Signed-off-by: Luxing Yin <root@tr0jan.top>
> Signed-off-by: Zihan Xi <zihanx@nebusec.ai>
> ---
>  include/net/gso.h      | 9 +++++++++
>  net/core/gso.c         | 1 +
>  net/ipv4/af_inet.c     | 3 +++
>  net/ipv6/ip6_offload.c | 3 +++
>  4 files changed, 16 insertions(+)
> 
> diff --git a/include/net/gso.h b/include/net/gso.h
> index 29975440cad5..2665acbb9205 100644
> --- a/include/net/gso.h
> +++ b/include/net/gso.h
> @@ -19,10 +19,19 @@ struct skb_gso_cb {
>  	int	encap_level;
>  	__wsum	csum;
>  	__u16	csum_start;
> +	/* Number of GSO callbacks this packet already went through. */
> +	u8	recursion_counter;
>  };
>  #define SKB_GSO_CB_OFFSET	32
>  #define SKB_GSO_CB(skb) ((struct skb_gso_cb *)((skb)->cb + SKB_GSO_CB_OFFSET))
>  
> +#define GSO_RECURSION_LIMIT	15	/* First callback depth to reject. */
> +static inline int gso_recursion_inc_test(struct sk_buff *skb)

What is 15 based on? Is that where in your test stack overflow occurs?

A realistic practical limit would likely already be smaller.