Re: [PATCH bpf] bpf: check for data_len before upgrading mss when 6 to 4

[Daniel Borkmann <daniel@iogearbox.net>]

On 4/29/21 12:08 PM, Dongseok Yi wrote:
> tcp_gso_segment check for the size of GROed payload if it is bigger
> than the mss. bpf_skb_proto_6_to_4 increases mss, but the mss can be
> bigger than the size of GROed payload unexpectedly if data_len is not
> big enough.
> 
> Assume that skb gso_size = 1372 and data_len = 8. bpf_skb_proto_6_to_4
> would increse the gso_size to 1392. tcp_gso_segment will get an error
> with 1380 <= 1392.
> 
> Check for the size of GROed payload if it is really bigger than target
> mss when increase mss.
> 
> Fixes: 6578171a7ff0 (bpf: add bpf_skb_change_proto helper)
> Signed-off-by: Dongseok Yi <dseok.yi@samsung.com>
> ---
>   net/core/filter.c | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 9323d34..3f79e3c 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -3308,7 +3308,9 @@ static int bpf_skb_proto_6_to_4(struct sk_buff *skb)
>   		}
>   
>   		/* Due to IPv4 header, MSS can be upgraded. */
> -		skb_increase_gso_size(shinfo, len_diff);
> +		if (skb->data_len > len_diff)

Could you elaborate some more on what this has to do with data_len specifically
here? I'm not sure I follow exactly your above commit description. Are you saying
that you're hitting in tcp_gso_segment():

         [...]
         mss = skb_shinfo(skb)->gso_size;
         if (unlikely(skb->len <= mss))
                 goto out;
         [...]

Please provide more context on the bug, thanks!

> +			skb_increase_gso_size(shinfo, len_diff);
> +
>   		/* Header must be checked, and gso_segs recomputed. */
>   		shinfo->gso_type |= SKB_GSO_DODGY;
>   		shinfo->gso_segs = 0;
>