Re: [Patch bpf] bpf: check negative offsets in __bpf_skb_min_len()

[Daniel Borkmann <daniel@iogearbox.net>]

On 10/8/24 7:33 AM, Cong Wang wrote:
> From: Cong Wang <cong.wang@bytedance.com>
> 
> skb_transport_offset() and skb_transport_offset() can be negative when

nit: I presume the 2nd one is skb_network_offset?

> they are called after we pull the transport header, for example, when
> we use eBPF sockmap (aka at the point of ->sk_data_ready()).
> 
> __bpf_skb_min_len() uses an unsigned int to get these offsets, this
> leads to a very large number which causes bpf_skb_change_tail() failed
> unexpectedly.
> 
> Fix this by using a signed int to get these offsets and test them
> against zero.
> 
> Fixes: 5293efe62df8 ("bpf: add bpf_skb_change_tail helper")
> Cc: Daniel Borkmann <daniel@iogearbox.net>
> Signed-off-by: Cong Wang <cong.wang@bytedance.com>

Is there any chance you could also extend the sockmap BPF selftest with
this case you're hitting so that BPF CI can run this regularly?

> ---
>   net/core/filter.c | 21 +++++++++++++++------
>   1 file changed, 15 insertions(+), 6 deletions(-)
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 4e3f42cc6611..10ef27639a5d 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -3737,13 +3737,22 @@ static const struct bpf_func_proto bpf_skb_adjust_room_proto = {
>   
>   static u32 __bpf_skb_min_len(const struct sk_buff *skb)
>   {
> -	u32 min_len = skb_network_offset(skb);
> +	int offset = skb_network_offset(skb);
> +	u32 min_len = 0;
>   
> -	if (skb_transport_header_was_set(skb))
> -		min_len = skb_transport_offset(skb);
> -	if (skb->ip_summed == CHECKSUM_PARTIAL)
> -		min_len = skb_checksum_start_offset(skb) +
> -			  skb->csum_offset + sizeof(__sum16);
> +	if (offset > 0)
> +		min_len = offset;
> +	if (skb_transport_header_was_set(skb)) {
> +		offset = skb_transport_offset(skb);
> +		if (offset > 0)
> +			min_len = offset;
> +	}
> +	if (skb->ip_summed == CHECKSUM_PARTIAL) {
> +		offset = skb_checksum_start_offset(skb) +
> +			 skb->csum_offset + sizeof(__sum16);
> +		if (offset > 0)
> +			min_len = offset;
> +	}
>   	return min_len;

I'll let John chime in, but does this mean in case of sockmap min_len always ends
up at 0? I just wonder whether we should pass a custom __bpf_skb_min_len to
__bpf_skb_change_tail for bpf_skb_change_tail vs sk_skb_change_tail assuming the
compiler is able to inlining all this (instead of indirect call).