Re: [PATCH] net: gro: mark frag_list GRO packets as SKB_GSO_DODGY when a list element exceeds gso_size

[Willem de Bruijn]

Shiming Cheng wrote:
> When RX LRO (or similar offload) is enabled, the TCP/IPv4 GRO path may
> aggregate traffic using frag_list. The resulting skb is later segmented
> via the frag_list segmentation path (skb_segment_list()).
> 
> However, some drivers can hand GRO/LRO-aggregated frames to the stack
> where individual frag_list elements are already larger than skb_shinfo(p)
> ->gso_size (i.e., an element itself contains multiple MSS worth of
> payload) and may be non-linear (nr_frags > 0). This shape is not
> naturally produced by the software GRO aggregation logic for devices
> without LRO, and can lead to unexpected behavior in the frag_list
> segmentation path.

Did you observe this with a specific driver?

We don't want to have to support every crazy driver scheme. The right
approach may be to fix the driver.

To understand the geometry: the driver passes a GSO skb with frag_list,
where frag_list members may be any size, not just gso_size? I.e., these
do not conform to SKB_GSO_FRAGLIST rules?

I don't recall immediately what the acceptable behavior for regular
GSO skbs with frag_list is. But for starters such a driver should not
advertiserr SKB_GSO_FRAGLIST.

> 
> Detect this condition during frag_list aggregation and mark the
> aggregated packet as SKB_GSO_DODGY when a list element’s length exceeds
> gso_size. This forces a more conservative segmentation/linearization
> behavior downstream and avoids relying on assumptions that do not hold
> for LRO-produced aggregates.
> 
> No change for normal software GRO aggregation: the new check only
> triggers when skb_shinfo(p)->gso_size is set and a frag_list element
> length exceeds that size.
> 
> Fixes: 3a1296a38d0c ("net: Support GRO/GSO fraglist chaining.")
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Shiming Cheng <shiming.cheng@xxxxxxxxxxxx>
> ---
>  net/core/gro.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/net/core/gro.c b/net/core/gro.c
> index 29b4d02bf519..e70ecf19b0a7 100644
> --- a/net/core/gro.c
> +++ b/net/core/gro.c
> @@ -259,6 +259,9 @@ int skb_gro_receive_list(struct sk_buff *p, struct sk_buff *skb)
>  	skb_shinfo(p)->flags |= skb_shinfo(skb)->flags & SKBFL_SHARED_FRAG;
>  
>  	NAPI_GRO_CB(skb)->same_flow = 1;
> +	/* frag_list element larger than gso_size (already coalesced before list-append) */
> +	if (skb_shinfo(p)->gso_size && skb->len > skb_shinfo(p)->gso_size)
> +		skb_shinfo(p)->gso_type |= SKB_GSO_DODGY;
>  
>  	return 0;
>  }
> -- 
> 2.45.2
>