Re: [PATCH net-next v2 1/2] net: Guard for gso_segs overflow in skb_segment

[Paolo Abeni <pabeni@redhat.com>]

On 8/13/26 7:46 PM, Alice Mikityanska wrote:
> From: Alice Mikityanska <alice@isovalent.com>
> 
> skb_segment calculates 32-bit partial_segs as len / gso_size, and then
> assigns it to the 16-bit gso_segs field. The division might overflow in
> some edge cases where the SKB is BIG TCP (65536 <= len <= 8*65535), and
> gso_size < TCP_MIN_GSO_SIZE = 8. While normally this can't happen due to
> TCP_MIN_GSO_SIZE, an AF_PACKET PACKET_VNET_HDR socket can generate such
> a malformed packet.
> 
> Blocking malformed virtio_net packets is implemented in the next patch,
> but this patch clamps partial_segs in skb_segment itself for more
> generic robustness. Should len / gso_size happen to be bigger than
> 65535 in partial GSO, skb_segment will now just produce more than two
> output SKBs, all of which will be valid with gso_segs <= 65535.

Minor nit: I think it would make sense to re-order the patches.

> Signed-off-by: Alice Mikityanska <alice@isovalent.com>
> ---
>  net/core/skbuff.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index c82a1472a5ea..439cbfeb02bd 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -4860,7 +4860,7 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
>  		 * doesn't fit into an MSS sized block, so take care of that
>  		 * now.
>  		 */
> -		partial_segs = len / mss;
> +		partial_segs = min(len / mss, GSO_MAX_SEGS);

Since on top of patch 2/2 the min() should always be a no-op, what about
instead:

		if (WARN_ON_ONCE(len/mss > GSO_MAX_SEGS))
			return ERR_PTR(-EINVAL);

?

/P