Re: [PATCH v3 net 1/3] flow_dissector: avoid u16 truncation of skb->len when computing thoff

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

Eric Dumazet wrote:
> __skb_flow_dissect() computes key_control->thoff using:
> 
>   key_control->thoff = min_t(u16, nhoff, skb ? skb->len : hlen);
> 
> min_t(u16, ...) casts both arguments to u16 before comparing them.
> Whenever skb->len (or hlen) modulo 65536 is smaller than nhoff, the
> truncated length wins and thoff is set to a bogus small value, even
> when the dissection succeeded. For example, an IPv4/TCP frame with
> skb->len == 65540 and nhoff == 34 gets thoff == 4, so callers such as
> skb_probe_transport_header() point the transport header inside the
> Ethernet header.
> 
> Such skbs are not exotic:
> - At the time of commit d0c081b49137 ("flow_dissector: properly cap
>   thoff field"), AF_PACKET with PACKET_VNET_HDR could already build GSO
>   skbs larger than 64KB (MTU checks are skipped for GSO, and
>   alloc_skb_with_frags() accepted up to MAX_SKB_FRAGS (17) order-0
>   pages on top of the linear part).
> - BIG TCP now makes skbs larger than 64KB common.
> - The following patch makes tun_get_user() dissect IFF_TAP frames
>   before eth_type_trans() pulls the Ethernet header, so a GSO frame
>   carrying a 65522..65535 byte L3 packet will be dissected with
>   skb->len in [65536, 65549].
> 
> Compare as u32 instead. If the resulting offset cannot be represented
> in the u16 key_control->thoff, cap it to U16_MAX and report the
> dissection as failed rather than silently returning a wrong transport
> offset. thoff is still set on failure, as some callers (such as
> eth_get_headlen()) use it regardless of the return value.
> 
> Fixes: d0c081b49137 ("flow_dissector: properly cap thoff field")
> Assisted-by: LLM
> Signed-off-by: Eric Dumazet <edumazet@kernel.org>

Reviewed-by: Willem de Bruijn <willemb@google.com>