netdev - Re: [PATCH net-next v3 1/8] udp: fixup csum for GSO receive slow path

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Date:   Tue, 30 Mar 2021 11:15:39 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Steffen Klassert <steffen.klassert@...unet.com>,
        Alexander Lobakin <alobakin@...me>
Subject: Re: [PATCH net-next v3 1/8] udp: fixup csum for GSO receive slow path

On Tue, Mar 30, 2021 at 6:29 AM Paolo Abeni <pabeni@...hat.com> wrote:
>
> When UDP packets generated locally by a socket with UDP_SEGMENT
> traverse the following path:
>
> UDP tunnel(xmit) -> veth (segmentation) -> veth (gro) ->
>         UDP tunnel (rx) -> UDP socket (no UDP_GRO)
>
> ip_summed will be set to CHECKSUM_PARTIAL at creation time and
> such checksum mode will be preserved in the above path up to the
> UDP tunnel receive code where we have:
>
>  __iptunnel_pull_header() -> skb_pull_rcsum() ->
> skb_postpull_rcsum() -> __skb_postpull_rcsum()
>
> The latter will convert the skb to CHECKSUM_NONE.
>
> The UDP GSO packet will be later segmented as part of the rx socket
> receive operation, and will present a CHECKSUM_NONE after segmentation.
>
> Additionally the segmented packets UDP CB still refers to the original
> GSO packet len. Overall that causes unexpected/wrong csum validation
> errors later in the UDP receive path.
>
> We could possibly address the issue with some additional checks and
> csum mangling in the UDP tunnel code. Since the issue affects only
> this UDP receive slow path, let's set a suitable csum status there.
>
> Note that SKB_GSO_UDP_L4 or SKB_GSO_FRAGLIST packets lacking an UDP
> encapsulation present a valid checksum when landing to udp_queue_rcv_skb(),
> as the UDP checksum has been validated by the GRO engine.
>
> v2 -> v3:
>  - even more verbose commit message and comments
>
> v1 -> v2:
>  - restrict the csum update to the packets strictly needing them
>  - hopefully clarify the commit message and code comments
>
> Signed-off-by: Paolo Abeni <pabeni@...hat.com>

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