netdev - Re: [PATCH net-next 01/14] netfilter: conntrack: mark UDP zero checksum as CHECKSUM

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Date:   Wed, 16 Feb 2022 16:10:25 +0200
From:   Gal Pressman <gal@...dia.com>
To:     Pablo Neira Ayuso <pablo@...filter.org>,
        netfilter-devel@...r.kernel.org
Cc:     davem@...emloft.net, netdev@...r.kernel.org, kuba@...nel.org
Subject: Re: [PATCH net-next 01/14] netfilter: conntrack: mark UDP zero
 checksum as CHECKSUM_UNNECESSARY

On 09/02/2022 15:36, Pablo Neira Ayuso wrote:
> From: Kevin Mitchell <kevmitch@...sta.com>
>
> The udp_error function verifies the checksum of incoming UDP packets if
> one is set. This has the desirable side effect of setting skb->ip_summed
> to CHECKSUM_COMPLETE, signalling that this verification need not be
> repeated further up the stack.
>
> Conversely, when the UDP checksum is empty, which is perfectly legal (at least
> inside IPv4), udp_error previously left no trace that the checksum had been
> deemed acceptable.
>
> This was a problem in particular for nf_reject_ipv4, which verifies the
> checksum in nf_send_unreach() before sending ICMP_DEST_UNREACH. It makes
> no accommodation for zero UDP checksums unless they are already marked
> as CHECKSUM_UNNECESSARY.
>
> This commit ensures packets with empty UDP checksum are marked as
> CHECKSUM_UNNECESSARY, which is explicitly recommended in skbuff.h.
>
> Signed-off-by: Kevin Mitchell <kevmitch@...sta.com>
> Acked-by: Florian Westphal <fw@...len.de>
> Signed-off-by: Pablo Neira Ayuso <pablo@...filter.org>
> ---
>  net/netfilter/nf_conntrack_proto_udp.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/net/netfilter/nf_conntrack_proto_udp.c b/net/netfilter/nf_conntrack_proto_udp.c
> index 3b516cffc779..12f793d8fe0c 100644
> --- a/net/netfilter/nf_conntrack_proto_udp.c
> +++ b/net/netfilter/nf_conntrack_proto_udp.c
> @@ -63,8 +63,10 @@ static bool udp_error(struct sk_buff *skb,
>  	}
>  
>  	/* Packet with no checksum */
> -	if (!hdr->check)
> +	if (!hdr->check) {
> +		skb->ip_summed = CHECKSUM_UNNECESSARY;
>  		return false;
> +	}
>  
>  	/* Checksum invalid? Ignore.
>  	 * We skip checking packets on the outgoing path

Hey,

I think this patch broke geneve tunnels, or possibly all udp tunnels?

A simple test that creates two geneve tunnels and runs tcp iperf fails
and results in checksum errors (TcpInCsumErrors).


Any idea how to solve that? Maybe 'skb->csum_level' needs some adjustments?