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Date:   Tue, 28 Jan 2020 10:59:09 +0100 (CET)
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To:     willemdebruijn.kernel@gmail.com
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Subject: Re: [PATCH net] udp: segment looped gso packets correctly
From:   David Miller <davem@davemloft.net>
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From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date: Mon, 27 Jan 2020 15:40:31 -0500

> From: Willem de Bruijn <willemb@google.com>
> 
> Multicast and broadcast packets can be looped from egress to ingress
> pre segmentation with dev_loopback_xmit. That function unconditionally
> sets ip_summed to CHECKSUM_UNNECESSARY.
> 
> udp_rcv_segment segments gso packets in the udp rx path. Segmentation
> usually executes on egress, and does not expect packets of this type.
> __udp_gso_segment interprets !CHECKSUM_PARTIAL as CHECKSUM_NONE. But
> the offsets are not correct for gso_make_checksum.
> 
> UDP GSO packets are of type CHECKSUM_PARTIAL, with their uh->check set
> to the correct pseudo header checksum. Reset ip_summed to this type.
> (CHECKSUM_PARTIAL is allowed on ingress, see comments in skbuff.h)
> 
> Reported-by: syzbot <syzkaller@googlegroups.com>
> Fixes: cf329aa42b66 ("udp: cope with UDP GRO packet misdirection")
> Signed-off-by: Willem de Bruijn <willemb@google.com>

Applied and queued up for -stable, but I have to say:

> +	if (skb->pkt_type == PACKET_LOOPBACK)
> +		skb->ip_summed = CHECKSUM_PARTIAL;
> +

There are a lot of implementation detail assumptions encoded into that
conditional statement :-)

Feel free to follow-up with a patch adding a comment containing a
condensed version of your commit log here.

Thanks.