netdev - Re: [PATCH net] udp: segment looped gso packets correctly

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Date:   Tue, 28 Jan 2020 12:27:14 +0100
From:   Paolo Abeni <pabeni@...hat.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        netdev@...r.kernel.org
Cc:     davem@...emloft.net, Willem de Bruijn <willemb@...gle.com>,
        syzbot <syzkaller@...glegroups.com>
Subject: Re: [PATCH net] udp: segment looped gso packets correctly

On Mon, 2020-01-27 at 15:40 -0500, Willem de Bruijn wrote:
> From: Willem de Bruijn <willemb@...gle.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@...glegroups.com>
> Fixes: cf329aa42b66 ("udp: cope with UDP GRO packet misdirection")
> Signed-off-by: Willem de Bruijn <willemb@...gle.com>
> ---
>  include/net/udp.h | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/include/net/udp.h b/include/net/udp.h
> index bad74f7808311..8f163d674f072 100644
> --- a/include/net/udp.h
> +++ b/include/net/udp.h
> @@ -476,6 +476,9 @@ static inline struct sk_buff *udp_rcv_segment(struct sock *sk,
>  	if (!inet_get_convert_csum(sk))
>  		features |= NETIF_F_IP_CSUM | NETIF_F_IPV6_CSUM;
>  
> +	if (skb->pkt_type == PACKET_LOOPBACK)
> +		skb->ip_summed = CHECKSUM_PARTIAL;
> +
>  	/* the GSO CB lays after the UDP one, no need to save and restore any
>  	 * CB fragment
>  	 */

LGTM, Thanks!

Acked-by: Paolo Abeni <pabeni@...hat.com>

Out of sheer curiosity, do you know what was the kernel behaviour
before the 'fixes' commit ? GSO packet delivered to user-space stillaggregated ?!? 

Thanks,

Paolo