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Date:   Mon, 27 Jan 2020 15:40:31 -0500
From:   Willem de Bruijn <>
        Willem de Bruijn <>,
        syzbot <>
Subject: [PATCH net] udp: segment looped gso packets correctly

From: Willem de Bruijn <>

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 <>
Fixes: cf329aa42b66 ("udp: cope with UDP GRO packet misdirection")
Signed-off-by: Willem de Bruijn <>
 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

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