netdev - [PATCH net] net: mangle zero checksum in skb_checksum

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Message-ID: <1477764156.7065.282.camel@edumazet-glaptop3.roam.corp.google.com>
Date:   Sat, 29 Oct 2016 11:02:36 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     David Miller <davem@...emloft.net>
Cc:     netdev <netdev@...r.kernel.org>,
        Maciej Żenczykowski <maze@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: [PATCH net] net: mangle zero checksum in skb_checksum_help()

From: Eric Dumazet <edumazet@...gle.com>

Sending zero checksum is ok for TCP, but not for UDP.

UDPv6 receiver should by default drop a frame with a 0 checksum,
and UDPv4 would not verify the checksum and might accept a corrupted
packet.

Simply replace such checksum by 0xffff, regardless of transport.

This error was caught on SIT tunnels, but seems generic.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Cc: Maciej Żenczykowski <maze@...gle.com>
Cc: Willem de Bruijn <willemb@...gle.com>
---
 net/core/dev.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index dbc871306910..899a0f00e721 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -2484,7 +2484,7 @@ int skb_checksum_help(struct sk_buff *skb)
 			goto out;
 	}
 
-	*(__sum16 *)(skb->data + offset) = csum_fold(csum);
+	*(__sum16 *)(skb->data + offset) = csum_fold(csum) ?: CSUM_MANGLED_0;
 out_set_summed:
 	skb->ip_summed = CHECKSUM_NONE;
 out: