Subject: [PATCH 03/16] net: mangle zero checksum in skb_checksum_help()

From: Eric Dumazet <[email-masked]>
Date: Sat, 29 Oct 2016 11:02:36 -0700

[ Upstream commit 4f2e4ad56a65f3b7d64c258e373cb71e8d2499f4 ]

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 <[email-masked]>
Cc: Maciej Żenczykowski <[email-masked]>
Cc: Willem de Bruijn <[email-masked]>
Acked-by: Maciej Żenczykowski <[email-masked]>
Signed-off-by: David S. Miller <[email-masked]>
---
 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 b3fa4b8..9ca749c 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -2462,7 +2462,7 @@ 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:
-- 
2.7.4