[PATCH 3.2 169/199] net: mangle zero checksum in skb_checksum_help()

[Ben Hutchings]

3.2.87-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Eric Dumazet <edumazet@xxxxxxxxxx>

[ 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 <edumazet@xxxxxxxxxx>
Cc: Maciej Åenczykowski <maze@xxxxxxxxxx>
Cc: Willem de Bruijn <willemb@xxxxxxxxxx>
Acked-by: Maciej Åenczykowski <maze@xxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 net/core/dev.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1918,7 +1918,7 @@ int skb_checksum_help(struct sk_buff *sk
 			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: