net: mangle zero checksum in skb_checksum_help()

[ 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@google.com>
Cc: Maciej Żenczykowski <maze@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Acked-by: Maciej Żenczykowski <maze@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/net/core/dev.c b/net/core/dev.c
index 91018eb..164958d 100644 (file)
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1918,7 +1918,7 @@ int skb_checksum_help(struct sk_buff *skb)
                        goto out;
        }
 
                        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:
 out_set_summed:
        skb->ip_summed = CHECKSUM_NONE;
 out: