net: mangle zero checksum in skb_checksum_help()

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 protected]>
Cc: Maciej Żenczykowski <[email protected]>
Cc: Willem de Bruijn <[email protected]>
Acked-by: Maciej Żenczykowski <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

diff --git a/net/core/dev.c b/net/core/dev.c
index 820bac239738eb021354ac95ca5bbdff1840cb8e..eaad4c28069ff523ac784bf2dffd0acff82341a0 100644 (file)
--- 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: