The udp_error function verifies the checksum of incoming UDP packets if
one is set. This has the desirable side effect of setting skb->ip_summed
to CHECKSUM_COMPLETE, signalling that this verification need not be
repeated further up the stack.
Conversely, when the UDP checksum is empty, which is perfectly legal (at least
inside IPv4), udp_error previously left no trace that the checksum had been
deemed acceptable.
This was a problem in particular for nf_reject_ipv4, which verifies the
checksum in nf_send_unreach() before sending ICMP_DEST_UNREACH. It makes
no accommodation for zero UDP checksums unless they are already marked
as CHECKSUM_UNNECESSARY.
This commit ensures packets with empty UDP checksum are marked as
CHECKSUM_UNNECESSARY, which is explicitly recommended in skbuff.h.
Signed-off-by: Kevin Mitchell <kevmitch@xxxxxxxxxx>
---
net/netfilter/nf_conntrack_proto_udp.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/net/netfilter/nf_conntrack_proto_udp.c b/net/netfilter/nf_conntrack_proto_udp.c
index 3b516cffc779..12f793d8fe0c 100644
--- a/net/netfilter/nf_conntrack_proto_udp.c
+++ b/net/netfilter/nf_conntrack_proto_udp.c
@@ -63,8 +63,10 @@ static bool udp_error(struct sk_buff *skb,
}
/* Packet with no checksum */
- if (!hdr->check)
+ if (!hdr->check) {
+ skb->ip_summed = CHECKSUM_UNNECESSARY;
return false;
+ }
/* Checksum invalid? Ignore.
* We skip checking packets on the outgoing path
--
2.34.1
