linux-kernel - [PATCH 1/1] netfilter: conntrack: mark UDP zero checksum as CHECKSUM

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Date:   Fri, 14 Jan 2022 20:00:50 -0800
From:   Kevin Mitchell <kevmitch@...sta.com>
To:     unlisted-recipients:; (no To-header on input)
Cc:     kevmitch@...sta.com, Pablo Neira Ayuso <pablo@...filter.org>,
        Jozsef Kadlecsik <kadlec@...filter.org>,
        Florian Westphal <fw@...len.de>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        netfilter-devel@...r.kernel.org, coreteam@...filter.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH 1/1] netfilter: conntrack: mark UDP zero checksum as CHECKSUM_UNNECESSARY

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@...sta.com>
---
 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