lists  /  announce  owl-users  owl-dev  john-users  john-dev  passwdqc-users  yescrypt  popa3d-users  /  oss-security  kernel-hardening  musl  sabotage  tlsify  passwords  /  crypt-dev  xvendor  /  Bugtraq  Full-Disclosure  linux-kernel  linux-netdev  linux-ext4  linux-hardening  PHC 
Open Source and information security mailing list archives
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date:   Fri, 14 Jan 2022 20:00:50 -0800
From:   Kevin Mitchell <>
To:     unlisted-recipients:; (no To-header on input)
Cc:, Pablo Neira Ayuso <>,
        Jozsef Kadlecsik <>,
        Florian Westphal <>,
        "David S. Miller" <>,
        Jakub Kicinski <>,,,,
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

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 <>
 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

Powered by blists - more mailing lists