[PATCH net v3 4/8] selftests: net: tuntap: fix UDP checksum length in build_udp_packet_csum()

[Eric Dumazet <edumazet@kernel.org>]

build_udp_packet_csum() passes udph->len, which is in network byte
order, as the number of bytes to checksum. On little-endian hosts this
reads up to 64KB past the UDP header and computes a wrong checksum.

Existing callers did not crash only because they use packet buffers
of MAX_VNET_TUNNEL_PACKET_SZ bytes.

Use ntohs(udph->len), as tap.c does in its own build_udp_packet().

While at it, clear udph->check before summing instead of relying on
the caller, and send 0xffff instead of a computed zero checksum, as
the kernel does with CSUM_MANGLED_0.

Fixes: a942fcd72e97 ("selftest: tun: Introduce tuntap_helpers.h header for TUN/TAP testing")
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Eric Dumazet <edumazet@kernel.org>
---
 tools/testing/selftests/net/tuntap_helpers.h | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/net/tuntap_helpers.h b/tools/testing/selftests/net/tuntap_helpers.h
index d6c0437136ec..ecad83fae158 100644
--- a/tools/testing/selftests/net/tuntap_helpers.h
+++ b/tools/testing/selftests/net/tuntap_helpers.h
@@ -330,14 +330,22 @@ static inline void build_udp_packet_csum(uint8_t *buf, int family,
 	size_t ipalen = ip_addr_len(family);
 	uint32_t sum;
 
+	udph->check = 0;
+
 	/* No extension IPv4 and IPv6 headers addresses are the last fields */
 	sum = add_csum(buf - 2 * ipalen, 2 * ipalen);
 	sum += htons(IPPROTO_UDP) + udph->len;
 
 	if (!csum_off)
-		sum += add_csum(buf, udph->len);
+		sum += add_csum(buf, ntohs(udph->len));
 
 	udph->check = finish_ip_csum(sum);
+
+	/* A zero UDP checksum means no checksum: send 0xffff instead,
+	 * as the kernel does with CSUM_MANGLED_0.
+	 */
+	if (!csum_off && !udph->check)
+		udph->check = 0xffff;
 }
 
 static inline int build_udp_packet(uint8_t *buf, uint16_t sport, uint16_t dport,
-- 
2.53.0