udp: Explicitly set checksum in guest-bound UDP headers

For IPv4, UDP checksums are optional and can just be set to 0.
udp_update_hdr4() ignores the checksum field entirely.  Since these are set
to 0 during startup, this works as intended for now.

However, we'd like to share payload and UDP header buffers betweem IPv4 and
IPv6, which does calculate UDP checksums.  Therefore, for robustness, we
should explicitly set the checksum field to 0 for guest-bound UDP packets.

In the tap_udp4_send() slow path, however, we do allow IPv4 UDP checksums
to be calculated as a compile time option.  For consistency, use the same
thing in the udp_update_hdr4() path, which will typically initialize to 0,
but calculate a real checksum if configured to do so.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

1 files changed, 4 insertions, 3 deletions

diff --git a/udp.c b/udp.c
index bb7d161..cc938bb 100644
--- a/udp.c
+++ b/udp.c
@@ -592,6 +592,7 @@ static size_t udp_update_hdr4(const struct ctx *c, struct udp4_l2_buf_t *b,
 			      in_port_t dstport, size_t dlen,
 			      const struct timespec *now)
 {
+	const struct in_addr dst = c->ip4.addr_seen;
 	size_t l4len = dlen + sizeof(b->uh);
 	size_t l3len = l4len + sizeof(b->iph);
 	in_port_t srcport = ntohs(b->s_in.sin_port);
@@ -617,14 +618,14 @@ static size_t udp_update_hdr4(const struct ctx *c, struct udp4_l2_buf_t *b,
 	}
 
 	b->iph.tot_len = htons(l3len);
-	b->iph.daddr = c->ip4.addr_seen.s_addr;
+	b->iph.daddr = dst.s_addr;
 	b->iph.saddr = src.s_addr;
-	b->iph.check = csum_ip4_header(l3len, IPPROTO_UDP,
-				       src, c->ip4.addr_seen);
+	b->iph.check = csum_ip4_header(l3len, IPPROTO_UDP, src, dst);
 
 	b->uh.source = b->s_in.sin_port;
 	b->uh.dest = htons(dstport);
 	b->uh.len = htons(l4len);
+	csum_udp4(&b->uh, src, dst, b->data, dlen);
 
 	tap_hdr_update(&b->taph, l3len + sizeof(b->eh));
 	return l4len;