udp: Correctly look up outbound socket with port remappings

Commit bb9bf0bb ("tcp, udp: Don't precompute port remappings in epoll
references") changed the epoll reference for UDP sockets to include the
bound port as seen by the socket itself, rather than the bound port it
would be translated to on the guest side.  As a side effect, it also means
that udp_tap_map[] is indexed by the bound port on the host side, rather
than on the guest side.  This is consistent and a good idea, however we
forgot to account for it when finding the correct outgoing socket for
packets originating in the guest.  This means that if forwarding UDP
inbound with a port number change, reply packets would be misdirected.

Fix this by applying the reverse mapping before looking up the socket in
udp_tap_handler().  While we're at it, use 'port' directly instead of
'uref.port' in udp_sock_init().  Those now always have the same value -
failing to realise that is the same error as above.

Reported-by: Laurent Jacquot <jk@lutty.net>
Link: https://bugs.passt.top/show_bug.cgi?id=87
Fixes: bb9bf0bb8f57 ("tcp, udp: Don't precompute port remappings in epoll references")
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

1 files changed, 3 insertions, 2 deletions

diff --git a/udp.c b/udp.c
index 694424a..594ea19 100644
--- a/udp.c
+++ b/udp.c
@@ -829,6 +829,7 @@ int udp_tap_handler(struct ctx *c, uint8_t pif,
 	 * and destination, so we can just take those from the first message.
 	 */
 	src = ntohs(uh->source);
+	src += c->udp.fwd_in.rdelta[src];
 	dst = ntohs(uh->dest);
 
 	if (af == AF_INET) {
@@ -1005,7 +1006,7 @@ int udp_sock_init(const struct ctx *c, int ns, sa_family_t af,
 			r4 = s = sock_l4(c, AF_INET, IPPROTO_UDP, addr,
 					 ifname, port, uref.u32);
 
-			udp_tap_map[V4][uref.port].sock = s < 0 ? -1 : s;
+			udp_tap_map[V4][port].sock = s < 0 ? -1 : s;
 			udp_splice_init[V4][port].sock = s < 0 ? -1 : s;
 		} else {
 			r4 = s = sock_l4(c, AF_INET, IPPROTO_UDP,
@@ -1022,7 +1023,7 @@ int udp_sock_init(const struct ctx *c, int ns, sa_family_t af,
 			r6 = s = sock_l4(c, AF_INET6, IPPROTO_UDP, addr,
 					 ifname, port, uref.u32);
 
-			udp_tap_map[V6][uref.port].sock = s < 0 ? -1 : s;
+			udp_tap_map[V6][port].sock = s < 0 ? -1 : s;
 			udp_splice_init[V6][port].sock = s < 0 ? -1 : s;
 		} else {
 			r6 = s = sock_l4(c, AF_INET6, IPPROTO_UDP,