[PATCH v3 18/20] tap: Disallow loopback addresses on tap interface

[David Gibson <david@gibson.dropbear.id.au>]

The "tap" interface, whether it's actually a tuntap device or a qemu
socket, presents a virtual external link between different network hosts.
Hence, loopback addresses make no sense there.  However, nothing prevents
the guest from putting bogus packets with loopback addresses onto the
interface and it's not entirely clear what effect that will have on passt.

Explicitly test for such packets and drop them.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 tap.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/tap.c b/tap.c
index 8a9a68b7..3a666212 100644
--- a/tap.c
+++ b/tap.c
@@ -610,6 +610,16 @@ resume:
 
 		l4_len = htons(iph->tot_len) - hlen;
 
+		if (IN4_IS_ADDR_LOOPBACK(&iph->saddr) ||
+		    IN4_IS_ADDR_LOOPBACK(&iph->daddr)) {
+			char sstr[INET_ADDRSTRLEN], dstr[INET_ADDRSTRLEN];
+
+			debug("Loopback address on tap interface: %s -> %s",
+			      inet_ntop(AF_INET, &iph->saddr, sstr, sizeof(sstr)),
+			      inet_ntop(AF_INET, &iph->daddr, dstr, sizeof(dstr)));
+			continue;
+		}
+
 		if (iph->saddr && c->ip4.addr_seen.s_addr != iph->saddr)
 			c->ip4.addr_seen.s_addr = iph->saddr;
 
@@ -766,6 +776,15 @@ resume:
 		if (!(l4h = ipv6_l4hdr(in, i, sizeof(*eh), &proto, &l4_len)))
 			continue;
 
+		if (IN6_IS_ADDR_LOOPBACK(saddr) || IN6_IS_ADDR_LOOPBACK(daddr)) {
+			char sstr[INET6_ADDRSTRLEN], dstr[INET6_ADDRSTRLEN];
+
+			debug("Loopback address on tap interface: %s -> %s",
+			      inet_ntop(AF_INET6, saddr, sstr, sizeof(sstr)),
+			      inet_ntop(AF_INET6, daddr, dstr, sizeof(dstr)));
+			continue;
+		}
+
 		if (IN6_IS_ADDR_LINKLOCAL(saddr)) {
 			c->ip6.addr_ll_seen = *saddr;
 
-- 
@@ -610,6 +610,16 @@ resume:
 
 		l4_len = htons(iph->tot_len) - hlen;
 
+		if (IN4_IS_ADDR_LOOPBACK(&iph->saddr) ||
+		    IN4_IS_ADDR_LOOPBACK(&iph->daddr)) {
+			char sstr[INET_ADDRSTRLEN], dstr[INET_ADDRSTRLEN];
+
+			debug("Loopback address on tap interface: %s -> %s",
+			      inet_ntop(AF_INET, &iph->saddr, sstr, sizeof(sstr)),
+			      inet_ntop(AF_INET, &iph->daddr, dstr, sizeof(dstr)));
+			continue;
+		}
+
 		if (iph->saddr && c->ip4.addr_seen.s_addr != iph->saddr)
 			c->ip4.addr_seen.s_addr = iph->saddr;
 
@@ -766,6 +776,15 @@ resume:
 		if (!(l4h = ipv6_l4hdr(in, i, sizeof(*eh), &proto, &l4_len)))
 			continue;
 
+		if (IN6_IS_ADDR_LOOPBACK(saddr) || IN6_IS_ADDR_LOOPBACK(daddr)) {
+			char sstr[INET6_ADDRSTRLEN], dstr[INET6_ADDRSTRLEN];
+
+			debug("Loopback address on tap interface: %s -> %s",
+			      inet_ntop(AF_INET6, saddr, sstr, sizeof(sstr)),
+			      inet_ntop(AF_INET6, daddr, dstr, sizeof(dstr)));
+			continue;
+		}
+
 		if (IN6_IS_ADDR_LINKLOCAL(saddr)) {
 			c->ip6.addr_ll_seen = *saddr;
 
-- 
2.43.2