icmp6-util: make icmp6_receive() accept the null source address

Fixes #29050.

src/libsystemd-network/fuzz-ndisc-rs.c

@@ -24,6 +24,13 @@ int icmp6_bind_router_advertisement(int index) {
         return -ENOSYS;
 }
 
+static struct in6_addr dummy_link_local = {
+        .s6_addr = {
+                0xfe, 0x80, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+                0x12, 0x34, 0x56, 0xff, 0xfe, 0x78, 0x9a, 0xbc,
+        },
+};
+
 int icmp6_receive(
                 int fd,
                 void *iov_base,
@@ -36,6 +43,9 @@ int icmp6_receive(
         if (ret_timestamp)
                 triple_timestamp_get(ret_timestamp);
 
+        if (ret_sender)
+                *ret_sender = dummy_link_local;
+
         return 0;
 }
 

src/libsystemd-network/icmp6-util.c

@@ -182,7 +182,7 @@ int icmp6_receive(
             sa.in6.sin6_family == AF_INET6)  {
 
                 addr = sa.in6.sin6_addr;
-                if (!in6_addr_is_link_local(&addr))
+                if (!in6_addr_is_link_local(&addr) && !in6_addr_is_null(&addr))
                         return -EADDRNOTAVAIL;
 
         } else if (msg.msg_namelen > 0)

src/libsystemd-network/sd-ndisc.c

@@ -225,7 +225,7 @@ static int ndisc_recv(sd_event_source *s, int fd, uint32_t revents, void *userda
         if (r < 0)
                 switch (r) {
                 case -EADDRNOTAVAIL:
-                        log_ndisc(nd, "Received RA from non-link-local address. Ignoring.");
+                        log_ndisc(nd, "Received RA from neither link-local nor null address. Ignoring.");
                         return 0;
 
                 case -EMULTIHOP:
@@ -241,6 +241,11 @@ static int ndisc_recv(sd_event_source *s, int fd, uint32_t revents, void *userda
                         return 0;
                 }
 
+        /* The function icmp6_receive() accepts the null source address, but RFC 4861 Section 6.1.2 states
+         * that hosts MUST discard messages with the null source address. */
+        if (in6_addr_is_null(&rt->address))
+                log_ndisc(nd, "Received RA from null address. Ignoring.");
+
         (void) event_source_disable(nd->timeout_event_source);
         (void) ndisc_handle_datagram(nd, rt);
         return 0;

src/libsystemd-network/sd-radv.c

@@ -264,7 +264,7 @@ static int radv_recv(sd_event_source *s, int fd, uint32_t revents, void *userdat
         if (r < 0)
                 switch (r) {
                 case -EADDRNOTAVAIL:
-                        log_radv(ra, "Received RS from non-link-local address. Ignoring");
+                        log_radv(ra, "Received RS from neither link-local nor null address. Ignoring");
                         return 0;
 
                 case -EMULTIHOP:
@@ -285,6 +285,9 @@ static int radv_recv(sd_event_source *s, int fd, uint32_t revents, void *userdat
                 return 0;
         }
 
+        /* TODO: if the sender address is null, check that the message does not have the source link-layer
+         * address option. See RFC 4861 Section 6.1.1. */
+
         const char *addr = IN6_ADDR_TO_STRING(&src);
 
         r = radv_send(ra, &src, ra->lifetime_usec);

src/libsystemd-network/test-ndisc-rs.c

@@ -41,7 +41,8 @@ static void router_dump(sd_ndisc_router *rt) {
         assert_se(rt);
 
         log_info("--");
-        assert_se(sd_ndisc_router_get_address(rt, &addr) == -ENODATA);
+        assert_se(sd_ndisc_router_get_address(rt, &addr) >= 0);
+        log_info("Sender: %s", IN6_ADDR_TO_STRING(&addr));
 
         assert_se(sd_ndisc_router_get_timestamp(rt, CLOCK_REALTIME, &t) >= 0);
         log_info("Timestamp: %s", FORMAT_TIMESTAMP(t));
@@ -176,6 +177,13 @@ int icmp6_bind_router_advertisement(int ifindex) {
         return -ENOSYS;
 }
 
+static struct in6_addr dummy_link_local = {
+        .s6_addr = {
+                0xfe, 0x80, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+                0x12, 0x34, 0x56, 0xff, 0xfe, 0x78, 0x9a, 0xbc,
+        },
+};
+
 int icmp6_receive(
                 int fd,
                 void *iov_base,
@@ -188,6 +196,9 @@ int icmp6_receive(
         if (ret_timestamp)
                 triple_timestamp_get(ret_timestamp);
 
+        if (ret_sender)
+                *ret_sender = dummy_link_local;
+
         return 0;
 }