nd6: Update RA link-local addr option length check

lwIP might support different hardware address lengths (when using
Ethernet and 6LoWPAN for instance). Match provided lladdr length
from Router Advertisement to the current network interface instead
of comparing against longest that can be stored.

CHANGELOG

@@ -6,6 +6,11 @@ HISTORY
 
   * [Enter new changes just after this line - do not remove this line]
 
+  ++ Bugfixes:
+
+  2023-09-28: Erik Ekman
+  * Fix ND6 Router Advertisement parsing when NETIF_MAX_HWADDR_LEN is above 6.
+
 (STABLE-2.2.0):
 
   2018-10-02: Dirk Ziegelmeier

src/core/ipv6/nd6.c

@@ -687,11 +687,12 @@ nd6_input(struct pbuf *p, struct netif *inp)
       case ND6_OPTION_TYPE_SOURCE_LLADDR:
       {
         struct lladdr_option *lladdr_opt;
-        if (option_len < sizeof(struct lladdr_option)) {
+        if (option_len < ND6_LLADDR_OPTION_MIN_LENGTH) {
           goto lenerr_drop_free_return;
         }
         lladdr_opt = (struct lladdr_option *)buffer;
-        if ((default_router_list[i].neighbor_entry != NULL) &&
+        if ((lladdr_opt->length == inp->hwaddr_len) &&
+            (default_router_list[i].neighbor_entry != NULL) &&
             (default_router_list[i].neighbor_entry->state == ND6_INCOMPLETE)) {
           SMEMCPY(default_router_list[i].neighbor_entry->lladdr, lladdr_opt->addr, inp->hwaddr_len);
           default_router_list[i].neighbor_entry->state = ND6_REACHABLE;

src/include/lwip/prot/nd6.h

@@ -153,6 +153,7 @@ PACK_STRUCT_END
 /** Link-layer address option. */
 #define ND6_OPTION_TYPE_SOURCE_LLADDR (0x01)
 #define ND6_OPTION_TYPE_TARGET_LLADDR (0x02)
+#define ND6_LLADDR_OPTION_MIN_LENGTH (2)
 #ifdef PACK_STRUCT_USE_INCLUDES
 #  include "arch/bpstruct.h"
 #endif