[posix] more robust netlink message handler (#9613)

Check and ensure at least the netlink header is received in the netlink
socket to be safe before intepreting the buffer as a netlink message.

Per https://linux.die.net/man/3/netlink, should make sure
current message type is not NLMSG_DONE before calling NLMSG_NEXT
to properly handle multi-part netlink message.

src/posix/platform/netif.cpp

@@ -1636,7 +1636,18 @@ static void processNetlinkEvent(otInstance *aInstance)
 
     length = recv(sNetlinkFd, msgBuffer.buffer, sizeof(msgBuffer.buffer), 0);
 
-    VerifyOrExit(length > 0);
+#if defined(__linux__)
+#define HEADER_SIZE sizeof(nlmsghdr)
+#else
+#define HEADER_SIZE sizeof(rt_msghdr)
+#endif
+
+    // Ensures full netlink header is received
+    if (length < static_cast<ssize_t>(HEADER_SIZE))
+    {
+        otLogWarnPlat("[netif] Unexpected netlink recv() result: %ld", static_cast<long>(length));
+        ExitNow();
+    }
 
 #if defined(__linux__)
     for (struct nlmsghdr *msg = &msgBuffer.nlMsg; NLMSG_OK(msg, static_cast<size_t>(length));
@@ -1654,6 +1665,12 @@ static void processNetlinkEvent(otInstance *aInstance)
 #endif
         switch (msg->nlmsg_type)
         {
+#if defined(__linux__)
+        case NLMSG_DONE:
+            // NLMSG_DONE indicates the end of the netlink message, exits now
+            ExitNow();
+#endif
+
         case RTM_NEWADDR:
         case RTM_DELADDR:
             processNetifAddrEvent(aInstance, msg);