extmod/modlwip: Ensure socket is finalisable if error during creation.

Because socket objects have a finaliser they must be created carefully, in
case an exception is raised during the population of their members, eg
invalid input argument or out-of-memory when allocating additional arrays.

Prior to the fix in this commit, the finaliser would crash due to
`incoming.udp_raw.array` being an invalid pointer in the following cases:
- if a SOCK_RAW was created with a proto argument that was not an integer
- if a SOCK_DGRAM or SOCK_RAW was created where the allocation of
  `lwip_incoming_packet_t` failed
- if an integer was passed in for the socket type but it was not one of
  SOCK_STREAM, SOCK_DGRAM or SOCK_RAW

Furthermore, if the allocation of `lwip_incoming_packet_t` failed then it
may have led to corruption within lwIP when freeing `socket->pcb.raw`
because that PCB was not fully set up with its callbacks.

This commit fixes all of these issues by ensuring:
- `pcb.tcp` and `incoming.udp_raw.array` are initialised to NULL early on
- the proto argument is parsed before allocating the PCB
- the allocation of `lwip_incoming_packet_t` occurs befor allocating the
  PCB
- `incoming.udp_raw.array` is checked for NULL in the finaliser code

The corresponding test (which already checked most of these causes of
failure) has been updated to include a previously-uncovered scenario.

Signed-off-by: Damien George <damien@micropython.org>

extmod/modlwip.c

@@ -384,7 +384,7 @@ static void lwip_socket_free_incoming(lwip_socket_obj_t *socket, bool free_queue
                 pbuf_free(socket->incoming.tcp.pbuf);
                 socket->incoming.tcp.pbuf = NULL;
             }
-        } else {
+        } else if (socket->incoming.udp_raw.array != NULL) {
             for (size_t i = 0; i < LWIP_INCOMING_PACKET_QUEUE_LEN; ++i) {
                 lwip_incoming_packet_t *slot = &socket->incoming.udp_raw.array[i];
                 if (slot->pbuf != NULL) {
@@ -938,7 +938,12 @@ static void lwip_socket_print(const mp_print_t *print, mp_obj_t self_in, mp_prin
 static mp_obj_t lwip_socket_make_new(const mp_obj_type_t *type, size_t n_args, size_t n_kw, const mp_obj_t *args) {
     mp_arg_check_num(n_args, n_kw, 0, 4, false);
 
+    // Once the socket is allocated it must be in a valid state to be finalised:
+    // - `incoming.udp_raw.array` is NULL or a valid heap pointer
+    // - `pcb` is NULL or a valid lwIP PCB that has been fully initialised
     lwip_socket_obj_t *socket = mp_obj_malloc_with_finaliser(lwip_socket_obj_t, &lwip_socket_type);
+    socket->pcb.tcp = NULL;
+    socket->incoming.udp_raw.array = NULL;
     socket->timeout = -1;
     socket->recv_offset = 0;
     socket->domain = MOD_NETWORK_AF_INET;
@@ -946,10 +951,16 @@ static mp_obj_t lwip_socket_make_new(const mp_obj_type_t *type, size_t n_args, s
     socket->callback = MP_OBJ_NULL;
     socket->state = STATE_NEW;
 
+    // Parse given arguments.
+    uint8_t socket_proto = 0;
+    (void)socket_proto;
     if (n_args >= 1) {
         socket->domain = mp_obj_get_int(args[0]);
         if (n_args >= 2) {
             socket->type = mp_obj_get_int(args[1]);
+            if (n_args >= 3) {
+                socket_proto = mp_obj_get_int(args[2]);
+            }
         }
     }
 
@@ -963,18 +974,17 @@ static mp_obj_t lwip_socket_make_new(const mp_obj_type_t *type, size_t n_args, s
         #if MICROPY_PY_LWIP_SOCK_RAW
         case MOD_NETWORK_SOCK_RAW:
         #endif
+            socket->incoming.udp_raw.array = m_new0(lwip_incoming_packet_t, LWIP_INCOMING_PACKET_QUEUE_LEN);
             if (socket->type == MOD_NETWORK_SOCK_DGRAM) {
                 socket->pcb.udp = udp_new();
             }
             #if MICROPY_PY_LWIP_SOCK_RAW
             else {
-                mp_int_t proto = n_args <= 2 ? 0 : mp_obj_get_int(args[2]);
-                socket->pcb.raw = raw_new(proto);
+                socket->pcb.raw = raw_new(socket_proto);
             }
             #endif
             socket->incoming.udp_raw.iget = 0;
             socket->incoming.udp_raw.iput = 0;
-            socket->incoming.udp_raw.array = m_new0(lwip_incoming_packet_t, LWIP_INCOMING_PACKET_QUEUE_LEN);
             break;
         default:
             mp_raise_OSError(MP_EINVAL);

tests/extmod/socket_badconstructor.py

@@ -16,6 +16,11 @@ try:
 except TypeError:
     print("TypeError")
 
+try:
+    s = socket.socket(socket.AF_INET, 123456)
+except OSError:
+    print("OSError")
+
 try:
     s = socket.socket(socket.AF_INET, socket.SOCK_RAW, None)
 except TypeError: