extmod/moductypes: Validate the descriptor tuple.

Fixes various null dereferencing, out-of-bounds memory accesses and
`assert(0)` failures in the case of an invalid `uctypes` descriptor.

By design `uctypes` can crash because it accesses arbitrary memory, but at
least describing the descriptor layout should be forced to be correct and
not crash.

Fixes issue micropython#12702.

Signed-off-by: stijn <stijn@ignitron.net>

extmod/moductypes.c

@@ -143,15 +143,26 @@ static inline mp_uint_t uctypes_struct_scalar_size(int val_type) {
 
 // Get size of aggregate type descriptor
 static mp_uint_t uctypes_struct_agg_size(mp_obj_tuple_t *t, int layout_type, mp_uint_t *max_field_size) {
+    if (t->len == 0) {
+        syntax_error();
+    }
+
     mp_uint_t total_size = 0;
 
     mp_int_t offset_ = MP_OBJ_SMALL_INT_VALUE(t->items[0]);
     mp_uint_t agg_type = GET_TYPE(offset_, AGG_TYPE_BITS);
 
     switch (agg_type) {
         case STRUCT:
+            if (t->len != 2) {
+                syntax_error();
+            }
             return uctypes_struct_size(t->items[1], layout_type, max_field_size);
         case PTR:
+            // Second field ignored, but should still be present for consistency.
+            if (t->len != 2) {
+                syntax_error();
+            }
             if (sizeof(void *) > *max_field_size) {
                 *max_field_size = sizeof(void *);
             }
@@ -167,15 +178,17 @@ static mp_uint_t uctypes_struct_agg_size(mp_obj_tuple_t *t, int layout_type, mp_
                 if (item_s > *max_field_size) {
                     *max_field_size = item_s;
                 }
-            } else {
+            } else if (t->len == 3) {
                 // Elements of array are aggregates
                 item_s = uctypes_struct_size(t->items[2], layout_type, max_field_size);
+            } else {
+                syntax_error();
             }
 
             return item_s * arr_sz;
         }
         default:
-            assert(0);
+            syntax_error();
     }
 
     return total_size;

tests/extmod/uctypes_sizeof.py

@@ -43,8 +43,47 @@
 print(uctypes.sizeof(S.sub))
 assert uctypes.sizeof(S.sub) == 1
 
-# invalid descriptor
+# invalid descriptors
 try:
     print(uctypes.sizeof([]))
 except TypeError:
     print("TypeError")
+
+try:
+    print(uctypes.sizeof(()))
+except TypeError:
+    print("TypeError")
+
+try:
+    print(uctypes.sizeof(("garbage",)))
+except TypeError:
+    print("TypeError")
+
+try:
+    # PTR * 3 is intended to be an invalid agg_type (STRUCT, PTR, ARRAY are valid ones).
+    print(uctypes.sizeof((uctypes.PTR * 3,)))
+except TypeError:
+    print("TypeError")
+
+try:
+    print(uctypes.sizeof((0, {}, "garbage")))
+except TypeError:
+    print("TypeError")
+
+try:
+    print(uctypes.sizeof((uctypes.PTR | 0,)))
+except TypeError:
+    print("TypeError")
+
+try:
+    print(uctypes.sizeof((uctypes.ARRAY | 0,)))
+except TypeError:
+    print("TypeError")
+
+try:
+    print(uctypes.sizeof((uctypes.ARRAY | 0, 1, {}, "garbage")))
+except TypeError:
+    print("TypeError")
+
+# empty descriptor
+print(uctypes.sizeof({}))

tests/extmod/uctypes_sizeof.py.exp

@@ -5,3 +5,11 @@ TypeError
 6
 1
 TypeError
+TypeError
+TypeError
+TypeError
+TypeError
+TypeError
+TypeError
+TypeError
+0

tests/extmod/uctypes_sizeof_od.py

@@ -45,9 +45,3 @@
 
 print(uctypes.sizeof(S.sub))
 assert uctypes.sizeof(S.sub) == 1
-
-# invalid descriptor
-try:
-    print(uctypes.sizeof([]))
-except TypeError:
-    print("TypeError")

tests/extmod/uctypes_sizeof_od.py.exp

@@ -4,4 +4,3 @@
 TypeError
 6
 1
-TypeError