json: Enforce token count and size limits more tightly

Token count and size limits exist to guard against excessive heap
usage.  We check them only after we created the token on the heap.
That's assigning a cowboy to the barn to lasso the horse after it has
bolted.  Close the barn door instead: check before we create the
token.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Message-Id: <20180823164025.12553-49-armbru@redhat.com>

qobject/json-streamer.c

@@ -20,7 +20,7 @@
 
 #define MAX_TOKEN_SIZE (64ULL << 20)
 #define MAX_TOKEN_COUNT (2ULL << 20)
-#define MAX_NESTING (1ULL << 10)
+#define MAX_NESTING (1 << 10)
 
 static void json_message_free_token(void *token, void *opaque)
 {
@@ -71,6 +71,23 @@ void json_message_process_token(JSONLexer *lexer, GString *input,
         break;
     }
 
+    /*
+     * Security consideration, we limit total memory allocated per object
+     * and the maximum recursion depth that a message can force.
+     */
+    if (parser->token_size + input->len + 1 > MAX_TOKEN_SIZE) {
+        error_setg(&err, "JSON token size limit exceeded");
+        goto out_emit;
+    }
+    if (g_queue_get_length(parser->tokens) + 1 > MAX_TOKEN_COUNT) {
+        error_setg(&err, "JSON token count limit exceeded");
+        goto out_emit;
+    }
+    if (parser->bracket_count + parser->brace_count > MAX_NESTING) {
+        error_setg(&err, "JSON nesting depth limit exceeded");
+        goto out_emit;
+    }
+
     token = g_malloc(sizeof(JSONToken) + input->len + 1);
     token->type = type;
     memcpy(token->str, input->str, input->len);
@@ -91,23 +108,6 @@ void json_message_process_token(JSONLexer *lexer, GString *input,
         goto out_emit;
     }
 
-    /*
-     * Security consideration, we limit total memory allocated per object
-     * and the maximum recursion depth that a message can force.
-     */
-    if (parser->token_size > MAX_TOKEN_SIZE) {
-        error_setg(&err, "JSON token size limit exceeded");
-        goto out_emit;
-    }
-    if (g_queue_get_length(parser->tokens) > MAX_TOKEN_COUNT) {
-        error_setg(&err, "JSON token count limit exceeded");
-        goto out_emit;
-    }
-    if (parser->bracket_count + parser->brace_count > MAX_NESTING) {
-        error_setg(&err, "JSON nesting depth limit exceeded");
-        goto out_emit;
-    }
-
     return;
 
 out_emit: