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: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180823164025.12553-49-armbru@redhat.com>
---
 qobject/json-streamer.c | 36 ++++++++++++++++++------------------
 1 file changed, 18 insertions(+), 18 deletions(-)

diff --git a/qobject/json-streamer.c b/qobject/json-streamer.c
index 674dfe6e85..810aae521f 100644
--- a/qobject/json-streamer.c
+++ b/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:
-- 
2.17.1