[Qemu-devel] [PATCH 4/4] json-streamer: Limit number of tokens in addition to total size

[Markus Armbruster <armbru@redhat.com>]

Commit 29c75dd "json-streamer: limit the maximum recursion depth and
maximum token count" attempts to guard against excessive heap usage by
limiting total token size (it says "token count", but that's a lie).

Total token size is a rather imprecise predictor of heap usage: many
small tokens use more space than few large tokens with the same input
size, because there's a constant per-token overhead.

Tighten this up: limit the token count to 128Ki.

If you think 128Ki is too stingy: check-qjson's large_dict test eats a
sweet 500MiB and pegs a core for four minutes on my machine to parse
~100K tokens.  Absurdly wasteful.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
---
 qobject/json-streamer.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/qobject/json-streamer.c b/qobject/json-streamer.c
index 755c74d..df2b4c1 100644
--- a/qobject/json-streamer.c
+++ b/qobject/json-streamer.c
@@ -19,6 +19,7 @@
 #include "qapi/qmp/json-streamer.h"
 
 #define MAX_TOKEN_SIZE (64ULL << 20)
+#define MAX_TOKEN_COUNT (128ULL << 10)
 #define MAX_NESTING (1ULL << 10)
 
 static void json_message_process_token(JSONLexer *lexer, QString *token, JSONTokenType type, int x, int y)
@@ -64,6 +65,7 @@ static void json_message_process_token(JSONLexer *lexer, QString *token, JSONTok
          parser->bracket_count == 0)) {
         goto out_emit;
     } else if (parser->token_size > MAX_TOKEN_SIZE ||
+               qlist_size(parser->tokens) > MAX_TOKEN_COUNT ||
                parser->bracket_count + parser->brace_count > MAX_NESTING) {
         /* Security consideration, we limit total memory allocated per object
          * and the maximum recursion depth that a message can force.
-- 
2.4.3