Re: [Qemu-devel] [PATCH 1/8] qdict: fix unbounded stack for qdict_array_entries

[Peter Xu <peterx@redhat.com>]

On Tue, Mar 08, 2016 at 11:19:44AM +0100, Kevin Wolf wrote:
> Am 08.03.2016 um 09:22 hat Markus Armbruster geschrieben:
> > Same arguments as for PATCH 2, except here an argument on the maximum
> > length of subqdict would probably be easier.
> 
> Yes, these are constant string literals in all callers, including the
> one non-test case in quorum.
> 
> Let's simply assert a reasonable maximum for subqdict_length. The
> minimum we need to allow with the existing callers is 9, and I expect
> we'll never get keys longer than 16 characters.

Hi, Kevin, Markus,

The patch should be trying to do as mentioned above. To make it
clearer, how about the following one:

diff --git a/qobject/qdict.c b/qobject/qdict.c
index 9833bd0..dde99e0 100644
--- a/qobject/qdict.c
+++ b/qobject/qdict.c
@@ -704,17 +704,16 @@ int qdict_array_entries(QDict *src, const char *subqdict)
     for (i = 0; i < INT_MAX; i++) {
         QObject *subqobj;
         int subqdict_entries;
-        size_t slen = 32 + subqdict_len;
-        char indexstr[slen], prefix[slen];
+        char indexstr[128], prefix[128];
         size_t snprintf_ret;

-        snprintf_ret = snprintf(indexstr, slen, "%s%u", subqdict, i);
-        assert(snprintf_ret < slen);
+        snprintf_ret = snprintf(indexstr, ARRAY_SIZE(indexstr), "%s%u", subqdict, i);
+        assert(snprintf_ret < ARRAY_SIZE(indexstr));

         subqobj = qdict_get(src, indexstr);

-        snprintf_ret = snprintf(prefix, slen, "%s%u.", subqdict, i);
-        assert(snprintf_ret < slen);
+        snprintf_ret = snprintf(prefix, ARRAY_SIZE(prefix), "%s%u.", subqdict, i);
+        assert(snprintf_ret < ARRAY_SIZE(prefix));

         subqdict_entries = qdict_count_prefixed_entries(src, prefix);
         if (subqdict_entries < 0) {

Two assertions on the snprintf_ret should make sure we are safe,
right?

Thanks.
Peter