hmp: fix sendkey out of bounds write (CVE-2015-8619)

When processing 'sendkey' command, hmp_sendkey routine null
terminates the 'keyname_buf' array. This results in an OOB
write issue, if 'keyname_len' was to fall outside of
'keyname_buf' array.

Since the keyname's length is known the keyname_buf can be
removed altogether by adding a length parameter to
index_from_key() and using it for the error output as well.

Reported-by: Ling Liu <liuling-it@360.cn>
Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
Message-Id: <20160113080958.GA18934@olga>
[Comparison with "<" dumbed down, test for junk after strtoul()
tweaked]
Signed-off-by: Markus Armbruster <armbru@redhat.com>

ui/input-legacy.c

@@ -57,12 +57,13 @@ struct QEMUPutLEDEntry {
 static QTAILQ_HEAD(, QEMUPutLEDEntry) led_handlers =
     QTAILQ_HEAD_INITIALIZER(led_handlers);
 
-int index_from_key(const char *key)
+int index_from_key(const char *key, size_t key_length)
 {
     int i;
 
     for (i = 0; QKeyCode_lookup[i] != NULL; i++) {
-        if (!strcmp(key, QKeyCode_lookup[i])) {
+        if (!strncmp(key, QKeyCode_lookup[i], key_length) &&
+            !QKeyCode_lookup[i][key_length]) {
             break;
         }
     }