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Limit client cut text length to 1 MB

This patch constrains a client cut text length to 1 MB. Otherwise
a client could make server allocate 2 GB of memory and that seems to
be to much to classify it as a denial of service.

The limit also prevents from an integer overflow followed by copying
an uninitilized memory when processing msg.cct.length value larger
than SIZE_MAX or INT_MAX - sz_rfbClientCutTextMsg.

This patch also corrects accepting length value of zero (malloc(0) is
interpreted on differnet systems differently).

CVE-2018-7225
<https://github.com/LibVNC/libvncserver/issues/218>
pull/3/head
Petr Písař 4 years ago
parent
commit
28afb6c537
  1. 20
      libvncserver/rfbserver.c

20
libvncserver/rfbserver.c

@ -88,6 +88,8 @@
#include <errno.h>
/* strftime() */
#include <time.h>
/* PRIu32 */
#include <inttypes.h>
#ifdef LIBVNCSERVER_WITH_WEBSOCKETS
#include "rfbssl.h"
@ -2575,7 +2577,23 @@ rfbProcessClientNormalMessage(rfbClientPtr cl)
msg.cct.length = Swap32IfLE(msg.cct.length);
str = (char *)malloc(msg.cct.length);
/* uint32_t input is passed to malloc()'s size_t argument,
* to rfbReadExact()'s int argument, to rfbStatRecordMessageRcvd()'s int
* argument increased of sz_rfbClientCutTextMsg, and to setXCutText()'s int
* argument. Here we impose a limit of 1 MB so that the value fits
* into all of the types to prevent from misinterpretation and thus
* from accessing uninitialized memory (CVE-2018-7225) and also to
* prevent from a denial-of-service by allocating to much memory in
* the server. */
if (msg.cct.length > 1<<20) {
rfbLog("rfbClientCutText: too big cut text length requested: %" PRIu32 "\n",
msg.cct.length);
rfbCloseClient(cl);
return;
}
/* Allow zero-length client cut text. */
str = (char *)calloc(msg.cct.length ? msg.cct.length : 1, 1);
if (str == NULL) {
rfbLogPerror("rfbProcessClientNormalMessage: not enough memory");
rfbCloseClient(cl);

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