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From 28afb6c537dc82ba04d5f245b15ca7205c6dbb9c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Petr=20P=C3=ADsa=C5=99?= <ppisar@redhat.com>
Date: Mon, 26 Feb 2018 13:48:00 +0100
Subject: [PATCH] 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>
---
libvncserver/rfbserver.c | 20 +++++++++++++++++++-
1 file changed, 19 insertions(+), 1 deletion(-)
diff --git a/libvncserver/rfbserver.c b/libvncserver/rfbserver.c
index 116c488..4fc4d9d 100644
--- a/libvncserver/rfbserver.c
+++ b/libvncserver/rfbserver.c
@@ -85,6 +88,8 @@
#include <errno.h>
/* strftime() */
#include <time.h>
+/* PRIu32 */
+#include <inttypes.h>
#ifdef LIBVNCSERVER_WITH_WEBSOCKETS
#include "rfbssl.h"
@@ -2577,7 +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);
--
2.17.0
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