From 0073e4f694d5a51bb72ff12a5e8364b6e752e094 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] Validate client cut text length
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Client-provided unsigned 32-bit cut text length is passed to various
functions that expects argument of a different type.

E.g. "RFB 003.003\n\001\006\0\0\0\xff\xff\xff\xff" string sent to the
RFB server leads to 4294967295 msg.cct.length value that in turn is
interpreted as -1 by rfbReadExact() and thus uninitialized str buffer
with potentially sensitive data is passed to subsequent functions.

This patch fixes it by checking for a maximal value that still can be
processed correctly. It also corrects accepting length value of zero
(malloc(0) is interpreted on differnet systems differently).

Whether a client can make the server allocate up to 2 GB and cause
a denial of service on memory-tight systems is kept without answer.
A possible solution would be adding an arbitrary memory limit that is
deemed safe.

CVE-2018-7225
<https://github.com/LibVNC/libvncserver/issues/218>

Signed-off-by: Petr Písař <ppisar@redhat.com>
---
 libvncserver/rfbserver.c | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/libvncserver/rfbserver.c b/libvncserver/rfbserver.c
index 116c488..a9561fc 100644
--- a/libvncserver/rfbserver.c
+++ b/libvncserver/rfbserver.c
@@ -88,6 +88,12 @@
 #include <errno.h>
 /* strftime() */
 #include <time.h>
+/* SIZE_MAX */
+#include <stdint.h>
+/* PRIu32 */
+#include <inttypes.h>
+/* INT_MAX */
+#include <limits.h>
 
 #ifdef LIBVNCSERVER_WITH_WEBSOCKETS
 #include "rfbssl.h"
@@ -2575,7 +2581,21 @@ 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 check that the value fits into all of them to
+	 * prevent from misinterpretation and thus from accessing uninitialized
+	 * memory. CVE-2018-7225 */
+	if (msg.cct.length > SIZE_MAX || msg.cct.length > INT_MAX - sz_rfbClientCutTextMsg) {
+	    rfbLog("rfbClientCutText: too big cut text length requested: %" PRIu32 "\n",
+		    msg.cct.length);
+	    rfbCloseClient(cl);
+	    return;
+	}
+
+	/* Allow zero-length client cut text. */
+	str = (char *)malloc(msg.cct.length ? msg.cct.length : 1);
 	if (str == NULL) {
 		rfbLogPerror("rfbProcessClientNormalMessage: not enough memory");
 		rfbCloseClient(cl);
-- 
2.13.6