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From 0073e4f694d5a51bb72ff12a5e8364b6e752e094 Mon Sep 17 00:00:00 2001
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From: =?UTF-8?q?Petr=20P=C3=ADsa=C5=99?= <ppisar@redhat.com>
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Date: Mon, 26 Feb 2018 13:48:00 +0100
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Subject: [PATCH] Validate client cut text length
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MIME-Version: 1.0
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Content-Type: text/plain; charset=UTF-8
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Content-Transfer-Encoding: 8bit
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Client-provided unsigned 32-bit cut text length is passed to various
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functions that expects argument of a different type.
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E.g. "RFB 003.003\n\001\006\0\0\0\xff\xff\xff\xff" string sent to the
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RFB server leads to 4294967295 msg.cct.length value that in turn is
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interpreted as -1 by rfbReadExact() and thus uninitialized str buffer
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with potentially sensitive data is passed to subsequent functions.
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This patch fixes it by checking for a maximal value that still can be
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processed correctly. It also corrects accepting length value of zero
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(malloc(0) is interpreted on differnet systems differently).
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Whether a client can make the server allocate up to 2 GB and cause
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a denial of service on memory-tight systems is kept without answer.
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A possible solution would be adding an arbitrary memory limit that is
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deemed safe.
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CVE-2018-7225
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<https://github.com/LibVNC/libvncserver/issues/218>
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Signed-off-by: Petr Písař <ppisar@redhat.com>
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---
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libvncserver/rfbserver.c | 22 +++++++++++++++++++++-
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1 file changed, 21 insertions(+), 1 deletion(-)
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diff --git a/libvncserver/rfbserver.c b/libvncserver/rfbserver.c
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index 116c488..a9561fc 100644
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--- a/libvncserver/rfbserver.c
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+++ b/libvncserver/rfbserver.c
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@@ -88,6 +88,12 @@
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#include <errno.h>
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/* strftime() */
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#include <time.h>
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+/* SIZE_MAX */
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+#include <stdint.h>
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+/* PRIu32 */
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+#include <inttypes.h>
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+/* INT_MAX */
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+#include <limits.h>
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#ifdef LIBVNCSERVER_WITH_WEBSOCKETS
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#include "rfbssl.h"
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@@ -2575,7 +2581,21 @@ rfbProcessClientNormalMessage(rfbClientPtr cl)
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msg.cct.length = Swap32IfLE(msg.cct.length);
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- str = (char *)malloc(msg.cct.length);
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+ /* uint32_t input is passed to malloc()'s size_t argument,
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+ * to rfbReadExact()'s int argument, to rfbStatRecordMessageRcvd()'s int
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+ * argument increased of sz_rfbClientCutTextMsg, and to setXCutText()'s int
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+ * argument. Here we check that the value fits into all of them to
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+ * prevent from misinterpretation and thus from accessing uninitialized
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+ * memory. CVE-2018-7225 */
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+ if (msg.cct.length > SIZE_MAX || msg.cct.length > INT_MAX - sz_rfbClientCutTextMsg) {
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+ rfbLog("rfbClientCutText: too big cut text length requested: %" PRIu32 "\n",
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+ msg.cct.length);
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+ rfbCloseClient(cl);
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+ return;
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+ }
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+
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+ /* Allow zero-length client cut text. */
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+ str = (char *)malloc(msg.cct.length ? msg.cct.length : 1);
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if (str == NULL) {
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rfbLogPerror("rfbProcessClientNormalMessage: not enough memory");
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rfbCloseClient(cl);
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--
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2.13.6
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