From: Michal Srb <michalsrb@gmail.com>
Date: Thu, 6 Apr 2017 20:52:22 +0000 (+0300)
Subject: Limit size of cursor accepted by client.
X-Git-Tag: v1.7.90~3
X-Git-Url: https://source.dussan.org/?a=commitdiff_plain;h=c26b4b3bd20b40ca5f1ae9477164473fbd94995d;p=tigervnc.git

Limit size of cursor accepted by client.

Width and height of a cursor are received as U16 from network. Accepting full range of U16 values can cause integer overflows in multiple places.

The worst is probably VLA in CMsgReader::readSetXCursor:
  rdr::U8 buf[width*height*4];

The width*height*4 can be too big to fit on stack or it can overflow into negative numbers. Both cases are undefined behaviour. Following writes to buf can overwrite other data on stack.
---

diff --git a/common/rfb/CMsgReader.cxx b/common/rfb/CMsgReader.cxx
index 7233fbd7..9abe3f24 100644
--- a/common/rfb/CMsgReader.cxx
+++ b/common/rfb/CMsgReader.cxx
@@ -202,6 +202,9 @@ void CMsgReader::readRect(const Rect& r, int encoding)
 
 void CMsgReader::readSetXCursor(int width, int height, const Point& hotspot)
 {
+  if (width > maxCursorSize || height > maxCursorSize)
+    throw Exception("Too big cursor");
+
   rdr::U8 pr, pg, pb;
   rdr::U8 sr, sg, sb;
   int data_len = ((width+7)/8) * height;
@@ -257,6 +260,9 @@ void CMsgReader::readSetXCursor(int width, int height, const Point& hotspot)
 
 void CMsgReader::readSetCursor(int width, int height, const Point& hotspot)
 {
+  if (width > maxCursorSize || height > maxCursorSize)
+    throw Exception("Too big cursor");
+
   int data_len = width * height * (handler->cp.pf().bpp/8);
   int mask_len = ((width+7)/8) * height;
   rdr::U8Array data(data_len);
@@ -295,6 +301,9 @@ void CMsgReader::readSetCursor(int width, int height, const Point& hotspot)
 
 void CMsgReader::readSetCursorWithAlpha(int width, int height, const Point& hotspot)
 {
+  if (width > maxCursorSize || height > maxCursorSize)
+    throw Exception("Too big cursor");
+
   int encoding;
 
   const PixelFormat rgbaPF(32, 32, false, true, 255, 255, 255, 16, 8, 0);
diff --git a/common/rfb/CMsgReader.h b/common/rfb/CMsgReader.h
index ff73414e..7b52033f 100644
--- a/common/rfb/CMsgReader.h
+++ b/common/rfb/CMsgReader.h
@@ -69,6 +69,8 @@ namespace rfb {
     CMsgHandler* handler;
     rdr::InStream* is;
     int nUpdateRectsLeft;
+
+    static const int maxCursorSize = 256;
   };
 }
 #endif