Multiple Vendor X Server Xdbe Extension 'ProcDbeSwapBuffers()' Memory Corruption
Vulnerability

iDefense Security Advisory XX.XX.06
http://www.idefense.com/intelligence/vulnerabilities/
MMM DD, 2006

I. BACKGROUND

The X Window System is a graphical windowing system based on a client/server
model. More information about about The X Window system is available at the
following link:

http://en.wikipedia.org/wiki/X_Window_System

II. DESCRIPTION

Local exploitation of a memory corruption vulnerability in the
'ProcDbeSwapBuffers()' function in the X.Org and XFree86 X server could allow an
attacker to execute arbitrary code with privileges of the X server, typically
root.

xorg-server-X11R7.1-1.1.0/dbe/dbe.c

static int
ProcDbeSwapBuffers(ClientPtr client)
{
    REQUEST(xDbeSwapBuffersReq);
    WindowPtr       pWin;
    DbeScreenPrivPtr    pDbeScreenPriv;
    DbeSwapInfoPtr  swapInfo;
    xDbeSwapInfo    *dbeSwapInfo;
    int         error;
    register int    i, j;
    int         nStuff;


    REQUEST_AT_LEAST_SIZE(xDbeSwapBuffersReq);
1]  nStuff = stuff->n;  /* use local variable for performance. */

    if (nStuff == 0)
    {
        return(Success);
    }

    /* Get to the swap info appended to the end of the request. */
    dbeSwapInfo = (xDbeSwapInfo *)&stuff[1];

    /* Allocate array to record swap information. */  
2] swapInfo = (DbeSwapInfoPtr)ALLOCATE_LOCAL(nStuff * sizeof(DbeSwapInfoRec));
    if (swapInfo == NULL)                                
    {
        return(BadAlloc);
    }


    for (i = 0; i < nStuff; i++)
    {
        /* Check all windows to swap. */

        /* Each window must be a valid window - BadWindow. */
3]     if (!(pWin = SecurityLookupWindow(dbeSwapInfo[i].window, client,
                      SecurityWriteAccess)))
        {
            DEALLOCATE_LOCAL(swapInfo);
        return(BadWindow);
        }

....

        /* Everything checks out OK.  Fill in the swap info array. */
4]     swapInfo[i].pWindow    = pWin;
        swapInfo[i].swapAction = dbeSwapInfo[i].swapAction;



1) This value is taken directly from the client's request buffer.

2) This macro will call alloca() on systems that support it, or malloc() on
systems that do not.  There are two vulnerabilities here.  First there is an
integer overflow when multiplying the two operands.  If nStuff is > 0xffffffff /
sizeof(DbeSwapInfoRec) the operation will overflow, and too little memory will
be allocated.  If malloc() is being used then this is the only way to exploit
the vulnerability.  The second vulnerability is passing a user controlled value
to the alloca() function.  This function takes its argument and subtracts it
from the current stack pointer in order to allocate local storage dynamically
without using the heap.  By passing alloca() a large unsigned value the stack
pointer can be set to nearly anywhere in the address space, including an area
outside of the actual stack segment.  This address can then be overwritten with
arbitrary values.

3) If alloca() is used then an attacker can trigger an overwrite of an arbitrary
address with arbitrary data when this function pushes its dbeSwapInfo[i].window
argument onto the stack.  This value is supplied by the attacker.

4) If malloc() is used then an attacker can trigger a heap overflow with data
they partially control. 

III. ANALYSIS

Successful local exploitation allows an attacker to execute arbitrary as the
root user. In order to exploit this vulnerability an attacker would require the
ability to send commands to an affected X server. This typically requires access
to the console, or access to the same account as a user who is on the console.
One method of gaining the required access would be to remotely exploit a
vulnerability in, for example, a graphical web browser. Thiw would then allow an
attacker to exploit this vulnerability and elevate their privileges to root.

As exploitation requires access to the X server, but typically gives local root,
iDefense considers this a MEDIUM-severity vulnerability.

IV. DETECTION

iDefense has confirmed the existance of this vulnerability in the X.org server
version 7.1-1.1.0. Previous versions may also be affected.

V. WORKAROUND

Disable loading of the dbe extension in the X configuration file. Depending on
the version of X being used this file name varies. Common names are:

/etc/X11/xorg.conf
/etx/X11/XF86Config

Removing the line:

Load "dbe"

from this file will stop the DBE extension from being loaded.

VI. VENDOR RESPONSE

[Quoted vendor response if available. Otherwise include vendor fix
details.]

VII. CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CVE-2006-XXXX to this issue. This is a candidate for inclusion in
the CVE list (http://cve.mitre.org), which standardizes names for
security problems.

[OR]

A Mitre Corp. Common Vulnerabilities and Exposures (CVE) number has not
been assigned yet.

VIII. DISCLOSURE TIMELINE

XX/XX/2006  Initial vendor notification
XX/XX/2006  Initial vendor response
XX/XX/2006  Coordinated public disclosure

IX. CREDIT

This vulnerability was discovered by Sean Larsson, iDefense Labs.

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X. LEGAL NOTICES

Copyright © 2006 iDefense, Inc.

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Disclaimer: The information in the advisory is believed to be accurate
at the time of publishing based on currently available information. Use
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