Re: Remote Windows Kernel Exploitation - Step Into the Ring 0

From: Tony Mason (Mason_at_OSR.COM)
Date: 03/10/05

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    Date:         Thu, 10 Mar 2005 14:47:39 -0500
    To: NTBUGTRAQ@LISTSERV.NTBUGTRAQ.COM
    
    

    Kernel stacks are "no execute" by default on x86 platforms that support
    NX and are configured for NX support (AMD-64, em64t running Windows XP
    SP2 with NX enabled). Thus, NO kernel code is exempt on such
    platforms.

    Windows will bugcheck ( ATTEMPTED_EXECUTE_OF_NOEXECUTE_MEMORY - 0xFC)
    for an attempt to execute stack-based code in kernel mode. This is not
    raised as an exception and thus is not something that can be handled
    inside a driver using ordinary means (much like a
    PAGE_FAULT_IN_NONPAGED_AREA cannot be trapped either).

    For processors that do not have NX support, this protection is not
    provided.

    Once inside the OS ("kernel mode") you are inside the trusted computing
    base and thus can change the processor state - so of course NX can be
    disabled. The OS developers can make this more difficult, but
    ultimately cannot prevent such things from happening - once you allow
    someone to install software in your OS, you have given away the keys to
    the kingdom. Of course there are ways of mitigating and even preventing
    this. For example, you could construct monitoring logic to detect
    changes in critical protections and report when they change, or even
    restore them. Microsoft has used this technique for years in ensuring
    critical licensing related registry keys are not changed without
    permission. Other OS designs may utilize multiple levels of control to
    have semi-trusted code. However, partially due to its heritage, Windows
    assumes only two privilege levels (recall that Windows NT was targeted
    to i860 and MIPS R3000 initially, only adding x86 during the development
    cycle.) Of course, x64 architecture only provides 2 levels of
    protection in long mode.

    Regards,

    Tony
     
    Tony Mason
    Consulting Partner
    OSR Open Systems Resources, Inc.
    http://www.osr.com

    -----Original Message-----

    Den 02/17/05 10:00 skrev Marc Maiffret:

    Does anyone on this list know if all kernel code is excepted from NX
    restrictions by default ? Because surely a utility-function like the
    keyboard
    buffer, or icmp handler should be read-execute only (ie trying to patch
    it
    should throw an exception of some kind) ?

    And if the pages are marked read-only, is it possible to patch the
    error-handler
    for that ?

    As the NX handling is toggable, it is obvious that some part of the
    kernel must
    be allowed to change it -- and it sounds reasonable that would mean all
    kernel
    code can change it, due to the NT memory model.

    Does anyone have more information on this ?

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