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Date: Wed, 18 Apr 2001 15:07:30 -0400
From: Christopher Faylor <cgf AT redhat DOT com>
To: cygdev <cygwin-developers AT cygwin DOT com>
Subject: Re: handle protection - please comment
Message-ID: <20010418150730.C3648@redhat.com>
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In-Reply-To: <20010418164712.J15005@cygbert.vinschen.de>; from vinschen@redhat.com on Wed, Apr 18, 2001 at 04:47:12PM +0200

On Wed, Apr 18, 2001 at 04:47:12PM +0200, Corinna Vinschen wrote:
>On Wed, Apr 18, 2001 at 06:26:40PM +0400, egor duda wrote:
>> CV> Process A needs a handle to a thing T which is owned by process B.
>> CV> To get the handle, the owner B needs to get the process handle of
>> CV> A to duplicate the handle and return it to A. So if A is the attacker
>> CV> it has no chance to undergo the permissions of B since it never
>> CV> sees the process handle of B. OTOH, if B is a malicious server, it
>> CV> has no chance to use ReadProcessMemory() if A gives B the own process
>> CV> handle with only PROCESS_DUP_HANDLE access.
>> 
>> the problem is that if i (process B) have handle of process with only
>> PROCESS_DUP_HANDLE access, and process A have at least one private
>> handle of itself with full access, i can elevate my privileges
>> using the code quoted above. for cygwin programs the second thing is
>> true -- every process has hMainProcess handle of itself. It won't give
>> it to me, sure, but i can just try to guess it. i'll start duplicating
>> all values 0x4,0x8,0xc,0x10,... (remember, i can duplicate handles
>> from process  A), and sooner or later i'll find hMainProcess. voila,
>> process B have all access to process' A address space.
>
>I see. But that's a Cygwin specific problem only, AFAICS. Since
>`_dll_crt0' creates hMainProc as a duplicate of `GetCurrentProcess()'
>all processes have a real process handle to itself with full access
>rights which could be duplicated by other processes. I'm not sure
>why this is done because the return value of `GetCurrentProcess()'
>should fit our needs and it can neither be duplicated nor used by
>other processes.

This handle is inherited by child processes.

I also did it to avoid the overhead of calling GetCurrentProcess
repeatedly.

cgf