linux-kernel - Re: Question about binfmt

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Date:	Wed, 21 Jul 2010 16:30:59 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Rob Landley <rob@...dley.net>
Cc:	linux-kernel@...r.kernel.org, Jakub Jelinek <jakub@...hat.com>,
	Roland McGrath <roland@...hat.com>
Subject: Re: Question about binfmt_elf.c

On Fri, 16 Jul 2010 15:12:39 -0500
Rob Landley <rob@...dley.net> wrote:

> Could somebody please update this comment to explain why fiddling with 
> strangely protected bss is _not_ an easy way to leak arbitrary amounts of 
> uninitalized kernel memory (with whatever previous contents they have) to 
> userspace?
> 
>     nbyte = ELF_PAGEOFFSET(elf_bss);
>     if (nbyte) {
>             nbyte = ELF_MIN_ALIGN - nbyte;
>             if (nbyte > elf_brk - elf_bss)
>                         nbyte = elf_brk - elf_bss;
>             if (clear_user((void __user *)elf_bss +
>                                     load_bias, nbyte)) {
>                     /*
>                      * This bss-zeroing can fail if the ELF
>                      * file specifies odd protections. So
>                      * we don't check the return value
>                       */
>             }
>     }
> 
> Just curious.  Reading through the code and trying to understand it...
> 

In January 2005 davem added a check.  In Feb 2005 Pavel said "hey, my
Kylix application broke".  So we took the check out again.

http://www.mail-archive.com/linux-kernel@vger.kernel.org/msg60120.html
http://www.mail-archive.com/bk-commits-head@vger.kernel.org/msg01390.html

I don't kow how one would craft such an elf file.  I don't _think_ it
could leak unintialised memory, as we probably haven't faulted the page
in yet.  Perhaps a partial page could be exposed though.

Roland, Jakub: help!
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