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SubjectRe: Question about binfmt_elf.c
On Fri, 16 Jul 2010 15:12:39 -0500
Rob Landley <rob@landley.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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