Re: fixing -fPIE + -fstack-protector-all

["Anthony G. Basile" <basile@opensource.dyc.edu>]

On 11/05/14 10:43, Rich Felker wrote:
> On Wed, Nov 05, 2014 at 04:25:03PM +0100, John Spencer wrote:
>> using -fPIE + -fstack-protector-all is currently broken for a number
>> of architectures (most notably i386) in the default gcc setup
>> (including the musl-cross patches), as it depends on a
>> libssp_nonshared.a which provides __stack_chk_fail_local().
>
> As discussed on IRC, I would _like_ to be able to simply add the
> following to crt/i386/crti.s:
>
> __stack_chk_fail_local: hlt
>
> and equivalent for other archs. This has the added benefit of
> effecting a crash without going through the PLT (whereas
> libssp_nonshared.a's __stack_chk_fail_local calls __stack_chk_fail via
> the PLT) so it's not vulnerable to attacks that have overwritten the
> GOT with malicious pointers.

For what its worth, hardening in gentoo (PaX kernel + userland hardening 
with relro and bindnow) tries to prevent this kind of attack by making 
the GOT read only after initial linking.

>
> However, this proposed solution breaks one odd corner case: static
> linking when all the source files were compiled with -fPIC or -fPIE.
> In that case, there would be no references to __stack_chk_fail, only
> to __stack_chk_fail_local, and thereby __init_ssp would not get
> linked, and a zero canary would be used.

I would rather not see this solution.

>
> One possible way to handle this would be giving up the conditional
> linking of ssp init and just always initializing it. The .o file is 78
> bytes on i386 and 70 bytes on x86_64, but there would also be some
> savings to offset the cost simply from having the code inline in
> __init_libc rather than as an external function.
>
> I'm open to other ideas too.
>
> Rich
>


-- 
Anthony G. Basile, Ph. D.
Chair of Information Technology
D'Youville College
Buffalo, NY 14201
(716) 829-8197