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Date: Wed, 05 Nov 2014 17:01:31 +0100
From: John Spencer <maillist-musl@...fooze.de>
To: musl@...ts.openwall.com
Subject: Re: fixing -fPIE + -fstack-protector-all
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.
>
> 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.
>
> 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.
that sounds reasonable. do you intend add this symbol and the mandatory
init_ssp code even to archs that don't need it (for example x86_64) ?
--JS
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