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From: Thomas Garnier <thgarnie@chromium.org>
Date: Fri, 1 Feb 2019 15:56:07 -0800
Message-ID: <CAJcbSZGRFhQL56VnLwJn=r6MVjk9miYVeKgwdP+P45JT2q7QdA@mail.gmail.com>
Subject: Re: [PATCH v6 20/27] x86: Support global stack cookie
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To: Andy Lutomirski <luto@amacapital.net>
Cc: Andy Lutomirski <luto@kernel.org>, Kernel Hardening <kernel-hardening@lists.openwall.com>, Kristen Carlson Accardi <kristen@linux.intel.com>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>, X86 ML <x86@kernel.org>, Masahiro Yamada <yamada.masahiro@socionext.com>, Juergen Gross <jgross@suse.com>, Joerg Roedel <jroedel@suse.de>, Jia Zhang <qianyue.zj@alibaba-inc.com>, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>, Tim Chen <tim.c.chen@linux.intel.com>, LKML <linux-kernel@vger.kernel.org>
List-ID: <kernel-hardening.lists.openwall.com>

On Fri, Feb 1, 2019 at 2:36 PM Andy Lutomirski <luto@amacapital.net> wrote:
>
>
> > On Feb 1, 2019, at 12:21 PM, Thomas Garnier <thgarnie@chromium.org> wrote:
> >
> >> On Fri, Feb 1, 2019 at 11:27 AM Andy Lutomirski <luto@kernel.org> wrote:
> >>
> >>> On Thu, Jan 31, 2019 at 11:29 AM Thomas Garnier <thgarnie@chromium.org> wrote:
> >>>
> >>> Add an off-by-default configuration option to use a global stack cookie
> >>> instead of the default TLS. This configuration option will only be used
> >>> with PIE binaries.
> >>>
> >>> For kernel stack cookie, the compiler uses the mcmodel=kernel to switch
> >>> between the fs segment to gs segment. A PIE binary does not use
> >>> mcmodel=kernel because it can be relocated anywhere, therefore the
> >>> compiler will default to the fs segment register. This is fixed on the
> >>> latest version of gcc.
> >>
> >> I hate all these gcc-sucks-so-we-hack-it-and-change-nasty-semantics
> >> options.  How about just preventing use of both stack protector and
> >> PIE unless the version of gcc in use is new enough.
> >
> > So fail the build in this scenario?
>
> Fail the build or use some Kconfig magic to prevent this from being configured in the first place.

Ok, I can do that in next iteration.

>
> >
> >>
> >> Also, does -mstack-protector-guard-reg not solve this?  See
> >> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81708.  Or is there
> >> another bug?  Or are you worried about gcc versions that don't have
> >> that feature yet?
> >
> > I am worried about gcc versions that don't have this feature, yes.