linux-kernel - Re: [PATCH v6 20/27] x86: Support global stack cookie

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Message-ID: <CALCETrVcU_9cczwbfVg6E2cey5B8O9k4j+jYgp0T-CiX9HELtA@mail.gmail.com>
Date:   Fri, 1 Feb 2019 11:27:22 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Thomas Garnier <thgarnie@...omium.org>
Cc:     Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Kristen Carlson Accardi <kristen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Garnier <thgarnie@...gle.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Juergen Gross <jgross@...e.com>,
        Joerg Roedel <jroedel@...e.de>,
        Jia Zhang <qianyue.zj@...baba-inc.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 20/27] x86: Support global stack cookie

On Thu, Jan 31, 2019 at 11:29 AM Thomas Garnier <thgarnie@...omium.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.

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?