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Date:   Thu, 20 Jan 2022 17:57:44 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Kyrylo Tkachov <Kyrylo.Tkachov@....com>
Cc:     "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>,
        Richard Sandiford <Richard.Sandiford@....com>,
        "thomas.preudhomme@...est.fr" <thomas.preudhomme@...est.fr>,
        Keith Packard <keithpac@...zon.com>,
        Kyrylo Tkachov <kyryo.tkachov@....com>,
        Richard Earnshaw <Richard.Earnshaw@....com>,
        Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
        "gcc-patches@....gnu.org" <gcc-patches@....gnu.org>
Subject: Re: [PATCH v5 1/1] [ARM] Add support for TLS register based stack
 protector canary access

On Wed, 19 Jan 2022 at 18:02, Ard Biesheuvel <ardb@...nel.org> wrote:
>
> On Wed, 19 Jan 2022 at 17:54, Kyrylo Tkachov <Kyrylo.Tkachov@....com> wrote:
> >
> > Hi Ard,
> >
> > > -----Original Message-----
> > > From: Gcc-patches <gcc-patches-
> > > bounces+kyrylo.tkachov=arm.com@....gnu.org> On Behalf Of Ard
> > > Biesheuvel via Gcc-patches
> > > Sent: Monday, November 15, 2021 6:04 PM
> > > To: linux-hardening@...r.kernel.org
> > > Cc: Richard Sandiford <Richard.Sandiford@....com>;
> > > thomas.preudhomme@...est.fr; Keith Packard <keithpac@...zon.com>;
> > > gcc-patches@....gnu.org; Kyrylo Tkachov <kyryo.tkachov@....com>; Ard
> > > Biesheuvel <ardb@...nel.org>
> > > Subject: [PATCH v5 1/1] [ARM] Add support for TLS register based stack
> > > protector canary access
> > >
> > > Add support for accessing the stack canary value via the TLS register,
> > > so that multiple threads running in the same address space can use
> > > distinct canary values. This is intended for the Linux kernel running in
> > > SMP mode, where processes entering the kernel are essentially threads
> > > running the same program concurrently: using a global variable for the
> > > canary in that context is problematic because it can never be rotated,
> > > and so the OS is forced to use the same value as long as it remains up.
> > >
> > > Using the TLS register to index the stack canary helps with this, as it
> > > allows each CPU to context switch the TLS register along with the rest
> > > of the process, permitting each process to use its own value for the
> > > stack canary.
> >
> > I've tested this patch on an arm-none-linux-gnueabihf target and the results look clean.
> > Have you tested this patch with a kernel build as well? (since the functionality is intended for that use).
>
> Of course.
>
> > If so, the patch is okay but please rebase it and repost so that we can commit it taking into account....
> >
>
> Will do.

I have sent out my v6 - please let me know if there is anything else I
need to do to get this landed.

Thanks,
Ard.

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