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Date: Tue, 4 Dec 2018 16:04:33 +0100
From: Ard Biesheuvel <ard.biesheuvel@...aro.org>
To: Nicolas Pitre <nicolas.pitre@...aro.org>
Cc: Kees Cook <keescook@...omium.org>, 
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>, Russell King <linux@...linux.org.uk>, 
	Emese Revfy <re.emese@...il.com>, Arnd Bergmann <arnd@...db.de>, Laura Abbott <labbott@...hat.com>, 
	Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [RFC/RFT PATCH v2] ARM: smp: add support for per-task stack canaries

On Fri, 30 Nov 2018 at 17:37, Nicolas Pitre <nicolas.pitre@...aro.org> wrote:
>
> On Fri, 30 Nov 2018, Ard Biesheuvel wrote:
>
> > On Thu, 29 Nov 2018 at 23:58, Nicolas Pitre <nicolas.pitre@...aro.org> wrote:
> > >
> > > On Tue, 27 Nov 2018, Ard Biesheuvel wrote:
> > >
> > > > On ARM, we currently only change the value of the stack canary when
> > > > switching tasks if the kernel was built for UP. On SMP kernels, this
> > > > is impossible since the stack canary value is obtained via a global
> > > > symbol reference, which means
> > > > a) all running tasks on all CPUs must use the same value
> > > > b) we can only modify the value when no kernel stack frames are live
> > > >    on any CPU, which is effectively never.
> > > >
> > > > So instead, use a GCC plugin to add a RTL pass that replaces each
> > > > reference to the address of the __stack_chk_guard symbol with an
> > > > expression that produces the address of the 'stack_canary' field
> > > > that is added to struct thread_info. This way, each task will use
> > > > its own randomized value.
> > > >
> > > > Cc: Russell King <linux@...linux.org.uk>
> > > > Cc: Kees Cook <keescook@...omium.org>
> > > > Cc: Emese Revfy <re.emese@...il.com>
> > > > Cc: Arnd Bergmann <arnd@...db.de>
> > > > Cc: Nicolas Pitre <nicolas.pitre@...aro.org>
> > > > Cc: Laura Abbott <labbott@...hat.com>
> > > > Cc: kernel-hardening@...ts.openwall.com
> > > > Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> > > > ---
> > > > v2:
> > > > - adopt powerpc Makefile approach to pass stack canary offset and thread
> > > >   size to the plugin as command line arguments
> > > > - depend on !XIP_DEFLATED_DATA
> > > >
> > > >  arch/arm/Kconfig                              |  16 +++
> > > >  arch/arm/Makefile                             |  11 +++
> > > >  arch/arm/boot/compressed/Makefile             |   1 +
> > > >  arch/arm/include/asm/stackprotector.h         |  12 ++-
> > > >  arch/arm/include/asm/thread_info.h            |   3 +
> > > >  arch/arm/kernel/asm-offsets.c                 |   1 +
> > > >  arch/arm/kernel/process.c                     |   6 +-
> > > >  scripts/Makefile.gcc-plugins                  |   6 ++
> > > >  scripts/gcc-plugins/Kconfig                   |   4 +
> > > >  scripts/gcc-plugins/arm_ssp_per_task_plugin.c | 103 ++++++++++++++++++++
> > > >  10 files changed, 160 insertions(+), 3 deletions(-)
> > > >
> > > > diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> > > > index 91be74d8df65..6142e83b45da 100644
> > > > --- a/arch/arm/Kconfig
> > > > +++ b/arch/arm/Kconfig
> > > > @@ -1810,6 +1810,22 @@ config XEN
> > > >       help
> > > >         Say Y if you want to run Linux in a Virtual Machine on Xen on ARM.
> > > >
> > > > +config STACKPROTECTOR_PER_TASK
> > > > +     bool "Use a unique stack canary value for each task"
> > > > +     depends on GCC_PLUGINS && STACKPROTECTOR && SMP && !XIP_DEFLATED_DATA
> > > > +     select GCC_PLUGIN_ARM_SSP_PER_TASK
> > > > +     help
> > > > +       Due to the fact that GCC uses an ordinary symbol reference from
> > > > +       which to load the value of the stack canary, this value can only
> > > > +       change at reboot time on SMP systems, and all tasks running in the
> > > > +       kernel's address space are forced to use the same canary value for
> > > > +       the entire duration that the system is up.
> > > > +
> > > > +       Enable this option to switch to a different method that uses a
> > > > +       different canary value for each task.
> > > > +
> > > > +       This is not enabled by default since it relies on a GCC plugin.
> > >
> > > Why wouldn't you default it to y and let the dependencies disable it
> > > when not met?
> > >
> > > Other than that:
> > >
> > > Acked-by: Nicolas Pitre <nico@...aro.org>
> > >
> >
> > Thanks Nico.
> >
> > I wasn't sure about enabling it by default, though. Perhaps Kees has
> > any insights?
>
> I'd say that if you have CONFIG_GCC_PLUGINS=y and
> CONFIG_STACKPROTECTOR=y then it is pretty clear what the user expects
> already. The fact that CONFIG_STACKPROTECTOR requires a twist on SMP is
> an implementation detail not a feature.
>

Fair point.

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