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Date: Thu, 29 Nov 2018 17:58:50 -0500 (EST)
From: Nicolas Pitre <nicolas.pitre@...aro.org>
To: Ard Biesheuvel <ard.biesheuvel@...aro.org>
cc: linux-arm-kernel@...ts.infradead.org, Russell King <linux@...linux.org.uk>, 
    Kees Cook <keescook@...omium.org>, Emese Revfy <re.emese@...il.com>, 
    Arnd Bergmann <arnd@...db.de>, Laura Abbott <labbott@...hat.com>, 
    kernel-hardening@...ts.openwall.com
Subject: Re: [RFC/RFT PATCH v2] ARM: smp: add support for per-task stack
 canaries

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>


Nicolas

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