Date: Fri, 30 Nov 2018 11:37:20 -0500 (EST) From: Nicolas Pitre <nicolas.pitre@...aro.org> To: Ard Biesheuvel <ard.biesheuvel@...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, 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. Nicolas
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