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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