[PATCH] arm64: Add CONFIG_CC_STACKPROTECTOR

[will.deacon@arm.com (Will Deacon)]

Hi Laura,

On Tue, Jan 21, 2014 at 05:26:06PM +0000, Laura Abbott wrote:
> arm64 currently lacks support for -fstack-protector. Add
> similar functionality to arm to detect stack corruption.
> 
> Cc: Will Deacon <will.deacon@arm.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Signed-off-by: Laura Abbott <lauraa@codeaurora.org>
> ---
>  arch/arm64/Kconfig                      |   12 +++++++++
>  arch/arm64/Makefile                     |    4 +++
>  arch/arm64/include/asm/stackprotector.h |   38 +++++++++++++++++++++++++++++++
>  arch/arm64/kernel/process.c             |    9 +++++++
>  4 files changed, 63 insertions(+), 0 deletions(-)
>  create mode 100644 arch/arm64/include/asm/stackprotector.h
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 6d4dd22..4f86874 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -168,6 +168,18 @@ config HOTPLUG_CPU
>  	  Say Y here to experiment with turning CPUs off and on.  CPUs
>  	  can be controlled through /sys/devices/system/cpu.
>  
> +config CC_STACKPROTECTOR
> +	bool "Enable -fstack-protector buffer overflow detection"
> +	help
> +	  This option turns on the -fstack-protector GCC feature. This
> +	  feature puts, at the beginning of functions, a canary value on
> +	  the stack just before the return address, and validates
> +	  the value just before actually returning.  Stack based buffer
> +	  overflows (that need to overwrite this return address) now also
> +	  overwrite the canary, which gets detected and the attack is then
> +	  neutralized via a kernel panic.
> +	  This feature requires gcc version 4.2 or above.

You can remove that bit about GCC -- GCC 4.2 doesn't support AArch64.

> +
>  source kernel/Kconfig.preempt
>  
>  config HZ
> diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
> index 2fceb71..1ce221e 100644
> --- a/arch/arm64/Makefile
> +++ b/arch/arm64/Makefile
> @@ -48,6 +48,10 @@ core-$(CONFIG_XEN) += arch/arm64/xen/
>  libs-y		:= arch/arm64/lib/ $(libs-y)
>  libs-y		+= $(LIBGCC)
>  
> +ifeq ($(CONFIG_CC_STACKPROTECTOR),y)
> +KBUILD_CFLAGS	+=-fstack-protector
> +endif
> +
>  # Default target when executing plain make
>  KBUILD_IMAGE	:= Image.gz
>  KBUILD_DTBS	:= dtbs
> diff --git a/arch/arm64/include/asm/stackprotector.h b/arch/arm64/include/asm/stackprotector.h
> new file mode 100644
> index 0000000..de00332
> --- /dev/null
> +++ b/arch/arm64/include/asm/stackprotector.h
> @@ -0,0 +1,38 @@
> +/*
> + * GCC stack protector support.
> + *
> + * Stack protector works by putting predefined pattern at the start of
> + * the stack frame and verifying that it hasn't been overwritten when
> + * returning from the function.  The pattern is called stack canary
> + * and gcc expects it to be defined by a global variable called
> + * "__stack_chk_guard" on ARM.  This unfortunately means that on SMP
> + * we cannot have a different canary value per task.
> + */
> +
> +#ifndef _ASM_STACKPROTECTOR_H

__ASM_ for consistency.

> +#define _ASM_STACKPROTECTOR_H 1

Why #define explicitly to 1?

> +
> +#include <linux/random.h>
> +#include <linux/version.h>
> +
> +extern unsigned long __stack_chk_guard;
> +
> +/*
> + * Initialize the stackprotector canary value.
> + *
> + * NOTE: this must only be called from functions that never return,
> + * and it must always be inlined.
> + */
> +static __always_inline void boot_init_stack_canary(void)
> +{
> +	unsigned long canary;
> +
> +	/* Try to get a semi random initial value. */
> +	get_random_bytes(&canary, sizeof(canary));
> +	canary ^= LINUX_VERSION_CODE;
> +
> +	current->stack_canary = canary;
> +	__stack_chk_guard = current->stack_canary;
> +}
> +
> +#endif	/* _ASM_STACKPROTECTOR_H */
> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> index de17c89..592d630 100644
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -50,6 +50,12 @@
>  #include <asm/processor.h>
>  #include <asm/stacktrace.h>
>  
> +#ifdef CONFIG_CC_STACKPROTECTOR
> +#include <linux/stackprotector.h>
> +unsigned long __stack_chk_guard __read_mostly;
> +EXPORT_SYMBOL(__stack_chk_guard);
> +#endif
> +
>  static void setup_restart(void)
>  {
>  	/*
> @@ -288,6 +294,9 @@ struct task_struct *__switch_to(struct task_struct *prev,
>  {
>  	struct task_struct *last;
>  
> +#if defined(CONFIG_CC_STACKPROTECTOR) && !defined(CONFIG_SMP)
> +	__stack_chk_guard = next->stack_canary;
> +#endif

I don't get the dependency on !SMP. Assumedly, the update of
__stack_chk_guard would be racy otherwise, but that sounds solvable with
atomics. Is the stack_canary updated periodically somewhere else?

Will