Re: [PATCH] ARM: Add support for STACKLEAK gcc plugin

[Kees Cook <kees@kernel.org>]

On Thu, Jun 20, 2024 at 09:16:49PM +0800, Jinjie Ruan wrote:
> Add the STACKLEAK gcc plugin to arm32 by adding the helper used by
> stackleak common code: on_thread_stack(). It initialize the stack with the
> poison value before returning from system calls which improves the kernel
> security. Additionally, this disables the plugin in EFI stub code and
> decompress code, which are out of scope for the protection.

Oh very cool! Thanks for sending this!

> Before the test on Qemu versatilepb board:
> 	# echo STACKLEAK_ERASING  > /sys/kernel/debug/provoke-crash/DIRECT
> 	lkdtm: Performing direct entry STACKLEAK_ERASING
> 	lkdtm: XFAIL: stackleak is not supported on this arch (HAVE_ARCH_STACKLEAK=n)
> 
> After:
> 	# echo STACKLEAK_ERASING  > /sys/kernel/debug/provoke-crash/DIRECT
> 	lkdtm: Performing direct entry STACKLEAK_ERASING
> 	lkdtm: stackleak stack usage:
> 	  high offset: 80 bytes
> 	  current:     280 bytes
> 	  lowest:      696 bytes
> 	  tracked:     696 bytes
> 	  untracked:   192 bytes
> 	  poisoned:    7220 bytes
> 	  low offset:  4 bytes
> 	lkdtm: OK: the rest of the thread stack is properly erased
> 
> Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
> ---
>  arch/arm/Kconfig                      | 1 +
>  arch/arm/boot/compressed/Makefile     | 1 +
>  arch/arm/include/asm/stacktrace.h     | 5 +++++
>  arch/arm/kernel/entry-common.S        | 3 +++
>  drivers/firmware/efi/libstub/Makefile | 3 ++-
>  5 files changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index 036381c5d42f..b211b7f5a138 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -86,6 +86,7 @@ config ARM
>  	select HAVE_ARCH_PFN_VALID
>  	select HAVE_ARCH_SECCOMP
>  	select HAVE_ARCH_SECCOMP_FILTER if AEABI && !OABI_COMPAT
> +	select HAVE_ARCH_STACKLEAK
>  	select HAVE_ARCH_THREAD_STRUCT_WHITELIST
>  	select HAVE_ARCH_TRACEHOOK
>  	select HAVE_ARCH_TRANSPARENT_HUGEPAGE if ARM_LPAE
> diff --git a/arch/arm/boot/compressed/Makefile b/arch/arm/boot/compressed/Makefile
> index 6bca03c0c7f0..945b5975fce2 100644
> --- a/arch/arm/boot/compressed/Makefile
> +++ b/arch/arm/boot/compressed/Makefile
> @@ -9,6 +9,7 @@ OBJS		=
>  
>  HEAD	= head.o
>  OBJS	+= misc.o decompress.o
> +CFLAGS_decompress.o += $(DISABLE_STACKLEAK_PLUGIN)
>  ifeq ($(CONFIG_DEBUG_UNCOMPRESS),y)
>  OBJS	+= debug.o
>  AFLAGS_head.o += -DDEBUG
> diff --git a/arch/arm/include/asm/stacktrace.h b/arch/arm/include/asm/stacktrace.h
> index 360f0d2406bf..a9b4b72ed241 100644
> --- a/arch/arm/include/asm/stacktrace.h
> +++ b/arch/arm/include/asm/stacktrace.h
> @@ -26,6 +26,11 @@ struct stackframe {
>  #endif
>  };
>  
> +static inline bool on_thread_stack(void)
> +{
> +	return !(((unsigned long)(current->stack) ^ current_stack_pointer) & ~(THREAD_SIZE - 1));
> +}
> +
>  static __always_inline
>  void arm_get_current_stackframe(struct pt_regs *regs, struct stackframe *frame)
>  {
> diff --git a/arch/arm/kernel/entry-common.S b/arch/arm/kernel/entry-common.S
> index 5c31e9de7a60..f379c852dcb7 100644
> --- a/arch/arm/kernel/entry-common.S
> +++ b/arch/arm/kernel/entry-common.S
> @@ -119,6 +119,9 @@ no_work_pending:
>  
>  	ct_user_enter save = 0
>  
> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
> +	bl	stackleak_erase_on_task_stack
> +#endif
>  	restore_user_regs fast = 0, offset = 0
>  ENDPROC(ret_to_user_from_irq)
>  ENDPROC(ret_to_user)
> diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
> index 06f0428a723c..20d8a491f25f 100644
> --- a/drivers/firmware/efi/libstub/Makefile
> +++ b/drivers/firmware/efi/libstub/Makefile
> @@ -27,7 +27,8 @@ cflags-$(CONFIG_ARM64)		+= -fpie $(DISABLE_STACKLEAK_PLUGIN) \
>  cflags-$(CONFIG_ARM)		+= -DEFI_HAVE_STRLEN -DEFI_HAVE_STRNLEN \
>  				   -DEFI_HAVE_MEMCHR -DEFI_HAVE_STRRCHR \
>  				   -DEFI_HAVE_STRCMP -fno-builtin -fpic \
> -				   $(call cc-option,-mno-single-pic-base)
> +				   $(call cc-option,-mno-single-pic-base) \
> +				   $(DISABLE_STACKLEAK_PLUGIN)
>  cflags-$(CONFIG_RISCV)		+= -fpic -DNO_ALTERNATIVE -mno-relax
>  cflags-$(CONFIG_LOONGARCH)	+= -fpie

This looks very straight forward! If an ARM person can Ack this, I could
carry it via the hardening tree. Otherwise, it should probably go via
rmk's patch tracker?

-Kees

-- 
Kees Cook