Re: [PATCH 2/2] arm64: Clear the stack

[Alexander Popov <alex.popov@linux.com>]

Hello Laura,

Thanks again for your work.
Please see some comments below.

On 19.07.2018 00:10, Laura Abbott wrote:
> Implementation of stackleak based heavily on the x86 version
> 
> Signed-off-by: Laura Abbott <labbott@redhat.com>
> ---
> Since last time: Minor style cleanups. Re-wrote check_alloca to
> correctly handle all stack types. While doing that, I also realized
> current_top_of_stack was incorrect so I fixed that as well.
> ---
>  arch/arm64/Kconfig                    |  1 +
>  arch/arm64/include/asm/processor.h    | 17 ++++++++++++++
>  arch/arm64/kernel/entry.S             |  7 ++++++
>  arch/arm64/kernel/process.c           | 32 +++++++++++++++++++++++++++
>  arch/arm64/kvm/hyp/Makefile           |  3 ++-
>  drivers/firmware/efi/libstub/Makefile |  3 ++-
>  include/linux/stackleak.h             |  1 +
>  7 files changed, 62 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 42c090cf0292..216d36a49ab5 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -96,6 +96,7 @@ config ARM64
>  	select HAVE_ARCH_MMAP_RND_BITS
>  	select HAVE_ARCH_MMAP_RND_COMPAT_BITS if COMPAT
>  	select HAVE_ARCH_SECCOMP_FILTER
> +	select HAVE_ARCH_STACKLEAK
>  	select HAVE_ARCH_THREAD_STRUCT_WHITELIST
>  	select HAVE_ARCH_TRACEHOOK
>  	select HAVE_ARCH_TRANSPARENT_HUGEPAGE
> diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
> index a73ae1e49200..4f3062ee22c6 100644
> --- a/arch/arm64/include/asm/processor.h
> +++ b/arch/arm64/include/asm/processor.h
> @@ -266,5 +266,22 @@ extern void __init minsigstksz_setup(void);
>  #define SVE_SET_VL(arg)	sve_set_current_vl(arg)
>  #define SVE_GET_VL()	sve_get_current_vl()
>  
> +/*
> + * For CONFIG_STACKLEAK

Our config option is called CONFIG_GCC_PLUGIN_STACKLEAK.

> + *
> + * These need to be macros because otherwise we get stuck in a nightmare
> + * of header definitions for the use of task_stack_page.
> + */
> +
> +#define current_top_of_stack()		\
> +({					\
> +	unsigned long _low = 0;		\
> +	unsigned long _high = 0;	\
> +					\
> +	current_stack_type(current, current_stack_pointer, &_low, &_high); \
> +	_high;	\
> +})

Do you really need _low here? Ah, I see this in the previous patch:
+	if (stack_low && stack_high) {
+		*stack_low = low;
+		*stack_high = high;
+	}

How about checking them against NULL separately? That would allow
+	current_stack_type(current, current_stack_pointer, NULL, &_high);

Also a minor comment - how about aligning backslashes?

> +#define on_thread_stack()	(on_task_stack(current, current_stack_pointer, NULL, NULL))
> +
>  #endif /* __ASSEMBLY__ */
>  #endif /* __ASM_PROCESSOR_H */
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index 28ad8799406f..67d12016063d 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -431,6 +431,11 @@ tsk	.req	x28		// current thread_info
>  
>  	.text
>  
> +	.macro	stackleak_erase
> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
> +	bl	stackleak_erase
> +#endif
> +	.endm
>  /*
>   * Exception vectors.
>   */
> @@ -910,6 +915,7 @@ ret_fast_syscall:
>  	and	x2, x1, #_TIF_WORK_MASK
>  	cbnz	x2, work_pending
>  	enable_step_tsk x1, x2
> +	stackleak_erase
>  	kernel_exit 0
>  ret_fast_syscall_trace:
>  	enable_daif
> @@ -936,6 +942,7 @@ ret_to_user:
>  	cbnz	x2, work_pending
>  finish_ret_to_user:
>  	enable_step_tsk x1, x2
> +	stackleak_erase
>  	kernel_exit 0
>  ENDPROC(ret_to_user)
>  
> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> index e10bc363f533..904defa36689 100644
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -493,3 +493,35 @@ void arch_setup_new_exec(void)
>  {
>  	current->mm->context.flags = is_compat_task() ? MMCF_AARCH32 : 0;
>  }
> +
> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
> +void __used stackleak_check_alloca(unsigned long size)
> +{
> +	unsigned long stack_left;
> +	enum stack_type type;
> +	unsigned long current_sp = current_stack_pointer;
> +	unsigned long low, high;
> +
> +	type = current_stack_type(current, current_sp, &low, &high);
> +	BUG_ON(type == STACK_TYPE_UNKNOWN);
> +
> +	stack_left = current_sp - low;
> +
> +	if (size >= stack_left) {
> +		/*
> +		 * Kernel stack depth overflow is detected, let's report that.
> +		 * If CONFIG_VMAP_STACK is enabled, we can safely use BUG().
> +		 * If CONFIG_VMAP_STACK is disabled, BUG() handling can corrupt
> +		 * the neighbour memory. CONFIG_SCHED_STACK_END_CHECK calls
> +		 * panic() in a similar situation, so let's do the same if that
> +		 * option is on. Otherwise just use BUG() and hope for the best.
> +		 */
> +#if !defined(CONFIG_VMAP_STACK) && defined(CONFIG_SCHED_STACK_END_CHECK)
> +		panic("alloca() over the kernel stack boundary\n");
> +#else
> +		BUG();
> +#endif

This comment and #if logic should be dropped, we should always use panic() here
on arm64. Mark Rutland and I have worked out the solution for arm64 in this thread:
http://openwall.com/lists/kernel-hardening/2018/05/11/12

Rationale: on arm64 with CONFIG_VMAP_STACK, a stack overflow results in panic()
anyway.

> +	}
> +}
> +EXPORT_SYMBOL(stackleak_check_alloca);
> +#endif
> diff --git a/arch/arm64/kvm/hyp/Makefile b/arch/arm64/kvm/hyp/Makefile
> index 4313f7475333..2fabc2dc1966 100644
> --- a/arch/arm64/kvm/hyp/Makefile
> +++ b/arch/arm64/kvm/hyp/Makefile
> @@ -3,7 +3,8 @@
>  # Makefile for Kernel-based Virtual Machine module, HYP part
>  #
>  
> -ccflags-y += -fno-stack-protector -DDISABLE_BRANCH_PROFILING
> +ccflags-y += -fno-stack-protector -DDISABLE_BRANCH_PROFILING \
> +		$(DISABLE_STACKLEAK_PLUGIN)
>  
>  KVM=../../../../virt/kvm
>  
> diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
> index a34e9290a699..25dd2a14560d 100644
> --- a/drivers/firmware/efi/libstub/Makefile
> +++ b/drivers/firmware/efi/libstub/Makefile
> @@ -20,7 +20,8 @@ cflags-$(CONFIG_EFI_ARMSTUB)	+= -I$(srctree)/scripts/dtc/libfdt
>  KBUILD_CFLAGS			:= $(cflags-y) -DDISABLE_BRANCH_PROFILING \
>  				   -D__NO_FORTIFY \
>  				   $(call cc-option,-ffreestanding) \
> -				   $(call cc-option,-fno-stack-protector)
> +				   $(call cc-option,-fno-stack-protector) \
> +				   $(DISABLE_STACKLEAK_PLUGIN)
>  
>  GCOV_PROFILE			:= n
>  KASAN_SANITIZE			:= n
> diff --git a/include/linux/stackleak.h b/include/linux/stackleak.h
> index b911b973d328..08420ec6b7c3 100644
> --- a/include/linux/stackleak.h
> +++ b/include/linux/stackleak.h
> @@ -5,6 +5,7 @@
>  #include <linux/sched.h>
>  #include <linux/sched/task_stack.h>
>  
> +#include <asm/stacktrace.h>
>  /*
>   * Check that the poison value points to the unused hole in the
>   * virtual memory map for your platform.
>