Re: [PATCH 2/2] arm64: Clear the stack
From: Kees Cook
Date: Wed May 02 2018 - 17:31:35 EST
On Wed, May 2, 2018 at 1:33 PM, Laura Abbott <labbott@xxxxxxxxxx> wrote:
>
> Implementation of stackleak based heavily on the x86 version
Awesome! Notes below for both you and Alexander, since I think we can
create a common code base instead of having near-duplicates in the
arch/ trees...
>
> Signed-off-by: Laura Abbott <labbott@xxxxxxxxxx>
> ---
> Now written in C instead of a bunch of assembly.
> ---
> arch/arm64/Kconfig | 1 +
> arch/arm64/include/asm/processor.h | 6 ++++
> arch/arm64/kernel/Makefile | 3 ++
> arch/arm64/kernel/entry.S | 6 ++++
> arch/arm64/kernel/erase.c | 55 +++++++++++++++++++++++++++++++++++
> arch/arm64/kernel/process.c | 16 ++++++++++
> drivers/firmware/efi/libstub/Makefile | 3 +-
> scripts/Makefile.gcc-plugins | 5 +++-
> 8 files changed, 93 insertions(+), 2 deletions(-)
> create mode 100644 arch/arm64/kernel/erase.c
>
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index eb2cf4938f6d..b0221db95dc9 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -92,6 +92,7 @@ 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 767598932549..d31ab80ff647 100644
> --- a/arch/arm64/include/asm/processor.h
> +++ b/arch/arm64/include/asm/processor.h
> @@ -124,6 +124,12 @@ struct thread_struct {
> unsigned long fault_address; /* fault info */
> unsigned long fault_code; /* ESR_EL1 value */
> struct debug_info debug; /* debugging */
> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
> + unsigned long lowest_stack;
> +#ifdef CONFIG_STACKLEAK_METRICS
> + unsigned long prev_lowest_stack;
> +#endif
> +#endif
I wonder if x86 and arm64 could include a common struct here that was
empty when the plugin is disabled... it would keep the ifdefs in one
place. Maybe include/linux/stackleak.h could be:
---start---
/* Poison value points to the unused hole in the virtual memory map */
#define STACKLEAK_POISON -0xBEEF
#define STACKLEAK_POISON_CHECK_DEPTH 128
struct stackleak {
#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
unsigned long lowest;
#ifdef CONFIG_STACKLEAK_METRICS
unsigned long prev_lowest;
#endif
#endif
};
asmlinkage void erase_kstack(void);
---eof---
and arch/*/include/asm/processor.h could do:
@@ -124,6 +124,12 @@ struct thread_struct {
unsigned long fault_address; /* fault info */
unsigned long fault_code; /* ESR_EL1 value */
struct debug_info debug; /* debugging */
+ struct stackleak stackleak;
and arch/x86/entry/erase.c could move to maybe kernel/stackleak.c?
(Oh, I notice this needs an SPDX line too.)
> static inline void arch_thread_struct_whitelist(unsigned long *offset,
> diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
> index bf825f38d206..0ceea613c65b 100644
> --- a/arch/arm64/kernel/Makefile
> +++ b/arch/arm64/kernel/Makefile
> @@ -55,6 +55,9 @@ arm64-reloc-test-y := reloc_test_core.o reloc_test_syms.o
> arm64-obj-$(CONFIG_CRASH_DUMP) += crash_dump.o
> arm64-obj-$(CONFIG_ARM_SDE_INTERFACE) += sdei.o
>
> +arm64-obj-$(CONFIG_GCC_PLUGIN_STACKLEAK) += erase.o
> +KASAN_SANITIZE_erase.o := n
> +
> obj-y += $(arm64-obj-y) vdso/ probes/
> obj-m += $(arm64-obj-m)
> head-y := head.o
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index ec2ee720e33e..3144f1ebdc18 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -401,6 +401,11 @@ tsk .req x28 // current thread_info
>
> .text
>
> + .macro ERASE_KSTACK
> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
> + bl erase_kstack
> +#endif
> + .endm
> /*
> * Exception vectors.
> */
> @@ -906,6 +911,7 @@ ret_to_user:
> cbnz x2, work_pending
> finish_ret_to_user:
> enable_step_tsk x1, x2
> + ERASE_KSTACK
> kernel_exit 0
> ENDPROC(ret_to_user)
Nice. All of the return paths end up here (I went looking for
ret_from_fork's path). :)
>
> diff --git a/arch/arm64/kernel/erase.c b/arch/arm64/kernel/erase.c
> new file mode 100644
> index 000000000000..b8b5648d893b
> --- /dev/null
> +++ b/arch/arm64/kernel/erase.c
> @@ -0,0 +1,55 @@
> +#include <linux/bug.h>
> +#include <linux/sched.h>
> +#include <asm/current.h>
> +#include <asm/linkage.h>
> +#include <asm/processor.h>
> +
> +asmlinkage void erase_kstack(void)
> +{
> + unsigned long p = current->thread.lowest_stack;
> + unsigned long boundary = p & ~(THREAD_SIZE - 1);
> + unsigned long poison = 0;
> + const unsigned long check_depth = STACKLEAK_POISON_CHECK_DEPTH /
> + sizeof(unsigned long);
> +
> + /*
> + * Let's search for the poison value in the stack.
> + * Start from the lowest_stack and go to the bottom.
> + */
> + while (p > boundary && poison <= check_depth) {
> + if (*(unsigned long *)p == STACKLEAK_POISON)
> + poison++;
> + else
> + poison = 0;
> +
> + p -= sizeof(unsigned long);
> + }
> +
> + /*
> + * One long int at the bottom of the thread stack is reserved and
> + * should not be poisoned (see CONFIG_SCHED_STACK_END_CHECK).
> + */
> + if (p == boundary)
> + p += sizeof(unsigned long);
> +
> +#ifdef CONFIG_STACKLEAK_METRICS
> + current->thread.prev_lowest_stack = p;
> +#endif
> +
> + /*
> + * So let's write the poison value to the kernel stack.
> + * Start from the address in p and move up till the new boundary.
> + */
> + boundary = current_stack_pointer;
This is the only difference between x86 and arm64 in this code. What
do you think about implementing on_thread_stack() to match x86:
if (on_thread_stack())
boundary = current_stack_pointer;
else
boundary = current_top_of_stack();
then we could make this common code too instead of having two copies in arch/?
> + BUG_ON(boundary - p >= THREAD_SIZE);
> +
> + while (p < boundary) {
> + *(unsigned long *)p = STACKLEAK_POISON;
> + p += sizeof(unsigned long);
> + }
> +
> + /* Reset the lowest_stack value for the next syscall */
> + current->thread.lowest_stack = current_stack_pointer;
> +}
> +
> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> index f08a2ed9db0d..156fa0a0da19 100644
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -364,6 +364,9 @@ int copy_thread(unsigned long clone_flags, unsigned long stack_start,
> p->thread.cpu_context.pc = (unsigned long)ret_from_fork;
> p->thread.cpu_context.sp = (unsigned long)childregs;
>
> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
> + p->thread.lowest_stack = (unsigned long)task_stack_page(p);
> +#endif
> ptrace_hw_copy_thread(p);
>
> return 0;
> @@ -493,3 +496,16 @@ void arch_setup_new_exec(void)
> {
> current->mm->context.flags = is_compat_task() ? MMCF_AARCH32 : 0;
> }
> +
> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
> +void __used check_alloca(unsigned long size)
> +{
> + unsigned long sp, stack_left;
> +
> + sp = current_stack_pointer;
> +
> + stack_left = sp & (THREAD_SIZE - 1);
> + BUG_ON(stack_left < 256 || size >= stack_left - 256);
> +}
> +EXPORT_SYMBOL(check_alloca);
This is pretty different from x86. Is this just an artifact of ORC, or
something else?
> +#endif
> 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) \
> + $(DISABLE_STACKLEAK_PLUGIN)
>
> GCOV_PROFILE := n
> KASAN_SANITIZE := n
> diff --git a/scripts/Makefile.gcc-plugins b/scripts/Makefile.gcc-plugins
> index 8d6070fc538f..6cc0e35d3324 100644
> --- a/scripts/Makefile.gcc-plugins
> +++ b/scripts/Makefile.gcc-plugins
> @@ -37,11 +37,14 @@ ifdef CONFIG_GCC_PLUGINS
>
> gcc-plugin-$(CONFIG_GCC_PLUGIN_STACKLEAK) += stackleak_plugin.so
> gcc-plugin-cflags-$(CONFIG_GCC_PLUGIN_STACKLEAK) += -DSTACKLEAK_PLUGIN -fplugin-arg-stackleak_plugin-track-min-size=$(CONFIG_STACKLEAK_TRACK_MIN_SIZE)
> + ifdef CONFIG_GCC_PLUGIN_STACKLEAK
> + DISABLE_STACKLEAK_PLUGIN += -fplugin-arg-stackleak_plugin-disable
> + endif
>
> GCC_PLUGINS_CFLAGS := $(strip $(addprefix -fplugin=$(objtree)/scripts/gcc-plugins/, $(gcc-plugin-y)) $(gcc-plugin-cflags-y))
>
> export PLUGINCC GCC_PLUGINS_CFLAGS GCC_PLUGIN GCC_PLUGIN_SUBDIR
> - export SANCOV_PLUGIN DISABLE_LATENT_ENTROPY_PLUGIN
> + export SANCOV_PLUGIN DISABLE_LATENT_ENTROPY_PLUGIN DISABLE_STACKLEAK_PLUGIN
>
> ifneq ($(PLUGINCC),)
> # SANCOV_PLUGIN can be only in CFLAGS_KCOV because avoid duplication.
> --
> 2.14.3
>
-Kees
--
Kees Cook
Pixel Security