[PATCH] arm64: Clear the stack

On 07/02/2018 06:02 AM, Alexander Popov wrote:

Hello Laura,

Thanks for your work!
Please see my comments below.

On 29.06.2018 22:05, Laura Abbott wrote:

quoted

Implementation of stackleak based heavily on the x86 version

Signed-off-by: Laura Abbott <redacted>
---
Changes since last time:
- Minor name change in entry.S
- Converted to use the generic interfaces so there's minimal additions.
- Added the fast syscall path.
- Addition of on_thread_stack and current_top_of_stack
- Disable stackleak on hyp per suggestion
- Added a define for check_alloca. I'm still not sure about keeping it
   since the x86 version got reworked?

I've mostly kept this as one patch with a minimal commit text. I can
split it up and elaborate more before final merging.
---
  arch/arm64/Kconfig                    |  1 +
  arch/arm64/include/asm/processor.h    |  9 +++++++++
  arch/arm64/kernel/entry.S             |  7 +++++++
  arch/arm64/kernel/process.c           | 16 ++++++++++++++++
  arch/arm64/kvm/hyp/Makefile           |  3 ++-
  drivers/firmware/efi/libstub/Makefile |  3 ++-
  include/linux/stackleak.h             |  1 +
  scripts/Makefile.gcc-plugins          |  5 ++++-
  8 files changed, 42 insertions(+), 3 deletions(-)

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..73914fd7e142 100644
--- a/arch/arm64/include/asm/processor.h
+++ b/arch/arm64/include/asm/processor.h

@@ -248,5 +248,14 @@ void cpu_clear_disr(const struct arm64_cpu_capabilities *__unused);
  #define SVE_SET_VL(arg)	sve_set_current_vl(arg)
  #define SVE_GET_VL()	sve_get_current_vl()
  
+/*
+ * For stackleak

May I ask you to call it STACKLEAK here and in other places for easier grep?

Sure

quoted

+ *
+ * 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()	(task_stack_page(current) + THREAD_SIZE)
+#define on_thread_stack()	(on_task_stack(current, current_stack_pointer))
+
  #endif /* __ASSEMBLY__ */
  #endif /* __ASM_PROCESSOR_H */

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index ec2ee720e33e..31c9da7d401e 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	stackleak_erase

Could you rename the macro to STACKLEAK_ERASE for similarity with x86?

Mark Rutland had previously asked for this to be lowercase.
I really don't care one way or the other so I'll defer to
someone else to have the final word.

quoted

+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+	bl	stackleak_erase_kstack
+#endif
+	.endm
  /*
   * Exception vectors.
   */

@@ -880,6 +885,7 @@ 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

@@ -906,6 +912,7 @@ 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 f08a2ed9db0d..9f0f135f8b66 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c

@@ -493,3 +493,19 @@ void arch_setup_new_exec(void)
  {
  	current->mm->context.flags = is_compat_task() ? MMCF_AARCH32 : 0;
  }
+
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+#define MIN_STACK_LEFT	256
+
+void __used stackleak_check_alloca(unsigned long size)
+{
+	unsigned long sp, stack_left;
+
+	sp = current_stack_pointer;
+
+	stack_left = sp & (THREAD_SIZE - 1);
+	BUG_ON(stack_left < MIN_STACK_LEFT ||
+		size >= stack_left - MIN_STACK_LEFT);
+}
+EXPORT_SYMBOL(stackleak_check_alloca);
+#endif

This code should be updated.
You may remember the troubles I had with MIN_STACK_LEFT:
http://openwall.com/lists/kernel-hardening/2018/05/11/12
Please see that thread where Mark Rutland and I worked out the solution.

Ah yeah, I missed the details in that thread. Thanks for
that pointer.

By the way, different stacks on x86_64 have different sizes. Is it false for arm64?

Currently everything except the overflow stack looks to be
the same size but there's also another stack I missed. It might
be cleaner just to use on_accessible_stack and then another
function to get the top of stack. This also might just be
reimplementing what x86 already has? (Mark, Ard?)

Thanks,
Laura

quoted

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 e2da99b3a191..00d62b302efb 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.

diff --git a/scripts/Makefile.gcc-plugins b/scripts/Makefile.gcc-plugins
index a535742a1c06..972ce4ca7f6a 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.