[PATCH 2/2] arm64: Clear the stack

[Laura Abbott <labbott@redhat.com>]

Implementation of stackleak based heavily on the x86 version

Signed-off-by: Laura Abbott <labbott@redhat.com>
---
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
 };
 
 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)
 
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;
+
+	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);
+#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