[PATCH v3] arm64: enable per-task stack canaries

[Ard Biesheuvel]

This enables the use of per-task stack canary values if GCC has
support for emitting the stack canary reference relative to the
value of sp_el0, which holds the task struct pointer in the arm64
kernel.

The $(eval) extends KBUILD_CFLAGS at the moment the make rule is
applied, which means asm-offsets.o (which we rely on for the offset
value) is built without the arguments, and everything built afterwards
has the options set.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
---
v3: add all 3 options to the Kconfig cc-option check, to avoid relying on
    implementation defined logic in the compiler regarding which options
    need to appear together.

 arch/arm64/Kconfig                      |  7 +++++++
 arch/arm64/Makefile                     | 10 ++++++++++
 arch/arm64/include/asm/stackprotector.h |  3 ++-
 arch/arm64/kernel/asm-offsets.c         |  3 +++
 arch/arm64/kernel/process.c             |  2 +-
 5 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index ea2ab0330e3a..e355946cde97 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -1272,6 +1272,13 @@ config RANDOMIZE_MODULE_REGION_FULL
 	  a limited range that contains the [_stext, _etext] interval of the
 	  core kernel, so branch relocations are always in range.
 
+config CC_HAVE_STACKPROTECTOR_SYSREG
+	def_bool $(cc-option,-mstack-protector-guard=sysreg -mstack-protector-guard-reg=sp_el0 -mstack-protector-guard-offset=0)
+
+config STACKPROTECTOR_PER_TASK
+	def_bool y
+	depends on STACKPROTECTOR && CC_HAVE_STACKPROTECTOR_SYSREG
+
 endmenu
 
 menu "Boot options"
diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index 6cb9fc7e9382..79d927542322 100644
--- a/arch/arm64/Makefile
+++ b/arch/arm64/Makefile
@@ -56,6 +56,16 @@ KBUILD_AFLAGS	+= $(lseinstr) $(brokengasinst)
 KBUILD_CFLAGS	+= $(call cc-option,-mabi=lp64)
 KBUILD_AFLAGS	+= $(call cc-option,-mabi=lp64)
 
+ifeq ($(CONFIG_STACKPROTECTOR_PER_TASK),y)
+prepare: stack_protector_prepare
+stack_protector_prepare: prepare0
+	$(eval KBUILD_CFLAGS += -mstack-protector-guard=sysreg		  \
+				-mstack-protector-guard-reg=sp_el0	  \
+				-mstack-protector-guard-offset=$(shell	  \
+			awk '{if ($$2 == "TSK_STACK_CANARY") print $$3;}' \
+					include/generated/asm-offsets.h))
+endif
+
 ifeq ($(CONFIG_CPU_BIG_ENDIAN), y)
 KBUILD_CPPFLAGS	+= -mbig-endian
 CHECKFLAGS	+= -D__AARCH64EB__
diff --git a/arch/arm64/include/asm/stackprotector.h b/arch/arm64/include/asm/stackprotector.h
index 58d15be11c4d..5884a2b02827 100644
--- a/arch/arm64/include/asm/stackprotector.h
+++ b/arch/arm64/include/asm/stackprotector.h
@@ -34,7 +34,8 @@ static __always_inline void boot_init_stack_canary(void)
 	canary &= CANARY_MASK;
 
 	current->stack_canary = canary;
-	__stack_chk_guard = current->stack_canary;
+	if (!IS_ENABLED(CONFIG_STACKPROTECTOR_PER_TASK))
+		__stack_chk_guard = current->stack_canary;
 }
 
 #endif	/* _ASM_STACKPROTECTOR_H */
diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c
index 323aeb5f2fe6..65b8afc84466 100644
--- a/arch/arm64/kernel/asm-offsets.c
+++ b/arch/arm64/kernel/asm-offsets.c
@@ -46,6 +46,9 @@ int main(void)
   DEFINE(TSK_TI_TTBR0,		offsetof(struct task_struct, thread_info.ttbr0));
 #endif
   DEFINE(TSK_STACK,		offsetof(struct task_struct, stack));
+#ifdef CONFIG_STACKPROTECTOR
+  DEFINE(TSK_STACK_CANARY,	offsetof(struct task_struct, stack_canary));
+#endif
   BLANK();
   DEFINE(THREAD_CPU_CONTEXT,	offsetof(struct task_struct, thread.cpu_context));
   BLANK();
diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index d9a4c2d6dd8b..8a2d68f04e0d 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -59,7 +59,7 @@
 #include <asm/processor.h>
 #include <asm/stacktrace.h>
 
-#ifdef CONFIG_STACKPROTECTOR
+#if defined(CONFIG_STACKPROTECTOR) && !defined(CONFIG_STACKPROTECTOR_PER_TASK)
 #include <linux/stackprotector.h>
 unsigned long __stack_chk_guard __read_mostly;
 EXPORT_SYMBOL(__stack_chk_guard);
-- 
2.19.2

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