[RFC PATCH] riscv: enable per-task stack canaries

guoren
2020-07-05T14:15:14Z

From: Guo Ren <guoren@linux.alibaba.com>

After compare arm64 and x86 implementations, seems arm64's is more
flexible and readable. The key point is how gcc get the offset of
stack_canary from gs/el0_sp.

x86: Use a fix offset from gs, not flexible.

struct fixed_percpu_data {
/*
* GCC hardcodes the stack canary as %gs:40. Since the
* irq_stack is the object at %gs:0, we reserve the bottom
* 48 bytes of the irq stack for the canary.
*/
char gs_base[40]; // :(
unsigned long stack_canary;
};

arm64: Use -mstack-protector-guard-offset & guard-reg

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

I prefer arm64, but x86 percpu_data design needs to be considered ?

After the discussion, let's continue the work for riscv gcc
stack-protector.

Here is arm64 gcc's work [1].

[1] https://github.com/gcc-mirror/gcc/commit/cd0b2d361df82c848dc7e1c3078651bb0624c3c6

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Cc: Kees Cook <keescook@chromium.org>
---
arch/riscv/Kconfig | 7 +++++++
arch/riscv/Makefile | 10 ++++++++++
arch/riscv/include/asm/stackprotector.h | 3 ++-
arch/riscv/kernel/asm-offsets.c | 3 +++
arch/riscv/kernel/process.c | 2 +-
5 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 4b0e308..4b4e833 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -394,6 +394,13 @@ config CMDLINE_FORCE

endchoice

+config CC_HAVE_STACKPROTECTOR_SYSREG
+ def_bool $(cc-option,-mstack-protector-guard=gpr -mstack-protector-guard-reg=tp -mstack-protector-guard-offset=0)
+
+config STACKPROTECTOR_PER_TASK
+ def_bool y
+ depends on STACKPROTECTOR && CC_HAVE_STACKPROTECTOR_SYSREG
+
endmenu

config BUILTIN_DTB
diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
index fb6e37d..880a288 100644
--- a/arch/riscv/Makefile
+++ b/arch/riscv/Makefile
@@ -47,6 +47,16 @@ KBUILD_AFLAGS += -march=$(riscv-march-y)
KBUILD_CFLAGS += -mno-save-restore
KBUILD_CFLAGS += -DCONFIG_PAGE_OFFSET=$(CONFIG_PAGE_OFFSET)

+ifeq ($(CONFIG_STACKPROTECTOR_PER_TASK),y)
+prepare: stack_protector_prepare
+stack_protector_prepare: prepare0
+ $(eval KBUILD_CFLAGS += -mstack-protector-guard=gpr \
+ -mstack-protector-guard-reg=tp \
+ -mstack-protector-guard-offset=$(shell \
+ awk '{if ($$2 == "TSK_STACK_CANARY") print $$3;}' \
+ include/generated/asm-offsets.h))
+endif
+
ifeq ($(CONFIG_CMODEL_MEDLOW),y)
KBUILD_CFLAGS += -mcmodel=medlow
endif
diff --git a/arch/riscv/include/asm/stackprotector.h b/arch/riscv/include/asm/stackprotector.h
index 5962f88..09093af 100644
--- a/arch/riscv/include/asm/stackprotector.h
+++ b/arch/riscv/include/asm/stackprotector.h
@@ -24,6 +24,7 @@ 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_RISCV_STACKPROTECTOR_H */
diff --git a/arch/riscv/kernel/asm-offsets.c b/arch/riscv/kernel/asm-offsets.c
index 07cb9c1..999b465 100644
--- a/arch/riscv/kernel/asm-offsets.c
+++ b/arch/riscv/kernel/asm-offsets.c
@@ -29,6 +29,9 @@ void asm_offsets(void)
OFFSET(TASK_THREAD_S11, task_struct, thread.s[11]);
OFFSET(TASK_THREAD_SP, task_struct, thread.sp);
OFFSET(TASK_STACK, task_struct, stack);
+#ifdef CONFIG_STACKPROTECTOR
+ OFFSET(TSK_STACK_CANARY, task_struct, stack_canary);
+#endif
OFFSET(TASK_TI, task_struct, thread_info);
OFFSET(TASK_TI_FLAGS, task_struct, thread_info.flags);
OFFSET(TASK_TI_PREEMPT_COUNT, task_struct, thread_info.preempt_count);
diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
index 6548929..cb4ac65 100644
--- a/arch/riscv/kernel/process.c
+++ b/arch/riscv/kernel/process.c
@@ -24,7 +24,7 @@

register unsigned long gp_in_global __asm__("gp");

-#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);
--
2.7.4