[PATCH v3 2/2] riscv: Enable per-task stack canaries
Kees Cook
keescook at chromium.org
Sun Jul 12 22:39:57 EDT 2020
On Fri, Jul 10, 2020 at 04:19:58PM +0000, guoren at kernel.org wrote:
> From: Guo Ren <guoren at linux.alibaba.com>
>
> 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 tp, which holds the task struct pointer in the riscv
> kernel.
>
> 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
> gcc options:
> -mstack-protector-guard=sysreg
> -mstack-protector-guard-reg=sp_el0
> -mstack-protector-guard-offset=xxx
>
> riscv: Use -mstack-protector-guard-offset & guard-reg
> gcc options:
> -mstack-protector-guard=tls
> -mstack-protector-guard-reg=tp
> -mstack-protector-guard-offset=xxx
>
> Here is riscv gcc's work [1].
>
> [1] https://gcc.gnu.org/pipermail/gcc-patches/2020-July/549583.html
>
> In the end, these codes are inserted by gcc before return:
>
> * 0xffffffe00020b396 <+120>: ld a5,1008(tp) # 0x3f0
> * 0xffffffe00020b39a <+124>: xor a5,a5,a4
> * 0xffffffe00020b39c <+126>: mv a0,s5
> * 0xffffffe00020b39e <+128>: bnez a5,0xffffffe00020b61c <_do_fork+766>
> 0xffffffe00020b3a2 <+132>: ld ra,136(sp)
> 0xffffffe00020b3a4 <+134>: ld s0,128(sp)
> 0xffffffe00020b3a6 <+136>: ld s1,120(sp)
> 0xffffffe00020b3a8 <+138>: ld s2,112(sp)
> 0xffffffe00020b3aa <+140>: ld s3,104(sp)
> 0xffffffe00020b3ac <+142>: ld s4,96(sp)
> 0xffffffe00020b3ae <+144>: ld s5,88(sp)
> 0xffffffe00020b3b0 <+146>: ld s6,80(sp)
> 0xffffffe00020b3b2 <+148>: ld s7,72(sp)
> 0xffffffe00020b3b4 <+150>: addi sp,sp,144
> 0xffffffe00020b3b6 <+152>: ret
> ...
> * 0xffffffe00020b61c <+766>: auipc ra,0x7f8
> * 0xffffffe00020b620 <+770>: jalr -1764(ra) # 0xffffffe000a02f38 <__stack_chk_fail>
>
> Signed-off-by: Guo Ren <guoren at linux.alibaba.com>
> Signed-off-by: cooper <cooper.qu at linux.alibaba.com>
> Cc: cooper <cooper.qu at linux.alibaba.com>
> Cc: Kees Cook <keescook at chromium.org>
> ---
> Change v2:
> - Change to -mstack-protector-guard=tls for gcc final define
> - Solve compile error by changing position of KBUILD_CFLAGS in
> Makefile
>
> Signed-off-by: Guo Ren <guoren at linux.alibaba.com>
> ---
> 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..d98ce29 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -394,6 +394,13 @@ config CMDLINE_FORCE
>
> endchoice
>
> +config CC_HAVE_STACKPROTECTOR_TLS
> + def_bool $(cc-option,-mstack-protector-guard=tls -mstack-protector-guard-reg=tp -mstack-protector-guard-offset=0)
> +
> +config STACKPROTECTOR_PER_TASK
> + def_bool y
> + depends on STACKPROTECTOR && CC_HAVE_STACKPROTECTOR_TLS
> +
> endmenu
>
> config BUILTIN_DTB
> diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
> index fb6e37d..f5f8ee9 100644
> --- a/arch/riscv/Makefile
> +++ b/arch/riscv/Makefile
> @@ -68,6 +68,16 @@ KBUILD_CFLAGS_MODULE += $(call cc-option,-mno-relax)
> # architectures. It's faster to have GCC emit only aligned accesses.
> KBUILD_CFLAGS += $(call cc-option,-mstrict-align)
>
> +ifeq ($(CONFIG_STACKPROTECTOR_PER_TASK),y)
> +prepare: stack_protector_prepare
> +stack_protector_prepare: prepare0
> + $(eval KBUILD_CFLAGS += -mstack-protector-guard=tls \
> + -mstack-protector-guard-reg=tp \
> + -mstack-protector-guard-offset=$(shell \
> + awk '{if ($$2 == "TSK_STACK_CANARY") print $$3;}' \
> + include/generated/asm-offsets.h))
> +endif
> +
> # arch specific predefines for sparse
> CHECKFLAGS += -D__riscv -D__riscv_xlen=$(BITS)
>
> diff --git a/arch/riscv/include/asm/stackprotector.h b/arch/riscv/include/asm/stackprotector.h
> index d95f7b2..a895e07 100644
> --- a/arch/riscv/include/asm/stackprotector.h
> +++ b/arch/riscv/include/asm/stackprotector.h
> @@ -28,6 +28,7 @@ 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
Should this be CONFIG_STACKPROTECTOR_PER_TASK ?
> + 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
>
--
Kees Cook
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