Re: [PATCH v3 2/2] riscv: Enable per-task stack canaries

[Kees Cook <keescook@chromium.org>]

On Fri, Jul 10, 2020 at 04:19:58PM +0000, guoren@kernel.org wrote:
> From: Guo Ren <guoren@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@linux.alibaba.com>
> Signed-off-by: cooper <cooper.qu@linux.alibaba.com>
> Cc: cooper <cooper.qu@linux.alibaba.com>
> Cc: Kees Cook <keescook@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@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