Re: [PATCH v4] riscv: Enable per-task stack canaries

[Kees Cook]

On Sun, Oct 18, 2020 at 12:38:17PM +0000, guoren@xxxxxxxxxx wrote:
> From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> 
> 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
> 
>  GCC's implementation has been merged:
>  commit c931e8d5a96463427040b0d11f9c4352ac22b2b0
>  Author: Cooper Qu <cooper.qu@xxxxxxxxxxxxxxxxx>
>  Date:   Mon Jul 13 16:15:08 2020 +0800
> 
>      RISC-V: Add support for TLS stack protector canary access
> 
> 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@xxxxxxxxxxxxxxxxx>

Thanks for getting this working! It looks good to me. :)

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-- 
Kees Cook