Re: [PATCH v2] riscv: fix race when vmap stack overflow

[Andrea Parri]

On Thu, Oct 20, 2022 at 10:33:29PM +0800, Jisheng Zhang wrote:
> Currently, when detecting vmap stack overflow, riscv firstly switches
> to the so called shadow stack, then use this shadow stack to call the
> get_overflow_stack() to get the overflow stack. However, there's
> a race here if two or more harts use the same shadow stack at the same
> time.
> 
> To solve this race, we introduce spin_shadow_stack atomic var, which
> will be swap between its own address and 0 in atomic way, when the
> var is set, it means the shadow_stack is being used; when the var
> is cleared, it means the shadow_stack isn't being used.
> 
> Fixes: 31da94c25aea ("riscv: add VMAP_STACK overflow detection")
> Signed-off-by: Jisheng Zhang <jszhang@xxxxxxxxxx>
> Suggested-by: Guo Ren <guoren@xxxxxxxxxx>
> ---
> Since v1:
>  - use smp_store_release directly
>  - use unsigned int instead of atomic_t
> 
>  arch/riscv/kernel/entry.S | 4 ++++
>  arch/riscv/kernel/traps.c | 4 ++++
>  2 files changed, 8 insertions(+)
> 
> diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
> index b9eda3fcbd6d..7b924b16792b 100644
> --- a/arch/riscv/kernel/entry.S
> +++ b/arch/riscv/kernel/entry.S
> @@ -404,6 +404,10 @@ handle_syscall_trace_exit:
>  
>  #ifdef CONFIG_VMAP_STACK
>  handle_kernel_stack_overflow:
> +1:	la sp, spin_shadow_stack
> +	amoswap.w sp, sp, (sp)
> +	bnez sp, 1b
> +
>  	la sp, shadow_stack
>  	addi sp, sp, SHADOW_OVERFLOW_STACK_SIZE
>  
> diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
> index f3e96d60a2ff..f1f57c1241b6 100644
> --- a/arch/riscv/kernel/traps.c
> +++ b/arch/riscv/kernel/traps.c
> @@ -221,11 +221,15 @@ asmlinkage unsigned long get_overflow_stack(void)
>  		OVERFLOW_STACK_SIZE;
>  }
>  
> +unsigned int spin_shadow_stack;
> +
>  asmlinkage void handle_bad_stack(struct pt_regs *regs)
>  {
>  	unsigned long tsk_stk = (unsigned long)current->stack;
>  	unsigned long ovf_stk = (unsigned long)this_cpu_ptr(overflow_stack);
>  
> +	smp_store_release(&spin_shadow_stack, 0);
> +

Maybe add a comment integrating Guo's remarks,

  https://lore.kernel.org/all/CAJF2gTRAEX_jQ_w5H05dyafZzHq+P5j05TJ=C+v+OL__GQam4A@xxxxxxxxxxxxxx/T/#u
  https://lore.kernel.org/all/CAJF2gTRdtcpccL5W48O8VEXCMvxNAyyrKJzhwNJkc8js+H2iJg@xxxxxxxxxxxxxx/T/#u

It will come in handy to future reviewers (and it would address the
checkpatch.pl's "memory barrier without comment" warning).

Thanks,
  Andrea


>  	console_verbose();
>  
>  	pr_emerg("Insufficient stack space to handle exception!\n");
> -- 
> 2.37.2
> 
> 
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