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References: <20221030124517.2370-1-jszhang@kernel.org>
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From:   Guo Ren <guoren@kernel.org>
Date:   Sun, 30 Oct 2022 21:23:51 +0800
Message-ID: <CAJF2gTRPECoPFHXKZZ8bRc27YSYv7z4xkYh+hTmfzmZg8=2Ejg@mail.gmail.com>
Subject: Re: [PATCH v3] riscv: fix race when vmap stack overflow
To:     Jisheng Zhang <jszhang@kernel.org>
Cc:     Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org
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Reviewed-by: Guo Ren <guoren@kernel.org>

On Sun, Oct 30, 2022 at 8:54 PM Jisheng Zhang <jszhang@kernel.org> 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@kernel.org>
> Suggested-by: Guo Ren <guoren@kernel.org>
> ---
> Since v2:
>  - use REG_AMOSWAP
>  - add comment to the purpose of smp_store_release()
>
> Since v1:
>  - use smp_store_release directly
>  - use unsigned int instead of atomic_t
>
>
>  arch/riscv/include/asm/asm.h | 1 +
>  arch/riscv/kernel/entry.S    | 4 ++++
>  arch/riscv/kernel/traps.c    | 9 +++++++++
>  3 files changed, 14 insertions(+)
>
> diff --git a/arch/riscv/include/asm/asm.h b/arch/riscv/include/asm/asm.h
> index 1b471ff73178..acf563072b8b 100644
> --- a/arch/riscv/include/asm/asm.h
> +++ b/arch/riscv/include/asm/asm.h
> @@ -23,6 +23,7 @@
>  #define REG_L          __REG_SEL(ld, lw)
>  #define REG_S          __REG_SEL(sd, sw)
>  #define REG_SC         __REG_SEL(sc.d, sc.w)
> +#define REG_AMOSWAP    __REG_SEL(amoswap.d, amoswap.w)
>  #define REG_ASM                __REG_SEL(.dword, .word)
>  #define SZREG          __REG_SEL(8, 4)
>  #define LGREG          __REG_SEL(3, 2)
> diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
> index b9eda3fcbd6d..ea6b78dac739 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
> +       REG_AMOSWAP 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..dea47f329708 100644
> --- a/arch/riscv/kernel/traps.c
> +++ b/arch/riscv/kernel/traps.c
> @@ -221,11 +221,20 @@ asmlinkage unsigned long get_overflow_stack(void)
>                 OVERFLOW_STACK_SIZE;
>  }
>
> +unsigned long 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);
>
> +       /*
> +        * to ensure spin flag is set after the sp is used in entry.S:
> +        * //load per-cpu overflow stack.
> +        * REG_L sp, -8(sp)
> +        */
> +       smp_store_release(&spin_shadow_stack, 0);
> +
>         console_verbose();
>
>         pr_emerg("Insufficient stack space to handle exception!\n");
> --
> 2.37.2
>


-- 
Best Regards
 Guo Ren