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Date:   Wed, 23 Nov 2022 23:23:51 +0800
From:   Jisheng Zhang <jszhang@kernel.org>
To:     Guo Ren <guoren@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
Subject: Re: [PATCH v3] riscv: fix race when vmap stack overflow
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References: <20221030124517.2370-1-jszhang@kernel.org>
 <CAJF2gTRPECoPFHXKZZ8bRc27YSYv7z4xkYh+hTmfzmZg8=2Ejg@mail.gmail.com>
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On Sun, Oct 30, 2022 at 09:23:51PM +0800, Guo Ren wrote:
> Reviewed-by: Guo Ren <guoren@kernel.org>
> 

Gentle ping? This patch is still missing in mainline.

> 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