linux-kernel - Re: [PATCH] riscv: fix race when vmap stack overflow

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Date:   Thu, 20 Oct 2022 22:38:19 +0800
From:   Jisheng Zhang <jszhang@...nel.org>
To:     Guo Ren <guoren@...nel.org>
Cc:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] riscv: fix race when vmap stack overflow

On Thu, Oct 20, 2022 at 10:16:47AM +0800, Guo Ren wrote:
> On Wed, Oct 19, 2022 at 11:57 PM Jisheng Zhang <jszhang@...nel.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 make the shadow stack usage serialized.
> >
> > Fixes: 31da94c25aea ("riscv: add VMAP_STACK overflow detection")
> > Signed-off-by: Jisheng Zhang <jszhang@...nel.org>
> > Suggested-by: Guo Ren <guoren@...nel.org>
> > ---
> >  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)
> If CONFIG_64BIT=y, it would be broken. Because we only hold 32bit of
> the sp, and the next loop would get the wrong sp value of
> &spin_shadow_stack.

Hi Guo,

Don't worry about it. the spin_shadow_stack is just a flag used for
"spin", if hart is allowed to used the shadow_stack, we load its
address in next instruction by "la sp, shadow_stack".
But I agree with use unsigned int instead of atomic_t, and use
smp_store_release directly. V2 has been sent out, could you please
review it?

Thanks