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From:   Guo Ren <guoren@kernel.org>
Date:   Fri, 21 Oct 2022 08:35:39 +0800
Message-ID: <CAJF2gTTBYR-6xGo_SrORrjJw9rYQXLukHwBFCNq_kwNLrd_1VA@mail.gmail.com>
Subject: Re: [PATCH] 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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On Thu, Oct 20, 2022 at 10:47 PM Jisheng Zhang <jszhang@kernel.org> wrote:
>
> 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@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 make the shadow stack usage serialized.
> > >
> > > Fixes: 31da94c25aea ("riscv: add VMAP_STACK overflow detection")
> > > Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
> > > Suggested-by: Guo Ren <guoren@kernel.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".
Haha, yes, my brain is at fault :)

> 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?
Okay

>
> Thanks


-- 
Best Regards
 Guo Ren