Date:   Wed, 30 Nov 2022 08:54:43 -0800 (PST)
From:   Palmer Dabbelt <palmer@...osinc.com>
To:     guoren@...nel.org, Andrea Parri <andrea@...osinc.com>
CC:     jszhang@...nel.org, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject:     Re: [PATCH v4] riscv: fix race when vmap stack overflow

On Tue, 29 Nov 2022 23:15:40 PST (-0800), guoren@...nel.org wrote:
> The comment becomes better. Thx.
>
> On Wed, Nov 30, 2022 at 10:29 AM Palmer Dabbelt <palmer@...osinc.com> wrote:
>>
>> From: Jisheng Zhang <jszhang@...nel.org>
>>
>> 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@...nel.org>
>> Suggested-by: Guo Ren <guoren@...nel.org>
>> Reviewed-by: Guo Ren <guoren@...nel.org>
>> Link: https://lore.kernel.org/r/20221030124517.2370-1-jszhang@kernel.org
>> [Palmer: Add AQ to the swap, and also some comments.]
>> Signed-off-by: Palmer Dabbelt <palmer@...osinc.com>
>> ---
>> Sorry to just re-spin this one without any warning, but I'd read patch a
>> few times and every time I'd managed to convice myself there was a much
>> simpler way of doing this.  By the time I'd figured out why that's not
>> the case it seemed faster to just write the comments.
>>
>> I've stashed this, right on top of the offending commit, at
>> palmer/riscv-fix_vmap_stack.
>>
>> Since v3:
>>  - Add AQ to the swap.
>>  - Add a bunch of comments.
>>
>> 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    | 13 +++++++++++++
>>  arch/riscv/kernel/traps.c    | 18 ++++++++++++++++++
>>  3 files changed, 32 insertions(+)
>>
>> diff --git a/arch/riscv/include/asm/asm.h b/arch/riscv/include/asm/asm.h
>> index 618d7c5af1a2..e15a1c9f1cf8 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_AQ __REG_SEL(amoswap.d.aq, amoswap.w.aq)
> Below is the reason why I use the relax version here:
> https://lore.kernel.org/all/CAJF2gTRAEX_jQ_w5H05dyafZzHq+P5j05TJ=C+v+OL__GQam4A@mail.gmail.com/T/#u

Sorry, I hadn't seen that one.  Adding Andrea.  IMO the acquire/release 
pair is necessary here, with just relaxed the stack stores inside the 
lock could show up on the next hart trying to use the stack.

>>  #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 98f502654edd..5fdb6ba09600 100644
>> --- a/arch/riscv/kernel/entry.S
>> +++ b/arch/riscv/kernel/entry.S
>> @@ -387,6 +387,19 @@ handle_syscall_trace_exit:
>>
>>  #ifdef CONFIG_VMAP_STACK
>>  handle_kernel_stack_overflow:
>> +       /*
>> +        * Takes the psuedo-spinlock for the shadow stack, in case multiple
>> +        * harts are concurrently overflowing their kernel stacks.  We could
>> +        * store any value here, but since we're overflowing the kernel stack
>> +        * already we only have SP to use as a scratch register.  So we just
>> +        * swap in the address of the spinlock, as that's definately non-zero.
>> +        *
>> +        * Pairs with a store_release in handle_bad_stack().
>> +        */
>> +1:     la sp, spin_shadow_stack
>> +       REG_AMOSWAP_AQ 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 bb6a450f0ecc..be54ccea8c47 100644
>> --- a/arch/riscv/kernel/traps.c
>> +++ b/arch/riscv/kernel/traps.c
>> @@ -213,11 +213,29 @@ asmlinkage unsigned long get_overflow_stack(void)
>>                 OVERFLOW_STACK_SIZE;
>>  }
>>
>> +/*
>> + * A pseudo spinlock to protect the shadow stack from being used by multiple
>> + * harts concurrently.  This isn't a real spinlock because the lock side must
>> + * be taken without a valid stack and only a single register, it's only taken
>> + * while in the process of panicing anyway so the performance and error
>> + * checking a proper spinlock gives us doesn't matter.
>> + */
>> +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);
>>
>> +       /*
>> +        * We're done with the shadow stack by this point, as we're on the
>> +        * overflow stack.  Tell any other concurrent overflowing harts that
>> +        * they can proceed with panicing by releasing the pseudo-spinlock.
>> +        *
>> +        * This pairs with an amoswap.aq in handle_kernel_stack_overflow.
>> +        */
>> +       smp_store_release(&spin_shadow_stack, 0);
>> +
>>         console_verbose();
>>
>>         pr_emerg("Insufficient stack space to handle exception!\n");
>> --
>> 2.38.1
>>

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