[PATCH v3] riscv: fix race when vmap stack overflow

[Palmer Dabbelt]

On Wed, 23 Nov 2022 07:23:51 PST (-0800), jszhang at kernel.org wrote:
> On Sun, Oct 30, 2022 at 09:23:51PM +0800, Guo Ren wrote:
>> Reviewed-by: Guo Ren <guoren at kernel.org>
>>
>
> Gentle ping? This patch is still missing in mainline.

Sorry, this one took a few looks to figure out.  I just sent out a v4, 
it's pretty much the same thing but by the time I'd figured out what 
this was doing it seemed easier to just write up the comments directly.

https://lore.kernel.org/r/20221130022442.18358-1-palmer@rivosinc.com/

I'll try and watch the threads, LMK if I screwed anything up.  I've 
already got this queued up in staging so it's being tested, should be 
fast to take it onto fixes.

Thanks!

>
>> On Sun, Oct 30, 2022 at 8:54 PM Jisheng Zhang <jszhang at 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 at kernel.org>
>> > Suggested-by: Guo Ren <guoren at 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