Re: [PATCH 3/4] riscv: fix race when vmap stack overflow and remove shadow_stack

[Guo Ren <guoren@kernel.org>]

Good job, it's a good fixup.

On Mon, Sep 26, 2022 at 2:03 AM 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 rely on two facts:
> 1. the kernel thread pointer I.E "tp" register can still be gotten from
> the CSR_SCRATCH register, thus we can clobber tp under the condition
> that we restore tp from CSR_SCRATCH later.
>
> 2. Once vmap stack overflow happen, panic is coming soon, no
> performance concern at all, so we don't need to define the overflow
> stack as percpu var, we can simplify it into an array.
>
> Thus we can use tp as a tmp register to get the cpu id to calculate
> the offset of overflow stack for each cpu w/o shadow stack any more.
> Thus the race condition is removed as side effect.
>
> NOTE: we can use similar mechanism to let each cpu use different shadow
> stack to fix the race codition, but if we can remove shadow stack usage
> totally, why not.
>
> Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
> Fixes: 31da94c25aea ("riscv: add VMAP_STACK overflow detection")
> ---
>  arch/riscv/include/asm/asm-prototypes.h |  1 -
>  arch/riscv/include/asm/thread_info.h    |  4 +-
>  arch/riscv/kernel/asm-offsets.c         |  1 +
>  arch/riscv/kernel/entry.S               | 54 ++++---------------------
>  arch/riscv/kernel/traps.c               | 15 +------
>  5 files changed, 11 insertions(+), 64 deletions(-)
>
> diff --git a/arch/riscv/include/asm/asm-prototypes.h b/arch/riscv/include/asm/asm-prototypes.h
> index ef386fcf3939..4a06fa0f6493 100644
> --- a/arch/riscv/include/asm/asm-prototypes.h
> +++ b/arch/riscv/include/asm/asm-prototypes.h
> @@ -25,7 +25,6 @@ DECLARE_DO_ERROR_INFO(do_trap_ecall_s);
>  DECLARE_DO_ERROR_INFO(do_trap_ecall_m);
>  DECLARE_DO_ERROR_INFO(do_trap_break);
>
> -asmlinkage unsigned long get_overflow_stack(void);
>  asmlinkage void handle_bad_stack(struct pt_regs *regs);
>
>  #endif /* _ASM_RISCV_PROTOTYPES_H */
> diff --git a/arch/riscv/include/asm/thread_info.h b/arch/riscv/include/asm/thread_info.h
> index c970d41dc4c6..c604a5212a73 100644
> --- a/arch/riscv/include/asm/thread_info.h
> +++ b/arch/riscv/include/asm/thread_info.h
> @@ -28,14 +28,12 @@
>
>  #define THREAD_SHIFT            (PAGE_SHIFT + THREAD_SIZE_ORDER)
>  #define OVERFLOW_STACK_SIZE     SZ_4K
> -#define SHADOW_OVERFLOW_STACK_SIZE (1024)
> +#define OVERFLOW_STACK_SHIFT   12
>
>  #define IRQ_STACK_SIZE         THREAD_SIZE
>
>  #ifndef __ASSEMBLY__
>
> -extern long shadow_stack[SHADOW_OVERFLOW_STACK_SIZE / sizeof(long)];
> -
>  #include <asm/processor.h>
>  #include <asm/csr.h>
>
> diff --git a/arch/riscv/kernel/asm-offsets.c b/arch/riscv/kernel/asm-offsets.c
> index df9444397908..62bf3bacc322 100644
> --- a/arch/riscv/kernel/asm-offsets.c
> +++ b/arch/riscv/kernel/asm-offsets.c
> @@ -37,6 +37,7 @@ void asm_offsets(void)
>         OFFSET(TASK_TI_PREEMPT_COUNT, task_struct, thread_info.preempt_count);
>         OFFSET(TASK_TI_KERNEL_SP, task_struct, thread_info.kernel_sp);
>         OFFSET(TASK_TI_USER_SP, task_struct, thread_info.user_sp);
> +       OFFSET(TASK_TI_CPU, task_struct, thread_info.cpu);
>
>         OFFSET(TASK_THREAD_F0,  task_struct, thread.fstate.f[0]);
>         OFFSET(TASK_THREAD_F1,  task_struct, thread.fstate.f[1]);
> diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
> index a3e1ed2fa2ac..442d93beffcf 100644
> --- a/arch/riscv/kernel/entry.S
> +++ b/arch/riscv/kernel/entry.S
> @@ -223,54 +223,14 @@ END(ret_from_exception)
>
>  #ifdef CONFIG_VMAP_STACK
>  ENTRY(handle_kernel_stack_overflow)
> -       la sp, shadow_stack
> -       addi sp, sp, SHADOW_OVERFLOW_STACK_SIZE
> -
> -       //save caller register to shadow stack
> -       addi sp, sp, -(PT_SIZE_ON_STACK)
> -       REG_S x1,  PT_RA(sp)
> -       REG_S x5,  PT_T0(sp)
> -       REG_S x6,  PT_T1(sp)
> -       REG_S x7,  PT_T2(sp)
> -       REG_S x10, PT_A0(sp)
> -       REG_S x11, PT_A1(sp)
> -       REG_S x12, PT_A2(sp)
> -       REG_S x13, PT_A3(sp)
> -       REG_S x14, PT_A4(sp)
> -       REG_S x15, PT_A5(sp)
> -       REG_S x16, PT_A6(sp)
> -       REG_S x17, PT_A7(sp)
> -       REG_S x28, PT_T3(sp)
> -       REG_S x29, PT_T4(sp)
> -       REG_S x30, PT_T5(sp)
> -       REG_S x31, PT_T6(sp)
> -
> -       la ra, restore_caller_reg
> -       tail get_overflow_stack
> -
> -restore_caller_reg:
> -       //save per-cpu overflow stack
> -       REG_S a0, -8(sp)
> -       //restore caller register from shadow_stack
> -       REG_L x1,  PT_RA(sp)
> -       REG_L x5,  PT_T0(sp)
> -       REG_L x6,  PT_T1(sp)
> -       REG_L x7,  PT_T2(sp)
> -       REG_L x10, PT_A0(sp)
> -       REG_L x11, PT_A1(sp)
> -       REG_L x12, PT_A2(sp)
> -       REG_L x13, PT_A3(sp)
> -       REG_L x14, PT_A4(sp)
> -       REG_L x15, PT_A5(sp)
> -       REG_L x16, PT_A6(sp)
> -       REG_L x17, PT_A7(sp)
> -       REG_L x28, PT_T3(sp)
> -       REG_L x29, PT_T4(sp)
> -       REG_L x30, PT_T5(sp)
> -       REG_L x31, PT_T6(sp)
> +       la sp, overflow_stack
> +       /* use tp as tmp register since we can restore it from CSR_SCRATCH */
> +       REG_L tp, TASK_TI_CPU(tp)
> +       addi tp, tp, 1
> +       slli tp, tp, OVERFLOW_STACK_SHIFT
> +       add sp, sp, tp
> +       csrr tp, CSR_SCRATCH
>
> -       //load per-cpu overflow stack
> -       REG_L sp, -8(sp)
>         addi sp, sp, -(PT_SIZE_ON_STACK)
>
>         //save context to overflow stack
> diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
> index 73f06cd149d9..2a2a977c1eff 100644
> --- a/arch/riscv/kernel/traps.c
> +++ b/arch/riscv/kernel/traps.c
> @@ -216,23 +216,12 @@ int is_valid_bugaddr(unsigned long pc)
>  #endif /* CONFIG_GENERIC_BUG */
>
>  #ifdef CONFIG_VMAP_STACK
> -static DEFINE_PER_CPU(unsigned long [OVERFLOW_STACK_SIZE/sizeof(long)],
> -               overflow_stack)__aligned(16);
> -/*
> - * shadow stack, handled_ kernel_ stack_ overflow(in kernel/entry.S) is used
> - * to get per-cpu overflow stack(get_overflow_stack).
> - */
> -long shadow_stack[SHADOW_OVERFLOW_STACK_SIZE/sizeof(long)];
> -asmlinkage unsigned long get_overflow_stack(void)
> -{
> -       return (unsigned long)this_cpu_ptr(overflow_stack) +
> -               OVERFLOW_STACK_SIZE;
> -}
> +unsigned long overflow_stack[NR_CPUS][OVERFLOW_STACK_SIZE/sizeof(long)] __aligned(16);
Agree with using a custom per CPU definition for easy entry.S implementation.

Reviewed-by: Guo Ren <guoren@kernel.org>

>
>  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);
> +       unsigned long ovf_stk = (unsigned long)overflow_stack[raw_smp_processor_id()];
>
>         console_verbose();
>
> --
> 2.34.1
>


-- 
Best Regards
 Guo Ren