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Date: Thu, 29 Sep 2022 00:54:17 +0800
From: Jisheng Zhang <jszhang@...nel.org>
To: Paul Walmsley <paul.walmsley@...ive.com>,
Palmer Dabbelt <palmer@...belt.com>,
Albert Ou <aou@...s.berkeley.edu>,
Nathan Chancellor <nathan@...nel.org>,
Nick Desaulniers <ndesaulniers@...gle.com>,
Guo Ren <guoren@...nel.org>
Cc: linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
llvm@...ts.linux.dev
Subject: Re: [PATCH v2 3/4] riscv: fix race when vmap stack overflow and
remove shadow_stack
On Thu, Sep 29, 2022 at 12:20:06AM +0800, Jisheng Zhang 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 content of kernel thread pointer I.E "tp" register can still
> be gotten from the the CSR_SCRATCH register, thus we can clobber tp
> under the condtion that we restore tp from CSR_SCRATCH later.
>
> 2. Once vmap stack overflow happen, panic is comming soon, no
> performance concern at all, so we don't need to define the overflow
> stack as percpu var, we can simplify it into a pointer array which
> points to allocated pages.
>
> Thus we can use tp as a tmp register to get the cpu id to calculate
> the offset of overflow stack pointer array for each cpu w/o shadow
> stack any more. Thus the race condition is removed as a 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@...nel.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 | 56 ++++---------------------
> arch/riscv/kernel/traps.c | 31 ++++++++------
> 5 files changed, 29 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
oops, this should be removed, will update it in a newer version after
collecting review comments.
>
> #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..5a6171a90d81 100644
> --- a/arch/riscv/kernel/entry.S
> +++ b/arch/riscv/kernel/entry.S
> @@ -223,54 +223,16 @@ 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)
> + slli tp, tp, RISCV_LGPTR
> + add tp, sp, tp
> + REG_L sp, 0(tp)
> +
> + /* restore 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..b6c64f0fb70f 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;
> -}
> +void *overflow_stack[NR_CPUS] __ro_after_init __aligned(16);
>
> 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();
>
> @@ -248,4 +237,20 @@ asmlinkage void handle_bad_stack(struct pt_regs *regs)
> for (;;)
> wait_for_interrupt();
> }
> +
> +static int __init alloc_overflow_stacks(void)
> +{
> + u8 *s;
> + int cpu;
> +
> + for_each_possible_cpu(cpu) {
> + s = (u8 *)__get_free_pages(GFP_KERNEL, get_order(OVERFLOW_STACK_SIZE));
> + if (WARN_ON(!s))
> + return -ENOMEM;
> + overflow_stack[cpu] = &s[OVERFLOW_STACK_SIZE];
Since overflow_stack[cpu] points to the top of the slack, we need to
update the ovf_stack dumping in handle_bad_stack(). will take care
this in newer version.
> + printk("%px\n", overflow_stack[cpu]);
forget to remove this printk :(
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