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Date:   Thu, 29 Sep 2022 00:54:17 +0800
From:   Jisheng Zhang <jszhang@kernel.org>
To:     Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Guo Ren <guoren@kernel.org>
Cc:     linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
        llvm@lists.linux.dev
Subject: Re: [PATCH v2 3/4] riscv: fix race when vmap stack overflow and
 remove shadow_stack
Message-ID: <YzR8OUFuV+R1i1Y6@xhacker>
References: <20220928162007.3791-1-jszhang@kernel.org>
 <20220928162007.3791-4-jszhang@kernel.org>
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In-Reply-To: <20220928162007.3791-4-jszhang@kernel.org>
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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@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               | 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 :(