Message-ID: <mhng-6eadf458-2777-4b73-acd4-ff4339a32923@palmerdabbelt-glaptop>
Date:   Sat, 22 May 2021 18:49:21 -0700 (PDT)
From:   Palmer Dabbelt <palmer@...belt.com>
To:     tongtiangen@...wei.com
CC:     Paul Walmsley <paul.walmsley@...ive.com>, aou@...s.berkeley.edu,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        tongtiangen@...wei.com
Subject:     Re: [RFC PATCH] riscv: add VMAP_STACK overflow detection

On Fri, 07 May 2021 02:25:09 PDT (-0700), tongtiangen@...wei.com wrote:
> This patch adds stack overflow detection to riscv, usable when
> CONFIG_VMAP_STACK=y.
>
> Overflow is detected in kernel exception entry(kernel/entry.S), if the kernel
> stack is overflow and been detected, the overflow handler is invoked on a
> per-cpu overflow stack. This approach preserves GPRs and the original exception
> information.
>
> The overflow detect is performed before any attempt is made to access the stack
> and the principle of stack overflow detection: kernel stacks are aligned to
> double their size, enabling overflow to be detected with a single bit test. For
> example, a 16K stack is aligned to 32K, ensuring that bit 14 of the SP must be
> zero. On an overflow (or underflow), this bit is flipped. Thus, overflow (of
> less than the size of the stack) can be detected by testing whether this bit is
> set.
>
> This gives us a useful error message on stack overflow, as can be trigger with
> the LKDTM overflow test:
>
> [  388.053267] lkdtm: Performing direct entry EXHAUST_STACK
> [  388.053663] lkdtm: Calling function with 1024 frame size to depth 32 ...
> [  388.054016] lkdtm: loop 32/32 ...
> [  388.054186] lkdtm: loop 31/32 ...
> [  388.054491] lkdtm: loop 30/32 ...
> [  388.054672] lkdtm: loop 29/32 ...
> [  388.054859] lkdtm: loop 28/32 ...
> [  388.055010] lkdtm: loop 27/32 ...
> [  388.055163] lkdtm: loop 26/32 ...
> [  388.055309] lkdtm: loop 25/32 ...
> [  388.055481] lkdtm: loop 24/32 ...
> [  388.055653] lkdtm: loop 23/32 ...
> [  388.055837] lkdtm: loop 22/32 ...
> [  388.056015] lkdtm: loop 21/32 ...
> [  388.056188] lkdtm: loop 20/32 ...
> [  388.058145] Insufficient stack space to handle exception!
> [  388.058153] Task stack:     [0xffffffd014260000..0xffffffd014264000]
> [  388.058160] Overflow stack: [0xffffffe1f8d2c220..0xffffffe1f8d2d220]
> [  388.058168] CPU: 0 PID: 89 Comm: bash Not tainted 5.12.0-rc8-dirty #90
> [  388.058175] Hardware name: riscv-virtio,qemu (DT)
> [  388.058187] epc : number+0x32/0x2c0
> [  388.058247]  ra : vsnprintf+0x2ae/0x3f0
> [  388.058255] epc : ffffffe0002d38f6 ra : ffffffe0002d814e sp : ffffffd01425ffc0
> [  388.058263]  gp : ffffffe0012e4010 tp : ffffffe08014da00 t0 : ffffffd0142606e8
> [  388.058271]  t1 : 0000000000000000 t2 : 0000000000000000 s0 : ffffffd014260070
> [  388.058303]  s1 : ffffffd014260158 a0 : ffffffd01426015e a1 : ffffffd014260158
> [  388.058311]  a2 : 0000000000000013 a3 : ffff0a01ffffff10 a4 : ffffffe000c398e0
> [  388.058319]  a5 : 511b02ec65f3e300 a6 : 0000000000a1749a a7 : 0000000000000000
> [  388.058327]  s2 : ffffffff000000ff s3 : 00000000ffff0a01 s4 : ffffffe0012e50a8
> [  388.058335]  s5 : 0000000000ffff0a s6 : ffffffe0012e50a8 s7 : ffffffe000da1cc0
> [  388.058343]  s8 : ffffffffffffffff s9 : ffffffd0142602b0 s10: ffffffd0142602a8
> [  388.058351]  s11: ffffffd01426015e t3 : 00000000000f0000 t4 : ffffffffffffffff
> [  388.058359]  t5 : 000000000000002f t6 : ffffffd014260158
> [  388.058366] status: 0000000000000100 badaddr: ffffffd01425fff8 cause: 000000000000000f
> [  388.058374] Kernel panic - not syncing: Kernel stack overflow
> [  388.058381] CPU: 0 PID: 89 Comm: bash Not tainted 5.12.0-rc8-dirty #90
> [  388.058387] Hardware name: riscv-virtio,qemu (DT)
> [  388.058393] Call Trace:
> [  388.058400] [<ffffffe000004944>] walk_stackframe+0x0/0xce
> [  388.058406] [<ffffffe0006f0b28>] dump_backtrace+0x38/0x46
> [  388.058412] [<ffffffe0006f0b46>] show_stack+0x10/0x18
> [  388.058418] [<ffffffe0006f3690>] dump_stack+0x74/0x8e
> [  388.058424] [<ffffffe0006f0d52>] panic+0xfc/0x2b2
> [  388.058430] [<ffffffe0006f0acc>] print_trace_address+0x0/0x24
> [  388.058436] [<ffffffe0002d814e>] vsnprintf+0x2ae/0x3f0
> [  388.058956] SMP: stopping secondary CPUs
>
> Signed-off-by: Tong Tiangen <tongtiangen@...wei.com>
> ---
>  arch/riscv/Kconfig                   |   1 +
>  arch/riscv/include/asm/thread_info.h |  15 ++++
>  arch/riscv/kernel/entry.S            | 108 +++++++++++++++++++++++++++
>  arch/riscv/kernel/traps.c            |  35 +++++++++
>  4 files changed, 159 insertions(+)
>
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 4515a10c5d22..587f001e84f4 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -63,6 +63,7 @@ config RISCV
>  	select HAVE_ARCH_MMAP_RND_BITS if MMU
>  	select HAVE_ARCH_SECCOMP_FILTER
>  	select HAVE_ARCH_TRACEHOOK
> +	select HAVE_ARCH_VMAP_STACK
>  	select HAVE_ASM_MODVERSIONS
>  	select HAVE_CONTEXT_TRACKING
>  	select HAVE_DEBUG_KMEMLEAK
> diff --git a/arch/riscv/include/asm/thread_info.h b/arch/riscv/include/asm/thread_info.h
> index 0e549a3089b3..60da0dcacf14 100644
> --- a/arch/riscv/include/asm/thread_info.h
> +++ b/arch/riscv/include/asm/thread_info.h
> @@ -19,6 +19,21 @@
>  #endif
>  #define THREAD_SIZE		(PAGE_SIZE << THREAD_SIZE_ORDER)
>
> +/*
> + * By aligning VMAP'd stacks to 2 * THREAD_SIZE, we can detect overflow by
> + * checking sp & (1 << THREAD_SHIFT), which we can do cheaply in the entry
> + * assembly.
> + */
> +#ifdef CONFIG_VMAP_STACK
> +#define THREAD_ALIGN            (2 * THREAD_SIZE)
> +#else
> +#define THREAD_ALIGN            THREAD_SIZE
> +#endif
> +
> +#define THREAD_SHIFT            (PAGE_SHIFT + THREAD_SIZE_ORDER)
> +#define OVERFLOW_STACK_SIZE     SZ_4K
> +#define SHADOW_OVERFLOW_STACK_SIZE (1024)
> +
>  #ifndef __ASSEMBLY__
>
>  #include <asm/processor.h>
> diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
> index 83095faa680e..deadf4000b86 100644
> --- a/arch/riscv/kernel/entry.S
> +++ b/arch/riscv/kernel/entry.S
> @@ -29,6 +29,15 @@ ENTRY(handle_exception)
>  _restore_kernel_tpsp:
>  	csrr tp, CSR_SCRATCH
>  	REG_S sp, TASK_TI_KERNEL_SP(tp)
> +
> +#ifdef CONFIG_VMAP_STACK
> +	addi sp, sp, -(PT_SIZE_ON_STACK)
> +	srli sp, sp, THREAD_SHIFT
> +	andi sp, sp, 0x1
> +	bnez sp, handle_kernel_stack_overflow
> +	REG_L sp, TASK_TI_KERNEL_SP(tp)
> +#endif
> +
>  _save_context:
>  	REG_S sp, TASK_TI_USER_SP(tp)
>  	REG_L sp, TASK_TI_KERNEL_SP(tp)
> @@ -375,6 +384,105 @@ handle_syscall_trace_exit:
>  	call do_syscall_trace_exit
>  	j ret_from_exception
>
> +#ifdef CONFIG_VMAP_STACK
> +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
> +	sd 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)
> +
> +	//load per-cpu overflow stack
> +	ld sp, -8(sp)
> +	addi sp, sp, -(PT_SIZE_ON_STACK)
> +
> +	//save context to overflow stack
> +	REG_S x1,  PT_RA(sp)
> +	REG_S x3,  PT_GP(sp)
> +	REG_S x5,  PT_T0(sp)
> +	REG_S x6,  PT_T1(sp)
> +	REG_S x7,  PT_T2(sp)
> +	REG_S x8,  PT_S0(sp)
> +	REG_S x9,  PT_S1(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 x18, PT_S2(sp)
> +	REG_S x19, PT_S3(sp)
> +	REG_S x20, PT_S4(sp)
> +	REG_S x21, PT_S5(sp)
> +	REG_S x22, PT_S6(sp)
> +	REG_S x23, PT_S7(sp)
> +	REG_S x24, PT_S8(sp)
> +	REG_S x25, PT_S9(sp)
> +	REG_S x26, PT_S10(sp)
> +	REG_S x27, PT_S11(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)
> +
> +	REG_L s0, TASK_TI_KERNEL_SP(tp)
> +	csrr s1, CSR_STATUS
> +	csrr s2, CSR_EPC
> +	csrr s3, CSR_TVAL
> +	csrr s4, CSR_CAUSE
> +	csrr s5, CSR_SCRATCH
> +	REG_S s0, PT_SP(sp)
> +	REG_S s1, PT_STATUS(sp)
> +	REG_S s2, PT_EPC(sp)
> +	REG_S s3, PT_BADADDR(sp)
> +	REG_S s4, PT_CAUSE(sp)
> +	REG_S s5, PT_TP(sp)
> +	move a0, sp
> +	tail handle_bad_stack
> +#endif
> +
>  END(handle_exception)
>
>  ENTRY(ret_from_fork)
> diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
> index 1357abf79570..ff8033b9fdd6 100644
> --- a/arch/riscv/kernel/traps.c
> +++ b/arch/riscv/kernel/traps.c
> @@ -200,3 +200,38 @@ int is_valid_bugaddr(unsigned long pc)
>  void trap_init(void)
>  {
>  }
> +
> +#ifdef CONFIG_VMAP_STACK
> +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;
> +}
> +
> +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);
> +
> +	console_verbose();
> +
> +	pr_emerg("Insufficient stack space to handle exception!\n");
> +	pr_emerg("Task stack:     [0x%016lx..0x%016lx]\n",
> +			tsk_stk, tsk_stk + THREAD_SIZE);
> +	pr_emerg("Overflow stack: [0x%016lx..0x%016lx]\n",
> +			ovf_stk, ovf_stk + OVERFLOW_STACK_SIZE);
> +
> +	__show_regs(regs);
> +	panic("Kernel stack overflow");
> +
> +	for (;;)
> +		wait_for_interrupt();
> +}
> +#endif

This LGTM.  It seems good enough to take, I'm not sure why this is an 
RFC?

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