Date:   Thu, 24 Nov 2022 23:26:42 +0800
From:   Jisheng Zhang <jszhang@...nel.org>
To:     Deepak Gupta <debug@...osinc.com>
Cc:     palmer@...belt.com, linux-kernel@...r.kernel.org,
        linux-riscv@...ts.infradead.org, Guo Ren <guoren@...nel.org>
Subject: Re: [PATCH v2] riscv: VMAP_STACK overflow detection thread-safe

On Thu, Nov 24, 2022 at 01:48:45AM -0800, Deepak Gupta wrote:
> commit 31da94c25aea ("riscv: add VMAP_STACK overflow detection") added
> support for CONFIG_VMAP_STACK. If overflow is detected, CPU switches to
> `shadow_stack` temporarily before switching finally to per-cpu
> `overflow_stack`.
> 
> If two CPUs/harts are racing and end up in over flowing kernel stack, one
> or both will end up corrupting each other state because `shadow_stack` is
> not per-cpu. This patch optimizes per-cpu overflow stack switch by
> directly picking per-cpu `overflow_stack` and gets rid of `shadow_stack`.
> 
> Following are the changes in this patch
> 
>  - Defines an asm macro to obtain per-cpu symbols in destination
>    register.
>  - In entry.S, when overflow is detected, per-cpu overflow stack is
>    located using per-cpu asm macro. Computing per-cpu symbol requires
>    a temporary register. x31 is saved away into CSR_SCRATCH

This only works if CSR_SCRATCH doesn't contain any valid reg saving,
but.. see below.

>    (CSR_SCRATCH is anyways zero since we're in kernel).
> 

To be honest, before [1] I have similar idea to keep the percpu usage,
however, the solution doesn't work. The key here is that there's
another VMAP_STACK bug in current riscv implementation: it only checks
vmap stack overflow when comming from kernelspace, but vmap should
check when comming from both kernelspace and userspace. So we can't
assume CSR_SCRATCH is always zero and free to use. The only available
solution is my fix[1] which only makes use of tp. But since[1] modifies
lots of code, it's not idea to merge it as a fix, so [2] is suggested
and sent out.

PS: I planed to send a fix for the missing FROM_USERSPACE after the
race fix is merged.


[1]https://lore.kernel.org/linux-riscv/20220925175356.681-1-jszhang@kernel.org/T/#t
[2]https://lore.kernel.org/linux-riscv/Y347B0x4VUNOd6V7@xhacker/T/#t

> Please see Links for additional relevant disccussion and alternative
> solution.
> 
> Tested by `echo EXHAUST_STACK > /sys/kernel/debug/provoke-crash/DIRECT`
> Kernel crash log below
> 
>  Insufficient stack space to handle exception!/debug/provoke-crash/DIRECT
>  Task stack:     [0xff20000010a98000..0xff20000010a9c000]
>  Overflow stack: [0xff600001f7d98370..0xff600001f7d99370]
>  CPU: 1 PID: 205 Comm: bash Not tainted 6.1.0-rc2-00001-g328a1f96f7b9 #34
>  Hardware name: riscv-virtio,qemu (DT)
>  epc : __memset+0x60/0xfc
>   ra : recursive_loop+0x48/0xc6 [lkdtm]
>  epc : ffffffff808de0e4 ra : ffffffff0163a752 sp : ff20000010a97e80
>   gp : ffffffff815c0330 tp : ff600000820ea280 t0 : ff20000010a97e88
>   t1 : 000000000000002e t2 : 3233206874706564 s0 : ff20000010a982b0
>   s1 : 0000000000000012 a0 : ff20000010a97e88 a1 : 0000000000000000
>   a2 : 0000000000000400 a3 : ff20000010a98288 a4 : 0000000000000000
>   a5 : 0000000000000000 a6 : fffffffffffe43f0 a7 : 00007fffffffffff
>   s2 : ff20000010a97e88 s3 : ffffffff01644680 s4 : ff20000010a9be90
>   s5 : ff600000842ba6c0 s6 : 00aaaaaac29e42b0 s7 : 00fffffff0aa3684
>   s8 : 00aaaaaac2978040 s9 : 0000000000000065 s10: 00ffffff8a7cad10
>   s11: 00ffffff8a76a4e0 t3 : ffffffff815dbaf4 t4 : ffffffff815dbaf4
>   t5 : ffffffff815dbab8 t6 : ff20000010a9bb48
>  status: 0000000200000120 badaddr: ff20000010a97e88 cause: 000000000000000f
>  Kernel panic - not syncing: Kernel stack overflow
>  CPU: 1 PID: 205 Comm: bash Not tainted 6.1.0-rc2-00001-g328a1f96f7b9 #34
>  Hardware name: riscv-virtio,qemu (DT)
>  Call Trace:
>  [<ffffffff80006754>] dump_backtrace+0x30/0x38
>  [<ffffffff808de798>] show_stack+0x40/0x4c
>  [<ffffffff808ea2a8>] dump_stack_lvl+0x44/0x5c
>  [<ffffffff808ea2d8>] dump_stack+0x18/0x20
>  [<ffffffff808dec06>] panic+0x126/0x2fe
>  [<ffffffff800065ea>] walk_stackframe+0x0/0xf0
>  [<ffffffff0163a752>] recursive_loop+0x48/0xc6 [lkdtm]
>  SMP: stopping secondary CPUs
>  ---[ end Kernel panic - not syncing: Kernel stack overflow ]---
> 
> Cc: Guo Ren <guoren@...nel.org>
> Cc: Jisheng Zhang <jszhang@...nel.org>
> Link: https://lore.kernel.org/linux-riscv/Y347B0x4VUNOd6V7@xhacker/T/#t
> Signed-off-by: Deepak Gupta <debug@...osinc.com>
> 
> ---
> v1 --> v2:
>  - asm macro to locate per-cpu symbol requires a temp reg.
>    When stack overflow happens, in trap handler we don't have spare regs
>    except sp.
>    v1 had a place holder in `thread_info` to spill a register.
>    v2 instead uses CSR_SCRATCH register because it's free to use.
> 
>  - v2 made per-cpu macro more readable.
>  - v2 fixed a bug that would've broken 32bit support.
> 
>  - v1 called it a fix over 31da94c25aea. v2 calls it alternative/
>    optimization solution
> ---
>  arch/riscv/include/asm/asm.h    | 17 ++++++++++
>  arch/riscv/kernel/asm-offsets.c |  1 +
>  arch/riscv/kernel/entry.S       | 57 ++++++---------------------------
>  arch/riscv/kernel/traps.c       | 12 +------
>  4 files changed, 29 insertions(+), 58 deletions(-)
> 
> diff --git a/arch/riscv/include/asm/asm.h b/arch/riscv/include/asm/asm.h
> index 1b471ff73178..1eb479cb9ae4 100644
> --- a/arch/riscv/include/asm/asm.h
> +++ b/arch/riscv/include/asm/asm.h
> @@ -69,6 +69,7 @@
>  
>  #ifdef __ASSEMBLY__
>  
> +#include <asm/asm-offsets.h>
>  /* Common assembly source macros */
>  
>  /*
> @@ -80,6 +81,22 @@
>  	.endr
>  .endm
>  
> +#ifdef CONFIG_32BIT
> +#define PER_CPU_OFFSET_SHIFT 2
> +#else
> +#define PER_CPU_OFFSET_SHIFT 3
> +#endif
> +
> +.macro asm_per_cpu dst sym tmp
> +	REG_L \tmp, TASK_TI_CPU_NUM(tp)
> +	slli  \tmp, \tmp, PER_CPU_OFFSET_SHIFT
> +	la    \dst, __per_cpu_offset
> +	add   \dst, \dst, \tmp
> +	REG_L \tmp, 0(\dst)
> +	la    \dst, \sym
> +	add   \dst, \dst, \tmp
> +.endm
> +
>  #endif /* __ASSEMBLY__ */
>  
>  #endif /* _ASM_RISCV_ASM_H */
> diff --git a/arch/riscv/kernel/asm-offsets.c b/arch/riscv/kernel/asm-offsets.c
> index df9444397908..a7da051159cf 100644
> --- a/arch/riscv/kernel/asm-offsets.c
> +++ b/arch/riscv/kernel/asm-offsets.c
> @@ -38,6 +38,7 @@ void asm_offsets(void)
>  	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_NUM, 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]);
>  	OFFSET(TASK_THREAD_F2,  task_struct, thread.fstate.f[2]);
> diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
> index b9eda3fcbd6d..2e90d9ccddd0 100644
> --- a/arch/riscv/kernel/entry.S
> +++ b/arch/riscv/kernel/entry.S
> @@ -10,9 +10,11 @@
>  #include <asm/asm.h>
>  #include <asm/csr.h>
>  #include <asm/unistd.h>
> +#include <asm/page.h>
>  #include <asm/thread_info.h>
>  #include <asm/asm-offsets.h>
>  #include <asm/errata_list.h>
> +#include <linux/sizes.h>
>  
>  #if !IS_ENABLED(CONFIG_PREEMPTION)
>  .set resume_kernel, restore_all
> @@ -404,54 +406,15 @@ handle_syscall_trace_exit:
>  
>  #ifdef CONFIG_VMAP_STACK
>  handle_kernel_stack_overflow:
> -	la sp, shadow_stack
> -	addi sp, sp, SHADOW_OVERFLOW_STACK_SIZE
> +	/* we reach here from kernel context, sscratch must be 0 */
> +	csrrw x31, CSR_SCRATCH, x31
> +	asm_per_cpu sp, overflow_stack, x31
> +	li x31, OVERFLOW_STACK_SIZE
> +	add sp, sp, x31
> +	/* zero out x31 again and restore x31 */
> +	xor x31, x31, x31
> +	csrrw x31, CSR_SCRATCH, x31
>  
> -	//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)
> -
> -	//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 f3e96d60a2ff..eef3a87514c7 100644
> --- a/arch/riscv/kernel/traps.c
> +++ b/arch/riscv/kernel/traps.c
> @@ -208,18 +208,8 @@ 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)],
> +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)
>  {
> -- 
> 2.25.1
> 

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