Re: [PATCH v2] riscv: VMAP_STACK overflow detection thread-safe
From: Jisheng Zhang
Date: Thu Nov 24 2022 - 10:36:40 EST
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@xxxxxxxxxx/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@xxxxxxxxxx>
> Cc: Jisheng Zhang <jszhang@xxxxxxxxxx>
> Link: https://lore.kernel.org/linux-riscv/Y347B0x4VUNOd6V7@xhacker/T/#t
> Signed-off-by: Deepak Gupta <debug@xxxxxxxxxxxx>
>
> ---
> 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
>