Messages in this thread | | | From | Guo Ren <> | Date | Thu, 24 Nov 2022 14:31:25 +0800 | Subject | Re: [PATCH] riscv: VMAP_STACK overflow detection thread-safe |
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On Thu, Nov 24, 2022 at 1:57 PM Deepak Gupta <debug@rivosinc.com> wrote: > > Inline. > > On Wed, Nov 23, 2022 at 5:28 PM Guo Ren <guoren@kernel.org> wrote: > > > > > > > > On Thu, Nov 24, 2022 at 8:50 AM Deepak Gupta <debug@rivosinc.com> wrote: > >> > >> commit 31da94c25aea835ceac00575a9fd206c5a833fed 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. > >> > >> Following are the changes in this patch > >> > >> - Defines an asm macro to obtain per-cpu symbols in destination > >> register. > >> - Computing per-cpu symbol requires a temporary register. When stack is > >> out of question, a place is needed to spill a register. `thread_info` > >> is good location to have spill register. > >> - In entry.S when overflow is detected x31 is spilled into thread_info. > >> x31 is used as temp reg for asm macro to locate per-cpu overflow stack > >> > >> Other relevant disccussion on this > >> https://lore.kernel.org/linux-riscv/Y347B0x4VUNOd6V7@xhacker/T/#t > >> > >> Tested by `echo EXHAUST_STACK > /sys/kernel/debug/provoke-crash/DIRECT` > >> > >> [ 286.223273] Insufficient stack space to handle exception!/debug/provoke-crash/DIRECT > >> [ 286.223878] Task stack: [0xff20000010a98000..0xff20000010a9c000] > >> [ 286.224411] Overflow stack: [0xff600001f7d98370..0xff600001f7d99370] > >> [ 286.226057] CPU: 1 PID: 205 Comm: bash Not tainted 6.1.0-rc2-00001-g328a1f96f7b9 #34 > >> [ 286.227139] Hardware name: riscv-virtio,qemu (DT) > >> [ 286.228000] epc : __memset+0x60/0xfc > >> [ 286.229299] ra : recursive_loop+0x48/0xc6 [lkdtm] > >> [ 286.231457] epc : ffffffff808de0e4 ra : ffffffff0163a752 sp : ff20000010a97e80 > >> [ 286.232207] gp : ffffffff815c0330 tp : ff600000820ea280 t0 : ff20000010a97e88 > >> [ 286.233584] t1 : 000000000000002e t2 : 3233206874706564 s0 : ff20000010a982b0 > >> [ 286.234293] s1 : 0000000000000012 a0 : ff20000010a97e88 a1 : 0000000000000000 > >> [ 286.234998] a2 : 0000000000000400 a3 : ff20000010a98288 a4 : 0000000000000000 > >> [ 286.235697] a5 : 0000000000000000 a6 : fffffffffffe43f0 a7 : 00007fffffffffff > >> [ 286.236384] s2 : ff20000010a97e88 s3 : ffffffff01644680 s4 : ff20000010a9be90 > >> [ 286.237743] s5 : ff600000842ba6c0 s6 : 00aaaaaac29e42b0 s7 : 00fffffff0aa3684 > >> [ 286.238691] s8 : 00aaaaaac2978040 s9 : 0000000000000065 s10: 00ffffff8a7cad10 > >> [ 286.239591] s11: 00ffffff8a76a4e0 t3 : ffffffff815dbaf4 t4 : ffffffff815dbaf4 > >> [ 286.240537] t5 : ffffffff815dbab8 t6 : ff20000010a9bb48 > >> [ 286.241540] status: 0000000200000120 badaddr: ff20000010a97e88 cause: 000000000000000f > >> [ 286.242979] Kernel panic - not syncing: Kernel stack overflow > >> [ 286.244106] CPU: 1 PID: 205 Comm: bash Not tainted 6.1.0-rc2-00001-g328a1f96f7b9 #34 > >> [ 286.245276] Hardware name: riscv-virtio,qemu (DT) > >> [ 286.245929] Call Trace: > >> [ 286.246954] [<ffffffff80006754>] dump_backtrace+0x30/0x38 > >> [ 286.247813] [<ffffffff808de798>] show_stack+0x40/0x4c > >> [ 286.248429] [<ffffffff808ea2a8>] dump_stack_lvl+0x44/0x5c > >> [ 286.249439] [<ffffffff808ea2d8>] dump_stack+0x18/0x20 > >> [ 286.250056] [<ffffffff808dec06>] panic+0x126/0x2fe > >> [ 286.250642] [<ffffffff800065ea>] walk_stackframe+0x0/0xf0 > >> [ 286.251357] [<ffffffff0163a752>] recursive_loop+0x48/0xc6 [lkdtm] > >> [ 286.253321] SMP: stopping secondary CPUs > >> [ 286.256724] ---[ end Kernel panic - not syncing: Kernel stack overflow ]--- > >> > >> Fixes: 31da94c25aea835ceac00575a9fd206c5a833fed > > > > The patch gives more significant change than the Fixes, and Fixes would expand to the previous stable versions. Please don't set it as a Fixes, but an improved OVERSTACK dead path performance feature. > > > > Not a performance feature but more like correctness. > If kernel died and two CPUs raced to kernel stack overflow, > death post-mortem should be straightforward. We already have had a fixup, and your patch likes a feature with a significant change. https://lore.kernel.org/linux-riscv/20221030124517.2370-1-jszhang@kernel.org/ If it is for correctness, the simple lock is enough.
> > >> > >> Cc: Guo Ren <guoren@kernel.org> > >> Cc: Jisheng Zhang <jszhang@kernel.org> > >> > >> Signed-off-by: Deepak Gupta <debug@rivosinc.com> > >> --- > >> arch/riscv/include/asm/asm.h | 11 ++++++ > >> arch/riscv/include/asm/thread_info.h | 3 ++ > >> arch/riscv/kernel/asm-offsets.c | 4 +++ > >> arch/riscv/kernel/entry.S | 54 ++++------------------------ > >> arch/riscv/kernel/traps.c | 12 +------ > >> 5 files changed, 26 insertions(+), 58 deletions(-) > >> > >> diff --git a/arch/riscv/include/asm/asm.h b/arch/riscv/include/asm/asm.h > >> index 1b471ff73178..373eba843331 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 +81,16 @@ > >> .endr > >> .endm > >> > >> +.macro asm_per_cpu dst sym tmp > >> + REG_L \tmp, TASK_TI_CPU_NUM(tp) > >> + slli \tmp, \tmp, 0x3 > >> + la \dst, __per_cpu_offset > >> + add \dst, \dst, \tmp > >> + REG_L \tmp, 0(\dst) > >> + la \dst, \sym > >> + add \dst, \dst, \tmp > > > > Another tricky asm code of using percpu, I don't know how the percpu maintenance guy thinks. > > Pretty much the same exists for arm64 as well. That's not the reason.
> I don't see anything tricky here. I prefer using standard percpu API, not hacking one.
> > > > > Percpu is really complex to be used here. That's why I don't like Jisheng's previous idea, although I haven't given an explicitly opposed comment. > > > > Why is per-cpu really complex here? > That's why per-cpu exists, right? > Compiler generates pretty much same code everywhere for C. > > > Keep simple and direct. > > > >> > >> +.endm > >> + > >> #endif /* __ASSEMBLY__ */ > >> > >> #endif /* _ASM_RISCV_ASM_H */ > >> diff --git a/arch/riscv/include/asm/thread_info.h b/arch/riscv/include/asm/thread_info.h > >> index 67322f878e0d..7e17dc07cf11 100644 > >> --- a/arch/riscv/include/asm/thread_info.h > >> +++ b/arch/riscv/include/asm/thread_info.h > >> @@ -65,6 +65,9 @@ struct thread_info { > >> */ > >> long kernel_sp; /* Kernel stack pointer */ > >> long user_sp; /* User stack pointer */ > >> +#ifdef CONFIG_VMAP_STACK > >> + long spill_reg; /* per cpu scratch space to spill a single register */ > >> +#endif > > > > Is it worth expanding "struct thread_info" with another reg size? A dead path performance issue affects living cost. May we give a cheap funeral for the OVERFLOW_STACK death? > > > > Yes I had been thinking over it and your point is quite valid. > There is a death's charge (in a corner case) to living structures > (particularly to all threads) > Andrew Bresticker (abrestic@rivosinc.com) suggested using `sscratch` > to spill into. > I think that makes sense. > > I'll send a revised patch. I think that's not a fix because your patch broke the previous author's design.
Below is the fixup style: https://lore.kernel.org/linux-riscv/20221030124517.2370-1-jszhang@kernel.org/ Just fix the previous author's design with a simple spinlock?
> > >> > >> int cpu; > >> }; > >> > >> diff --git a/arch/riscv/kernel/asm-offsets.c b/arch/riscv/kernel/asm-offsets.c > >> index df9444397908..bed3c83bfb8f 100644 > >> --- a/arch/riscv/kernel/asm-offsets.c > >> +++ b/arch/riscv/kernel/asm-offsets.c > >> @@ -38,6 +38,10 @@ 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); > >> +#ifdef CONFIG_VMAP_STACK > >> + OFFSET(TASK_TI_SPILL_REG, task_struct, thread_info.spill_reg); > >> +#endif > >> 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..12f285cec136 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 +406,12 @@ handle_syscall_trace_exit: > >> > >> #ifdef CONFIG_VMAP_STACK > >> handle_kernel_stack_overflow: > >> - la sp, shadow_stack > >> - addi sp, sp, SHADOW_OVERFLOW_STACK_SIZE > >> + REG_S x31, TASK_TI_SPILL_REG(tp) > >> + asm_per_cpu sp, overflow_stack, x31 > >> + li x31, OVERFLOW_STACK_SIZE > >> + add sp, sp, x31 > >> + REG_L x31, TASK_TI_SPILL_REG(tp) > >> > >> - //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) > > > > Yes, your patch gives a good performance here. But what do we gain? > > > >> > >> 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 > >> > > > > > > -- > > Best Regards > > Guo Ren
-- Best Regards Guo Ren
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