Messages in this thread |  | | Date | Wed, 23 Nov 2022 09:02:50 -0800 | From | Deepak Gupta <> | Subject | Re: [PATCH 3/4] riscv: fix race when vmap stack overflow and remove shadow_stack |
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On Mon, Sep 26, 2022 at 01:53:55AM +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 kernel thread pointer I.E "tp" register can still be gotten from >the CSR_SCRATCH register, thus we can clobber tp under the condition >that we restore tp from CSR_SCRATCH later. > >2. Once vmap stack overflow happen, panic is coming soon, no >performance concern at all, so we don't need to define the overflow >stack as percpu var, we can simplify it into an array. > >Thus we can use tp as a tmp register to get the cpu id to calculate >the offset of overflow stack for each cpu w/o shadow stack any more. >Thus the race condition is removed as 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.
I've a different patch to solve the same problem. Using an asm macro which allows obtaining per cpu symbols. Thus we can switch to overflow_stack directly from entry.S Patch coming soon.
> >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 | 54 ++++--------------------- > arch/riscv/kernel/traps.c | 15 +------ > 5 files changed, 11 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 > > #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..442d93beffcf 100644 >--- a/arch/riscv/kernel/entry.S >+++ b/arch/riscv/kernel/entry.S >@@ -223,54 +223,14 @@ 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) >+ addi tp, tp, 1 >+ slli tp, tp, OVERFLOW_STACK_SHIFT >+ add sp, sp, 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..2a2a977c1eff 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; >-} >+unsigned long overflow_stack[NR_CPUS][OVERFLOW_STACK_SIZE/sizeof(long)] __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(); > >-- >2.34.1 >
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