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Date:   Wed, 19 Oct 2022 21:05:42 +0800
From:   Jisheng Zhang <jszhang@kernel.org>
To:     Guo Ren <guoren@kernel.org>
Cc:     Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
        llvm@lists.linux.dev
Subject: Re: [PATCH v2 3/4] riscv: fix race when vmap stack overflow and
 remove shadow_stack
Message-ID: <Y0/2Ji4/bKF1xtpz@xhacker>
References: <20220928162007.3791-1-jszhang@kernel.org>
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On Wed, Oct 19, 2022 at 10:52:16AM +0800, Guo Ren wrote:
> On Thu, Sep 29, 2022 at 1:54 PM Guo Ren <guoren@kernel.org> wrote:
> >
> > On Thu, Sep 29, 2022 at 1:03 AM Jisheng Zhang <jszhang@kernel.org> wrote:
> > >
> > > On Thu, Sep 29, 2022 at 12:20:06AM +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 content of kernel thread pointer I.E "tp" register can still
> > > > be gotten from the the CSR_SCRATCH register, thus we can clobber tp
> > > > under the condtion that we restore tp from CSR_SCRATCH later.
> > > >
> > > > 2. Once vmap stack overflow happen, panic is comming soon, no
> > > > performance concern at all, so we don't need to define the overflow
> > > > stack as percpu var, we can simplify it into a pointer array which
> > > > points to allocated pages.
> > > >
> > > > Thus we can use tp as a tmp register to get the cpu id to calculate
> > > > the offset of overflow stack pointer array for each cpu w/o shadow
> > > > stack any more. Thus the race condition is removed as a 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.
> > > >
> > > > 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               | 56 ++++---------------------
> > > >  arch/riscv/kernel/traps.c               | 31 ++++++++------
> > > >  5 files changed, 29 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
> > >
> > > oops, this should be removed, will update it in a newer version after
> > > collecting review comments.
> > >
> > > >
> > > >  #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..5a6171a90d81 100644
> > > > --- a/arch/riscv/kernel/entry.S
> > > > +++ b/arch/riscv/kernel/entry.S
> > > > @@ -223,54 +223,16 @@ 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)
> > > > +     slli tp, tp, RISCV_LGPTR
> > > > +     add tp, sp, tp
> > > > +     REG_L sp, 0(tp)
> > > > +
> > > > +     /* restore 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..b6c64f0fb70f 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;
> > > > -}
> > > > +void *overflow_stack[NR_CPUS] __ro_after_init __aligned(16);
> > Er... We've talked NR_CPUS = 8192, even a pointer would cause 64KB wasted.
> >
> > I prefer the previous solution with a atomic flag.
> >
> > diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
> > index 5cbd6684ef52..42a3b14a20ab 100644
> > --- a/arch/riscv/kernel/entry.S
> > +++ b/arch/riscv/kernel/entry.S
> > @@ -223,6 +223,9 @@ END(ret_from_exception)
> >
> >  #ifdef CONFIG_VMAP_STACK
> >  ENTRY(handle_kernel_stack_overflow)
> > +1:     la sp, spin_ shadow_stack
> > +       amoswap.w sp, sp, (sp)
> > +       bnez sp, 1b
> >         la sp, shadow_stack
> >         addi sp, sp, SHADOW_OVERFLOW_STACK_SIZE
> >
> > diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
> > index 73f06cd149d9..9058a05cac53 100644
> > --- a/arch/riscv/kernel/traps.c
> > +++ b/arch/riscv/kernel/traps.c
> > @@ -229,11 +229,15 @@ asmlinkage unsigned long get_overflow_stack(void)
> >                 OVERFLOW_STACK_SIZE;
> >  }
> >
> > +atomic_t spin_ shadow_stack __section(".sdata");
> > +
> >  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);
> >
> > +       atomic_set_release(spin_ shadow_stack, 0);
> > +
> >         console_verbose();
> >
> >         pr_emerg("Insufficient stack space to handle exception!\n");
> Hi Jisheng,

Hi Guo,

> 
> Could you give out a new patch based on the above style to fix the
> problem first? It's a minor modification and consolidate the entry.S
> procedure.
> 

I read the above msg as: A new patch following the above style to fix
the race condition. Then another patch to remove the shadow stack
usage. Will cook the fix patch soon. 

Thanks