Re: [PATCH] arm64: avoid race condition issue in dump_backtrace

[Mark Rutland]

On Fri, Mar 30, 2018 at 04:08:12PM +0800, Ji.Zhang wrote:
> On Wed, 2018-03-28 at 11:12 +0100, Mark Rutland wrote:
> > On Wed, Mar 28, 2018 at 05:33:32PM +0800, Ji.Zhang wrote:
> > 
> > I'm very much not keen on this.
> > 
> > I think that if we're going to do this, the only sane way to do it is to
> > have unwind_frame() verify the current fp against the previous one, and
> > verify that we have some strict nesting of stacks. Generally, that means
> > we can go:
> > 
> >   overflow -> irq -> task
> > 
> > ... though I'm not sure what to do about the SDEI stack vs the overflow
> > stack.
> Actually I have had the fp check in unwind_frame(), but since I use the
> in_entry_text() to determine if stack spans, and I did not want to
> include traps.h in stacktrace.c, so I move the check out to
> dump_backtrace.
> Anyway, It seems that the key point is how should we verify that there
> are some nesting of stacks. Since in unwind_frame() we already have the
> previous fp and current fp, could we assume that if these two fps are
> NOT belong to the same stack, there should be stack spans (no matter
> task->irq, or irq->overflow, etc), and we can do the tricky to bypass
> the fp check.The sample of the prosal just like:

Unfortuantely, this still allows for loops, like task -> irq -> task, so
I think if we're going to try to fix this, we must define a nesting
order and check against that.

Thanks,
Mark.

> diff --git a/arch/arm64/include/asm/stacktrace.h
> b/arch/arm64/include/asm/stacktrace.h
> index 902f9ed..fc2bf4d 100644
> --- a/arch/arm64/include/asm/stacktrace.h
> +++ b/arch/arm64/include/asm/stacktrace.h
> @@ -92,4 +92,18 @@ static inline bool on_accessible_stack(struct
> task_struct *tsk, unsigned long sp
>         return false;
>  }
> 
> +static inline bool on_same_stack(struct task_struct *tsk, unsigned long
> sp1, unsigned sp2)
> +{
> +       if (on_task_stack(tsk, sp1) && on_task_stack(tsk, sp2))
> +               return true;
> +       if (on_irq_stack(sp1) && on_irq_stack(sp2))
> +               return true;
> +       if (on_overflow_stack(sp1) && on_overflow_stack(sp2))
> +               return true;
> +       if (on_sdei_stack(sp1) && on_sdei_stack(sp2))
> +               return true;
> +
> +       return false;
> +}
> +
>  #endif /* __ASM_STACKTRACE_H */
> diff --git a/arch/arm64/kernel/stacktrace.c
> b/arch/arm64/kernel/stacktrace.c
> index d5718a0..4907a67 100644
> --- a/arch/arm64/kernel/stacktrace.c
> +++ b/arch/arm64/kernel/stacktrace.c
> @@ -56,6 +56,13 @@ int notrace unwind_frame(struct task_struct *tsk,
> struct stackframe *frame)
>         frame->fp = READ_ONCE_NOCHECK(*(unsigned long *)(fp));
>         frame->pc = READ_ONCE_NOCHECK(*(unsigned long *)(fp + 8));
> 
> +       if (!on_same_stack(fp, frame->fp))
> +               fp = frame->fp + 0x8;
> +       if (fp <= frame->fp) {
> +               pr_notice("fp invalid, stop unwind\n");
> +               return -EINVAL;
> +       }
> +
>  #ifdef CONFIG_FUNCTION_GRAPH_TRACER
>         if (tsk->ret_stack &&
>                         (frame->pc == (unsigned long)return_to_handler))
> {
> 
> Could this work?
> 
> Best Regards,
> Ji
>