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

[Mark Rutland]

On Wed, Apr 11, 2018 at 02:30:28PM +0800, Ji.Zhang wrote:
> On Mon, 2018-04-09 at 12:26 +0100, Mark Rutland wrote:
> > On Sun, Apr 08, 2018 at 03:58:48PM +0800, Ji.Zhang wrote:
> > > Yes, I see where the loop is, I have missed that the loop may cross
> > > different stacks.
> > > Define a nesting order and check against is a good idea, and it can
> > > resolve the issue exactly, but as you mentioned before, we have no idea
> > > how to handle with overflow and sdei stack, and the nesting order is
> > > strongly related with the scenario of the stack, which means if someday
> > > we add another stack, we should consider the relationship of the new
> > > stack with other stacks. From the perspective of your experts, is that
> > > suitable for doing this in unwind?
> > > 
> > > Or could we just find some way easier but not so accurate, eg.
> > > Proposal 1: 
> > > When we do unwind and detect that the stack spans, record the last fp of
> > > previous stack and next time if we get into the same stack, compare it
> > > with that last fp, the new fp should still smaller than last fp, or
> > > there should be potential loop.
> > > For example, when we unwind from irq to task, we record the last fp in
> > > irq stack such as last_irq_fp, and if it unwind task stack back to irq
> > > stack, no matter if it is the same irq stack with previous, just let it
> > > go and compare the new irq fp with last_irq_fp, although the process may
> > > be wrong since from task stack it could not unwind to irq stack, but the
> > > whole process will eventually stop.
> > 
> > I agree that saving the last fp per-stack could work.
> > 
> > > Proposal 2:
> > > So far we have four types of stack: task, irq, overflow and sdei, could
> > > we just assume that the MAX number of stack spanning is just 3
> > > times?(task->irq->overflow->sdei or task->irq->sdei->overflow), if yes,
> > > we can just check the number of stack spanning when we detect the stack
> > > spans.
> > 
> > I also agree that counting the number of stack transitions will prevent
> > an inifinite loop, even if less accurately than proposal 1.
> > 
> > I don't have a strong preference either way.
> Thank you for your comment.
> Compared with proposal 1 and 2, I decide to use proposal2 since
> proposal1 seems a little complicated and it is not as easy as proposal2
> when new stack is added.
> The sample is as below:
> diff --git a/arch/arm64/include/asm/stacktrace.h
> b/arch/arm64/include/asm/stacktrace.h
> index 902f9ed..72d1f34 100644
> --- a/arch/arm64/include/asm/stacktrace.h
> +++ b/arch/arm64/include/asm/stacktrace.h
> @@ -92,4 +92,22 @@ static inline bool on_accessible_stack(struct
> task_struct *tsk, unsigned long sp
>         return false;
>  }
>  
> +#define MAX_STACK_SPAN 3

Depending on configuration we can have:

* task
* irq
* overflow (optional with VMAP_STACK)
* sdei (optional with ARM_SDE_INTERFACE && VMAP_STACK)

So 3 isn't always correct.

Also, could we please call this something like MAX_NR_STACKS?

> +DECLARE_PER_CPU(int, num_stack_span);

I'm pretty sure we can call unwind_frame() in a preemptible context, so
this isn't safe.

Put this counter into the struct stackframe, and call it something like
nr_stacks;

[...]

> +DEFINE_PER_CPU(int, num_stack_span);

As above, this can go.

> +
>  /*
>   * AArch64 PCS assigns the frame pointer to x29.
>   *
> @@ -56,6 +58,20 @@ 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(tsk, fp, frame->fp)) {
> +               int num = (int)__this_cpu_read(num_stack_span);
> +
> +               if (num >= MAX_STACK_SPAN)
> +                       return -EINVAL;
> +               num++;
> +               __this_cpu_write(num_stack_span, num);
> +               fp = frame->fp + 0x8;
> +       }
> +       if (fp <= frame->fp) {
> +               pr_notice("fp invalid, stop unwind\n");
> +               return -EINVAL;
> +       }

I think this can be simplified to something like:

	bool same_stack;

	same_stack = on_same_stack(tsk, fp, frame->fp);

	if (fp <= frame->fp && same_stack)
		return -EINVAL;
	if (!same_stack && ++frame->nr_stacks > MAX_NR_STACKS)
		return -EINVAL;

... assuming we add nr_stacks to struct stackframe.

Thanks,
Mark.