Re: [PATCH] arm64: avoid potential infinity loop in dump_backtrace

[Dave Martin <Dave.Martin@arm.com>]

On Fri, Apr 20, 2018 at 01:38:33PM +0800, Ji Zhang wrote:
> When we dump the backtrace of some tasks there is a potential infinity
> loop if the content of the stack changed, no matter the change is
> because the task is running or other unexpected cases.
> 
> This patch add stronger check on frame pointer and set the max number
> of stack spanning to avoid infinity loop.

This looks like a good idea, but I wonder whether we can do a bit better.

I think that transitions between stacks are only valid in a particular
direction.  Task context cannot preempt IRQ context for example, so
we should probably abandon the backtrace whenever unwinding seems to
be leading us from the task stack to the IRQ stack.

I *think* (but I may be wrong) that we can put the different stacks
into a strict order: task < IRQ < overflow < SDEI.  If so, this provides
a nice way to check that the backtrace is travelling in the correct
direction.

I had a go at hacking something up: see [1].  If it works, it provides
some nice cleanups too.  But my logic may be wrong and I haven't
tested it at all.

Any thoughts?  Even if it's not quite right, it may be a useful starting
point.

Cheers
---Dave

[1] [RFC PATCH 0/3] arm64: stacktrace: Improve robustness and ensure
termination of backtraces
http://lists.infradead.org/pipermail/linux-arm-kernel/2018-April/572685.html

> 
> Signed-off-by: Ji Zhang <ji.zhang@mediatek.com>
> ---
>  arch/arm64/include/asm/stacktrace.h | 25 +++++++++++++++++++++++++
>  arch/arm64/kernel/stacktrace.c      |  8 ++++++++
>  arch/arm64/kernel/traps.c           |  1 +
>  3 files changed, 34 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/stacktrace.h b/arch/arm64/include/asm/stacktrace.h
> index 902f9ed..f235b86 100644
> --- a/arch/arm64/include/asm/stacktrace.h
> +++ b/arch/arm64/include/asm/stacktrace.h
> @@ -24,9 +24,18 @@
>  #include <asm/ptrace.h>
>  #include <asm/sdei.h>
>  
> +#ifndef CONFIG_VMAP_STACK
> +#define MAX_NR_STACKS	2
> +#elif !defined(CONFIG_ARM_SDE_INTERFACE)
> +#define MAX_NR_STACKS	3
> +#else
> +#define MAX_NR_STACKS	4
> +#endif
> +
>  struct stackframe {
>  	unsigned long fp;
>  	unsigned long pc;
> +	int nr_stacks;
>  #ifdef CONFIG_FUNCTION_GRAPH_TRACER
>  	int graph;
>  #endif
> @@ -92,4 +101,20 @@ 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 long 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..d75f59d 100644
> --- a/arch/arm64/kernel/stacktrace.c
> +++ b/arch/arm64/kernel/stacktrace.c
> @@ -43,6 +43,7 @@
>  int notrace unwind_frame(struct task_struct *tsk, struct stackframe *frame)
>  {
>  	unsigned long fp = frame->fp;
> +	bool same_stack;
>  
>  	if (fp & 0xf)
>  		return -EINVAL;
> @@ -56,6 +57,13 @@ 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));
>  
> +	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;
> +
>  #ifdef CONFIG_FUNCTION_GRAPH_TRACER
>  	if (tsk->ret_stack &&
>  			(frame->pc == (unsigned long)return_to_handler)) {
> diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
> index ba964da..ee0403d 100644
> --- a/arch/arm64/kernel/traps.c
> +++ b/arch/arm64/kernel/traps.c
> @@ -121,6 +121,7 @@ void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk)
>  		frame.fp = thread_saved_fp(tsk);
>  		frame.pc = thread_saved_pc(tsk);
>  	}
> +	frame.nr_stacks = 1;
>  #ifdef CONFIG_FUNCTION_GRAPH_TRACER
>  	frame.graph = tsk->curr_ret_stack;
>  #endif
> -- 
> 1.9.1
> 
> 
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