Re: [PATCH v4 1/3] tracing: kprobes: Prohibit probing on notrace function

[Masami Hiramatsu]

On Sat, 28 Jul 2018 22:55:27 +0900
Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:

> Prohibit kprobe-events probing on notrace function.
> Since probing on the notrace function can cause recursive
> event call. In most case those are just skipped, but
> in some case it falls into infinit recursive call.
> 
> This protection can be disabled by the kconfig
> CONFIG_KPROBE_EVENTS_ON_NOTRACE=y, but it is highly
> recommended to keep it "n" for normal kernel.
> 
> Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Tested-by: Francis Deslauriers <francis.deslauriers@xxxxxxxxxxxx>
> ---
>   Changes in v2
>    - Add CONFIG_KPROBE_EVENTS_ON_NOTRACE kconfig for knocking down
>      the protection.
>   Changes in v3
>    - Fix to check raw-address (no symbol) probe point correctly.
>   Changes in v4
>    - No notrace check if CONFIG_FUNCTION_TRACER=n. In that case
>      notrace is ignored.

Oops, this must be DYNAMIC_FTRACE, not FUNCTION_TRACER, since
ftrace_location_range() is provided only if CONFIG_DYNAMIC_FTACE=y.
(And CONFIG_DYNAMIC_FTRACE depends on FUNCTION_TRACER)

Thanks,

> ---
>  kernel/trace/Kconfig        |   18 ++++++++++++++++
>  kernel/trace/trace_kprobe.c |   47 +++++++++++++++++++++++++++++++++++--------
>  2 files changed, 56 insertions(+), 9 deletions(-)
> 
> diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
> index dcc0166d1997..24d5a58467a3 100644
> --- a/kernel/trace/Kconfig
> +++ b/kernel/trace/Kconfig
> @@ -456,6 +456,24 @@ config KPROBE_EVENTS
>  	  This option is also required by perf-probe subcommand of perf tools.
>  	  If you want to use perf tools, this option is strongly recommended.
>  
> +config KPROBE_EVENTS_ON_NOTRACE
> +	bool "Do NOT protect notrace function from kprobe events"
> +	depends on KPROBE_EVENTS
> +	default n
> +	help
> +	  This is only for the developers who want to debug ftrace itself
> +	  using kprobe events.
> +
> +	  Usually, ftrace related functions are protected from kprobe-events
> +	  to prevent an infinit recursion or any unexpected execution path
> +	  which leads to a kernel crash.
> +
> +	  This option disables such protection and allows you to put kprobe
> +	  events on ftrace functions for debugging ftrace by itself.
> +	  Note that this might let you shoot yourself in the foot.
> +
> +	  If unsure, say N.
> +
>  config UPROBE_EVENTS
>  	bool "Enable uprobes-based dynamic events"
>  	depends on ARCH_SUPPORTS_UPROBES
> diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
> index b37b92e7dbd4..a6ff37a9b65b 100644
> --- a/kernel/trace/trace_kprobe.c
> +++ b/kernel/trace/trace_kprobe.c
> @@ -87,6 +87,21 @@ static nokprobe_inline unsigned long trace_kprobe_nhit(struct trace_kprobe *tk)
>  	return nhit;
>  }
>  
> +static nokprobe_inline
> +unsigned long trace_kprobe_address(struct trace_kprobe *tk)
> +{
> +	unsigned long addr;
> +
> +	if (tk->symbol) {
> +		addr = (unsigned long)
> +			kallsyms_lookup_name(trace_kprobe_symbol(tk));
> +		addr += tk->rp.kp.offset;
> +	} else {
> +		addr = (unsigned long)tk->rp.kp.addr;
> +	}
> +	return addr;
> +}
> +
>  bool trace_kprobe_on_func_entry(struct trace_event_call *call)
>  {
>  	struct trace_kprobe *tk = (struct trace_kprobe *)call->data;
> @@ -99,16 +114,8 @@ bool trace_kprobe_on_func_entry(struct trace_event_call *call)
>  bool trace_kprobe_error_injectable(struct trace_event_call *call)
>  {
>  	struct trace_kprobe *tk = (struct trace_kprobe *)call->data;
> -	unsigned long addr;
>  
> -	if (tk->symbol) {
> -		addr = (unsigned long)
> -			kallsyms_lookup_name(trace_kprobe_symbol(tk));
> -		addr += tk->rp.kp.offset;
> -	} else {
> -		addr = (unsigned long)tk->rp.kp.addr;
> -	}
> -	return within_error_injection_list(addr);
> +	return within_error_injection_list(trace_kprobe_address(tk));
>  }
>  
>  static int register_kprobe_event(struct trace_kprobe *tk);
> @@ -487,6 +494,22 @@ disable_trace_kprobe(struct trace_kprobe *tk, struct trace_event_file *file)
>  	return ret;
>  }
>  
> +#if defined(CONFIG_KPROBE_EVENTS_ON_NOTRACE) || \
> +	!defined(CONFIG_FUNCTION_TRACER)
> +#define within_notrace_func(tk)	(false)
> +#else
> +static bool within_notrace_func(struct trace_kprobe *tk)
> +{
> +	unsigned long offset, size, addr;
> +
> +	addr = trace_kprobe_address(tk);
> +	if (!kallsyms_lookup_size_offset(addr, &size, &offset))
> +		return true;	/* Out of range. */
> +
> +	return !ftrace_location_range(addr - offset, addr - offset + size);
> +}
> +#endif
> +
>  /* Internal register function - just handle k*probes and flags */
>  static int __register_trace_kprobe(struct trace_kprobe *tk)
>  {
> @@ -495,6 +518,12 @@ static int __register_trace_kprobe(struct trace_kprobe *tk)
>  	if (trace_probe_is_registered(&tk->tp))
>  		return -EINVAL;
>  
> +	if (within_notrace_func(tk)) {
> +		pr_warn("Could not probe notrace function %s\n",
> +			trace_kprobe_symbol(tk));
> +		return -EINVAL;
> +	}
> +
>  	for (i = 0; i < tk->tp.nr_args; i++)
>  		traceprobe_update_arg(&tk->tp.args[i]);
>  
> 


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>