Re: [PATCH] kprobes: Fix possible warn in __arm_kprobe_ftrace()

[Google]

On Sun, 7 Apr 2024 11:59:04 +0800
Zheng Yejian <zhengyejian1@xxxxxxxxxx> wrote:

> There is once warn in __arm_kprobe_ftrace() on:
> 
>  ret = ftrace_set_filter_ip(ops, (unsigned long)p->addr, 0, 0);
>  if (WARN_ONCE(..., "Failed to arm kprobe-ftrace at %pS (error %d)\n", ...)
>    return ret;
> 
> This warning is due to 'p->addr' is not a valid ftrace_location and
> that invalid 'p->addr' was bypassed in check_kprobe_address_safe():

Ah, this is a guard warning to detect changes on ftrace_set_filter() and/or
preparation steps. (A kind of debug message.)
The ftrace address check is done by check_ftrace_location() at the beginning of
check_kprobe_address_safe(). At that point, ftrace_location(addr) == addr should
return true if the module is loaded. But there is a small window that we check
the ftrace_location() and get the module refcount, even if the "addr" was ftrace
location, the module is unloaded and failed to get the module refcount and fail
to register the kprobe.

>    check_kprobe_address_safe() {
>      ...
>      // 1. p->addr is in some module, this check passed
>      is_module_text_address((unsigned long) p->addr)
>      ...
>      // 2. If the moudle is removed, the *probed_mod is NULL, but then
>      //    the return value would still be 0 !!!
>      *probed_mod = __module_text_address((unsigned long) p->addr);
>      ...
>    }
> 
> So adjust the module text check to fix it.

This seems just optimizing code (skip the 2nd module search), right?
Also some comments needs to be updated.

> 
> Signed-off-by: Zheng Yejian <zhengyejian1@xxxxxxxxxx>
> ---
>  kernel/kprobes.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index 9d9095e81792..e0612cc3e2a3 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -1567,10 +1567,16 @@ static int check_kprobe_address_safe(struct kprobe *p,
>  	jump_label_lock();
>  	preempt_disable();
>

	/* Ensure the address is in a text area, and find a module if exists. */
  
> +	*probed_mod = NULL;
> +	if (!core_kernel_text((unsigned long) p->addr)) {
> +		*probed_mod = __module_text_address((unsigned long) p->addr);
> +		if (!(*probed_mod)) {
> +			ret = -EINVAL;
> +			goto out;
> +		}

nit: this should get the module refcount soon after getting probed_mod.
(I think this should be an atomic operation, but we don't have such interface.)

> +	}

>  	/* Ensure it is not in reserved area nor out of text */

	/* Ensure it is not in reserved area. */

> -	if (!(core_kernel_text((unsigned long) p->addr) ||
> -	    is_module_text_address((unsigned long) p->addr)) ||

Note that this part is doing same thing. If the probe address is !kernel_text
and !module_text, it will be rejected.

> -	    in_gate_area_no_mm((unsigned long) p->addr) ||
> +	if (in_gate_area_no_mm((unsigned long) p->addr) ||
>  	    within_kprobe_blacklist((unsigned long) p->addr) ||
>  	    jump_label_text_reserved(p->addr, p->addr) ||
>  	    static_call_text_reserved(p->addr, p->addr) ||
> @@ -1581,7 +1587,6 @@ static int check_kprobe_address_safe(struct kprobe *p,
>  	}
>  
>  	/* Check if 'p' is probing a module. */

	/* Get module refcount and reject __init functions for loaded modules. */

> -	*probed_mod = __module_text_address((unsigned long) p->addr);
>  	if (*probed_mod) {
>  		/*
>  		 * We must hold a refcount of the probed module while updating
> -- 
> 2.25.1
> 

Thank you,

-- 
Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>