Re: [PATCH -next v2] x86: Prevent KASAN false positive warnings in __show_regs

[Tengda Wu]

On 2025/8/30 10:18, Josh Poimboeuf wrote:
> On Sat, Aug 30, 2025 at 09:34:59AM +0800, Tengda Wu wrote:
>>
>>
>> On 2025/8/30 5:01, Josh Poimboeuf wrote:
>>> On Fri, Aug 29, 2025 at 09:47:44AM +0000, Tengda Wu wrote:
>>>>  static void show_regs_if_on_stack(struct stack_info *info, struct pt_regs *regs,
>>>>  				  bool partial, const char *log_lvl)
>>>>  {
>>>> +	bool kasan_disabled = false;
>>>> +
>>>> +	/*
>>>> +	 * When 'regs' resides in another task's stack space, KASAN should be
>>>> +	 * disabled to prevent false positives during 'regs->' operation, as
>>>> +	 * the 'regs' contents may change concurrently with task execution.
>>>> +	 */
>>>> +	if (!object_is_on_stack(regs)) {
>>>> +		kasan_disable_current();
>>>> +		kasan_disabled = true;
>>>> +	}
>>>
>>> I don't think this is right.  object_is_on_stack() only checks current's
>>> *task* stack.  However the regs might be on a different stack used by
>>> current (e.g., exception stack).  In which case there's no need to
>>> disable KASAN.
>>>
>>> What really determines the KASAN-safety is whether it's the current task
>>> or not.
>>>
>>
>> I see.
>>
>> Since operations walking other tasks' regs-> only occur in show_regs_if_on_stack,
>> to cover a more accurate range and targets, it seems we have no choice but to add
>> a task parameter to show_regs_if_on_stack and then perform the check, right?
> 
> Reading another task's stack while it's running is problematic in
> general -- not only for reading saved regs, but for reading any other
> values on the stack.  So why not just do it in show_trace_log_lvl()?
> 

Theoretically, that's correct. I was too focused on the regs itself and overlooked
this fact. Additionally, I just discovered a potentially missed fix: there is also
code in show_trace_log_lvl that directly accesses regs->:

		if (regs && stack == &regs->ip)
			goto next;

Simply disabling KASAN within show_regs_if_on_stack is indeed insufficient.

Thank you very much for the reminder. I will send out a v3 to move the KASAN check
back into show_trace_log_lvl.

-- Tengda