Re: [RFC PATCH v3 2/4] arm64: Check the return PC against unreliable code sections

["Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>]

On 5/5/21 11:46 AM, Mark Brown wrote:
> On Tue, May 04, 2021 at 02:32:35PM -0500, Madhavan T. Venkataraman wrote:
> 
>> If you prefer, I could do something like this:
>>
>> check_pc:
>> 	if (!__kernel_text_address(frame->pc))
>> 		frame->reliable = false;
>>
>> 	range = lookup_range(frame->pc);
>>
>> #ifdef CONFIG_FUNCTION_GRAPH_TRACER
>> 	if (tsk->ret_stack &&
>> 		frame->pc == (unsigned long)return_to_handler) {
>> 		...
>> 		frame->pc = ret_stack->ret;
>> 		frame->pc = ptrauth_strip_insn_pac(frame->pc);
>> 		goto check_pc;
>> 	}
>> #endif /* CONFIG_FUNCTION_GRAPH_TRACER */
> 
>> Is that acceptable?
> 
> I think that works even if it's hard to love the goto, might want some
> defensiveness to ensure we can't somehow end up in an infinite loop with
> a sufficiently badly formed stack.
> 

I could do something like this:

- Move all frame->pc checking code into a function called check_frame_pc().

	bool	check_frame_pc(frame)
	{
		Do all the checks including function graph
		return frame->pc changed
	}

- Then, in unwind_frame()

unwind_frame()
{
	int	i;
	...

	for (i = 0; i < MAX_CHECKS; i++) {
		if (!check_frame(tsk, frame))
			break;
	}

	if (i == MAX_CHECKS)
		frame->reliable = false;
	return 0;
}

The above would take care of future cases like kretprobe_trampoline().

If this is acceptable, then the only question is - what should be the value of
MAX_CHECKS (I will rename it to something more appropriate)?

Madhavan