linux-kernel - Re: [PATCH] tracepoint: Fix CFI failures with tp_stub

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Date:   Thu, 23 Mar 2023 14:37:15 -0400
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Steven Rostedt <rostedt@...dmis.org>,
        Mark Rutland <mark.rutland@....com>
Cc:     linux-kernel@...r.kernel.org, mhiramat@...nel.org,
        "Jose E. Marchesi" <jose.marchesi@...cle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] tracepoint: Fix CFI failures with tp_stub_func

On 2023-03-23 12:34, Steven Rostedt wrote:
> On Thu, 23 Mar 2023 16:27:37 +0000
> Mark Rutland <mark.rutland@....com> wrote:
> 
>> On Thu, Mar 23, 2023 at 01:45:34PM +0000, Mark Rutland wrote:
>>> On Thu, Mar 23, 2023 at 08:53:21AM -0400, Steven Rostedt wrote:
>>>> On Thu, 23 Mar 2023 11:40:12 +0000
>>>> Mark Rutland <mark.rutland@....com> wrote:
>>
>>>>> diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
>>>>> index 6811e43c1b5c2..1640926441910 100644
>>>>> --- a/include/linux/tracepoint.h
>>>>> +++ b/include/linux/tracepoint.h
>>>>> @@ -303,6 +303,7 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
>>>>>   	__section("__tracepoints_strings") = #_name;			\
>>>>>   	extern struct static_call_key STATIC_CALL_KEY(tp_func_##_name);	\
>>>>>   	int __traceiter_##_name(void *__data, proto);			\
>>>>> +	void __tracestub_##_name(void *, proto);			\
>>>>>   	struct tracepoint __tracepoint_##_name	__used			\
>>>>>   	__section("__tracepoints") = {					\
>>>>>   		.name = __tpstrtab_##_name,				\
>>>>> @@ -310,6 +311,7 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
>>>>>   		.static_call_key = &STATIC_CALL_KEY(tp_func_##_name),	\
>>>>>   		.static_call_tramp = STATIC_CALL_TRAMP_ADDR(tp_func_##_name), \
>>>>>   		.iterator = &__traceiter_##_name,			\
>>>>> +		.stub = &__tracestub_##_name,				\
>>>>>   		.regfunc = _reg,					\
>>>>>   		.unregfunc = _unreg,					\
>>>>>   		.funcs = NULL };					\
>>>>> @@ -330,6 +332,9 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
>>>>>   		}							\
>>>>>   		return 0;						\
>>>>>   	}								\
>>>>> +	void __tracestub_##_name(void *__data, proto)			\
>>>>> +	{								\
>>>>> +	}								\
>>>>
>>>> I purposely did not do this because this adds over a thousand stub
>>>> functions! It adds one for *every* tracepoint (and that is a superset of
>>>> trace events).
>>>>
>>>> Is there some other way we could do this?
>>>>
>>>> C really really needs a way to make a generic void do_nothing(...) function!
>>>>
>>>> I added some compiler folks to the Cc to hear our grievances.
>>>
>>> I pulled in Sami, who did much of the kCFI work, and PeterZ too...
>>>
>>> We can't have a generic function that's compatible will all function
>>> prototypes, since that mechanism would undermine the CFI scheme. Either callers
>>> would always have to omit the check, or we're have to have a special "always
>>> compatible" type hash, and both would be gigantic targets for attack.
>>>
>>> Can we avoid the stub entirely? e.g. make hte call conditional on the func
>>> pointer not being some bad value (e.g. like the error pointers?). That way we
>>> could avoid the call, and we wouldn't need the stub implementation.
>>
>> Along those lines (and as Peter also suggested over IRC), would something like
>> the below be preferable?
> 
> So we had this discussion a while ago:
> 
> See befe6d946551 ("tracepoint: Do not fail unregistering a probe due to memory failure")
> 
> Where I believe one answer was to use NULL instead of a stub.
> 
> I have to go back and re-read that thread. Mathieu was involved with all
> this too.
> 
> And as I mentioned in my other reply. There was a more complex solution
> that could handle this if the stub solution ended up being an issue.
> 
> Repeated again so Mathieu doesn't have to search for it.
> 
>      [ Note, this version does use undefined compiler behavior (assuming that
>        a stub function with no parameters or return, can be called by a location
>        that thinks it has parameters but still no return value. Static calls
>        do the same thing, so this trick is not without precedent.
> 
>        There's another solution that uses RCU tricks and is more complex, but
>        can be an alternative if this solution becomes an issue.
> 
>        Link: https://lore.kernel.org/lkml/20210127170721.58bce7cc@gandalf.local.home/
>      ]

Ugh. The last thing we need there is more RCU complexity. My brain is still recovering
from fixing the last time the introduction of static calls special-cases ended up subtly
breaking tracepoints.

> 
> -- Steve
> 
> 
>>
>> Mark.
>>
>> ---->8----
>> diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
>> index 6811e43c1b5c..b8017e906049 100644
>> --- a/include/linux/tracepoint.h
>> +++ b/include/linux/tracepoint.h
>> @@ -33,6 +33,8 @@ struct trace_eval_map {
>>   
>>   #define TRACEPOINT_DEFAULT_PRIO	10
>>   
>> +void tp_stub_func(void);
>> +
>>   extern struct srcu_struct tracepoint_srcu;
>>   
>>   extern int
>> @@ -324,6 +326,8 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
>>   		if (it_func_ptr) {					\
>>   			do {						\
>>   				it_func = READ_ONCE((it_func_ptr)->func); \
>> +				if (it_func == tp_stub_func)		\
>> +					continue;			\

Along the lines of my earlier proposal:

https://lore.kernel.org/all/1889971276.46615.1605559047845.JavaMail.zimbra@efficios.com/

We could reserve (void *)0x1 as a stub value rather than adding an explicit stub function
if we end up skipping the call anyway. The check could be:

#define TP_STUB_FN	((void *)0x1)

if (unlikely(it_func == TP_STUB_FN))
   continue;

And we would only need that check for CONFIG_HAVE_STATIC_CALL=y right ?

Thanks,

Mathieu


>>   				__data = (it_func_ptr)->data;		\
>>   				((void(*)(void *, proto))(it_func))(__data, args); \
>>   			} while ((++it_func_ptr)->func);		\
>> diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
>> index 8d1507dd0724..dcf5a637429f 100644
>> --- a/kernel/tracepoint.c
>> +++ b/kernel/tracepoint.c
>> @@ -99,7 +99,7 @@ struct tp_probes {
>>   };
>>   
>>   /* Called in removal of a func but failed to allocate a new tp_funcs */
>> -static void tp_stub_func(void)
>> +void tp_stub_func(void)
>>   {
>>   	return;
>>   }
> 

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com