Re: [patch 02/10] x86/mce: Disable tracing and kprobes on do_machine_check()

[Andy Lutomirski]

> On Feb 26, 2020, at 8:08 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> ïOn Wed, Feb 26, 2020 at 07:10:01AM -0800, Andy Lutomirski wrote:
>>> On Wed, Feb 26, 2020 at 5:28 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>>>> On Tue, Feb 25, 2020 at 09:29:00PM -0800, Andy Lutomirski wrote:
>>> 
>>>>>> +void notrace do_machine_check(struct pt_regs *regs, long error_code)
>>>>>> {
>>>>>>   DECLARE_BITMAP(valid_banks, MAX_NR_BANKS);
>>>>>>   DECLARE_BITMAP(toclear, MAX_NR_BANKS);
>>>>>> @@ -1360,6 +1366,7 @@ void do_machine_check(struct pt_regs *re
>>>>>>   ist_exit(regs);
>>>>>> }
>>>>>> EXPORT_SYMBOL_GPL(do_machine_check);
>>>>>> +NOKPROBE_SYMBOL(do_machine_check);
>>>>> 
>>>>> That won't protect all the function called by do_machine_check(), right?
>>>>> There are lots of them.
>>>>> 
>>>> 
>>>> It at least means we can survive to run actual C code in
>>>> do_machine_check(), which lets us try to mitigate this issue further.
>>>> PeterZ has patches for that, and maybe this series fixes it later on.
>>>> (I'm reading in order!)
>>> 
>>> Yeah, I don't cover that either. Making the kernel completely kprobe
>>> safe is _lots_ more work I think.
>>> 
>>> We really need some form of automation for this :/ The current situation
>>> is completely nonsatisfactory.
>> 
>> I've looked at too many patches lately and lost track a bit of which
>> is which.  Shouldn't a simple tracing_disable() or similar in
>> do_machine_check() be sufficient?
> 
> It entirely depends on what the goal is :-/ On the one hand I see why
> people might want function tracing / kprobes enabled, OTOH it's all
> mighty frigging scary. Any tracing/probing/whatever on an MCE has the
> potential to make a bad situation worse -- not unlike the same on #DF.
> 
> The same with that compiler instrumentation crap; allowing kprobes on
> *SAN code has the potential to inject probes in arbitrary random code.
> At the same time, if you're running a kernel with that on and injecting
> kprobes in it, you're welcome to own the remaining pieces.
> 

Agreed.

> How far do we want to go? At some point I think we'll have to give
> people rope, show then the knot and leave them be.

If someone puts a kprobe on some TLB flush thing and an MCE does memory failure handling, it would be polite to avoid crashing.  OTOH the x86 memory failure story is *so* bad that Iâm not sure how well we can ever really expect this to work.

I think we should aim to get the entry part correct, and if the meat of the function explodes, so be it.

> 
>> We'd maybe want automation to check
>> everything before it.  We still need to survive hitting a kprobe int3,
>> but that shouldn't have recursion issues.
> 
> Right, so I think avoiding the obvious recursion issues is a more
> tractable problem and yes some 'safe' spot annotation should be enough
> to get automation working for that -- mostly.