Messages in this thread |  | | From | Andy Lutomirski <> | Subject | Re: [patch 02/10] x86/mce: Disable tracing and kprobes on do_machine_check() | Date | Wed, 26 Feb 2020 09:28:51 -0800 |
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> On Feb 26, 2020, at 8:08 AM, Peter Zijlstra <peterz@infradead.org> wrote: > > On Wed, Feb 26, 2020 at 07:10:01AM -0800, Andy Lutomirski wrote: >>> On Wed, Feb 26, 2020 at 5:28 AM Peter Zijlstra <peterz@infradead.org> 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 I’m 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.
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