Messages in this thread |  | | Date | Wed, 26 Feb 2020 17:08:18 +0100 | From | Peter Zijlstra <> | Subject | Re: [patch 02/10] x86/mce: Disable tracing and kprobes on do_machine_check() |
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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.
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.
> 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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