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

Andy Lutomirski
2020-02-26T15:10:18Z

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? 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.

(Yes, that function doesn't exist in current kernels. And we'd need
to make sure that BPF respects it.)