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From:   Andy Lutomirski <luto@kernel.org>
Date:   Wed, 26 Feb 2020 07:10:01 -0800
Message-ID: <CALCETrVXzwmwNOB6qUk7aM=LQRBySrMJPdRZ244T3y1bpRBzaQ@mail.gmail.com>
Subject: Re: [patch 02/10] x86/mce: Disable tracing and kprobes on do_machine_check()
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Brian Gerst <brgerst@gmail.com>,
        Juergen Gross <jgross@suse.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Arnd Bergmann <arnd@arndb.de>
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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.)