linux-kernel - Re: [patch 02/10] x86/mce: Disable tracing and kprobes on do_machine

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Date:   Wed, 26 Feb 2020 19:42:37 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Brian Gerst <brgerst@...il.com>,
        Juergen Gross <JGross@...e.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [patch 02/10] x86/mce: Disable tracing and kprobes on
 do_machine_check()

On Wed, Feb 26, 2020 at 09:28:51AM -0800, Andy Lutomirski wrote:
> > 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.

FWIW, I had this at the beginning of the #MC handler in a feeble attempt
to poke at this:

+       hw_breakpoint_disable();
+       static_key_disable(&__tracepoint_read_msr.key);
+       tracing_off();

But then Tony noted that some recoverable errors do get reported with an
#MC exception so we would have to look at the error severity and then
decide whether to allow tracing or not.

But the error severity happens all the way down in __mc_scan_banks() -
i.e., we've executed the half handler already.

So, frankly, I wanna say, f*ck tracing etc - there are certain handlers
which simply don't allow it. And we'll only consider changing that when
a really good reason for it appears...

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette