linux-hardening - Re: [PATCH v3] arm64: smp: smp_send_stop() and crash_smp_send

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Message-ID: <CAD=FV=W2NDBCwTbLtuknejMhHeqgGc86Hb4RRWEsQc9BBasETg@mail.gmail.com>
Date: Fri, 23 Aug 2024 08:29:38 -0700
From: Doug Anderson <dianders@...omium.org>
To: Will Deacon <will@...nel.org>
Cc: Catalin Marinas <catalin.marinas@....com>, Yu Zhao <yuzhao@...gle.com>, 
	Mark Rutland <mark.rutland@....com>, Misono Tomohiro <misono.tomohiro@...itsu.com>, 
	Marc Zyngier <maz@...nel.org>, Sumit Garg <sumit.garg@...aro.org>, Chen-Yu Tsai <wens@...e.org>, 
	Daniel Thompson <daniel.thompson@...aro.org>, Stephen Boyd <swboyd@...omium.org>, 
	Frederic Weisbecker <frederic@...nel.org>, "Guilherme G. Piccoli" <gpiccoli@...lia.com>, 
	James Morse <james.morse@....com>, Jonathan Cameron <Jonathan.Cameron@...wei.com>, 
	Kees Cook <kees@...nel.org>, Puranjay Mohan <puranjay@...nel.org>, Tony Luck <tony.luck@...el.com>, 
	linux-arm-kernel@...ts.infradead.org, linux-hardening@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] arm64: smp: smp_send_stop() and crash_smp_send_stop()
 should try non-NMI first

Hi Will,

On Fri, Aug 23, 2024 at 3:46 AM Will Deacon <will@...nel.org> wrote:
>
> Hi Doug,
>
> On Wed, Aug 21, 2024 at 02:53:57PM -0700, Douglas Anderson wrote:
> > When testing hard lockup handling on my sc7180-trogdor-lazor device
> > with pseudo-NMI enabled, with serial console enabled and with kgdb
> > disabled, I found that the stack crawls printed to the serial console
> > ended up as a jumbled mess. After rebooting, the pstore-based console
> > looked fine though. Also, enabling kgdb to trap the panic made the
> > console look fine and avoided the mess.
>
> Just a small nit:
>
> >       while (num_other_online_cpus() && timeout--)
> >               udelay(1);
> >
> > -     if (num_other_online_cpus())
> > +     /*
> > +      * If CPUs are still online, try an NMI. There's no excuse for this to
> > +      * be slow, so we only give them an extra 10 ms to respond.
> > +      */
> > +     if (num_other_online_cpus() && ipi_should_be_nmi(IPI_CPU_STOP_NMI)) {
>
> We probably want an smp_rmb() here...
>
> > +             cpumask_copy(&mask, cpu_online_mask);
> > +             cpumask_clear_cpu(smp_processor_id(), &mask);
> > +
> > +             pr_info("SMP: retry stop with NMI for CPUs %*pbl\n",
> > +                     cpumask_pr_args(&mask));
> > +
> > +             smp_cross_call(&mask, IPI_CPU_STOP_NMI);
> > +             timeout = USEC_PER_MSEC * 10;
> > +             while (num_other_online_cpus() && timeout--)
> > +                     udelay(1);
> > +     }
> > +
> > +     if (num_other_online_cpus()) {
>
>
> ... and again here, just to make sure that the re-read of cpu_online_mask
> is ordered after the read of __num_online_cpus in num_other_online_cpus().
>
> I can add those when applying.

Sounds like a plan to me. Thanks!

-Doug