linux-kernel - Re: [PATCH RFC] Add support for core-wide protection of IRQ and softirq

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Date:   Mon, 11 May 2020 10:54:54 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Vineeth Pillai <vpillai@...italocean.com>,
        Allison Randal <allison@...utok.net>,
        Armijn Hemel <armijn@...ldur.nl>,
        Ben Segall <bsegall@...gle.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Mel Gorman <mgorman@...e.de>, Muchun Song <smuchun@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH RFC] Add support for core-wide protection of IRQ and softirq

On Mon, May 11, 2020 at 9:49 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Sun, May 10, 2020 at 07:46:52PM -0400, Joel Fernandes (Google) wrote:
> > With current core scheduling patchset, non-threaded IRQ and softirq
> > victims can leak data from its hyperthread to a sibling hyperthread
> > running an attacker.
> >
> > For MDS, it is possible for the IRQ and softirq handlers to leak data to
> > either host or guest attackers. For L1TF, it is possible to leak to
> > guest attackers. There is no possible mitigation involving flushing of
> > buffers to avoid this since the execution of attacker and victims happen
> > concurrently on 2 or more HTs.
> >
> > The solution in this patch is to monitor the outer-most core-wide
> > irq_enter() and irq_exit() executed by any sibling. In between these
> > two, we mark the core to be in a special core-wide IRQ state.
>
> Another possible option is force_irqthreads :-) That would cure it
> nicely.

Yes true, it was definitely my "plan B" at one point if this patch
showed any regression. Lastly, people not doing force_irqthreads would
still leave a hole open and it'd be nice to solve it by "default" than
depending on user/sysadmin configuration (same argument against
interrupt affinities, it is another knob for the sysadmin/designer to
configure correctly, Another argument being not all interrupts can be
threaded / affinitized).

Thanks in advance for reviewing the patch,

 - Joel