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From:   Peter Newman <peternewman@google.com>
Date:   Fri, 16 Dec 2022 11:26:44 +0100
Message-ID: <CALPaoCgt+epHFO=O8B0LWfps0NjJ1RvwmfGey22XyamkUbm0hg@mail.gmail.com>
Subject: Re: [PATCH v5 1/1] x86/resctrl: Fix task CLOSID/RMID update race
To:     Reinette Chatre <reinette.chatre@intel.com>
Cc:     fenghua.yu@intel.com, bp@alien8.de, derkling@google.com,
        eranian@google.com, hpa@zytor.com, james.morse@arm.com,
        jannh@google.com, kpsingh@google.com, linux-kernel@vger.kernel.org,
        mingo@redhat.com, tglx@linutronix.de, x86@kernel.org
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Hi Reinette,

On Fri, Dec 16, 2022 at 12:52 AM Reinette Chatre
<reinette.chatre@intel.com> wrote:
>
> For a fix a Fixes: tag is expected. It looks like the following
> may be relevant:
> Fixes: ae28d1aae48a ("x86/resctrl: Use an IPI instead of task_work_add() to update PQR_ASSOC MSR")
> Fixes: 0efc89be9471 ("x86/intel_rdt: Update task closid immediately on CPU in rmdir and unmount")

Thanks for preparing these lines. I'll include them.

>
> > Signed-off-by: Peter Newman <peternewman@google.com>
>
> Also, please do let the stable team know about this via:
> Cc: stable@vger.kernel.org

I wasn't sure if this fix met the criteria for backporting to stable,
because I found it by code inspection, so it doesn't meet the "bothers
people" criterion.

However I can make a case that it's exploitable:

"In a memory bandwidth-metered compute host, malicious jobs could
exploit this race to remain in a previous CLOSID or RMID in order to
dodge a class-of-service downgrade imposed by an admin or steal
bandwidth."


>
> Thank you very much for sticking with this and always paying attention
> to the details along the way.
>
> Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>

Thank you, Reinette! This has been a learning experience for me.

-Peter