linux-kernel - Re: [patch 20/24] x86/speculation: Split out TIF update

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Message-ID: <f976eb35-5dcd-387e-eb29-f7c6636fd17b@linux.intel.com>
Date:   Wed, 21 Nov 2018 18:13:56 -0800
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     x86@...nel.org, Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jiri Kosina <jkosina@...e.cz>,
        Tom Lendacky <thomas.lendacky@....com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Casey Schaufler <casey.schaufler@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Jon Masters <jcm@...hat.com>,
        Waiman Long <longman9394@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Dave Stewart <david.c.stewart@...el.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [patch 20/24] x86/speculation: Split out TIF update

On Wed, Nov 21, 2018 at 09:14:50PM +0100, Thomas Gleixner wrote:
> +static void task_update_spec_tif(struct task_struct *tsk, int tifbit, bool on)
>  {
>       bool update;
>
> +     if (on)
> +             update = !test_and_set_tsk_thread_flag(tsk, tifbit);
> +     else
> +             update = test_and_clear_tsk_thread_flag(tsk, tifbit);
> +
> +     /*
> +      * If being set on non-current task, delay setting the CPU
> +	 * mitigation until it is scheduled next.
> +	 */
> +     if (tsk == current && update)
> +             speculation_ctrl_update_current();

I think all the call paths from prctl and seccomp coming here
has tsk == current.

But if task_update_spec_tif gets used in the future
where tsk is running on a remote CPU, this could lead to the MSR
getting out of sync with the running task's TIF flag. This will break
either performance or security.

Should we add a
        WARN_ON(smp_processor_id() != task_cpu(tsk));

in case the assumption breaks that task is
on local CPU, or document this assumption?

Thanks.

Tim