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

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Date:   Tue, 27 Nov 2018 23:07:15 +0100 (CET)
From:   Jiri Kosina <jikos@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
cc:     Tim Chen <tim.c.chen@...ux.intel.com>,
        Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        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 Tue, 27 Nov 2018, Thomas Gleixner wrote:

> >  static int ssb_prctl_set(struct task_struct *task, unsigned long ctrl)
> > diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> > index 3f5e351bdd37..6c4fcef52b19 100644
> > --- a/arch/x86/kernel/process.c
> > +++ b/arch/x86/kernel/process.c
> > @@ -474,6 +474,21 @@ void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p)
> >  
> >  	tifn = READ_ONCE(task_thread_info(next_p)->flags);
> >  	tifp = READ_ONCE(task_thread_info(prev_p)->flags);
> > +
> > +	/*
> > +	 * SECCOMP tasks might have had their spec_ctrl flags updated during
> > +	 * runtime from a different CPU.
> > +	 *
> > +	 * When switching to such a task, populate thread flags with the ones
> > +	 * that have been temporarily saved in spec_flags by task_update_spec_tif()
> > +	 * in order to make sure MSR value is always kept up to date.
> > +	 *
> > +	 * SECCOMP tasks never disable the mitigation for other threads, only enable.
> > +	 */
> > +	if (IS_ENABLED(CONFIG_SECCOMP) &&
> > +			test_and_clear_tsk_thread_flag(next_p, TIF_SPEC_UPDATE))
> > +		tifp |= READ_ONCE(task_thread_info(next_p)->spec_flags);
> 
> And how does that get folded into task_thread_info(next_p)->flags for the
> next context switch? 

Does it really have to? 

We need this special handling only if the next task has TIF_SPEC_UPDATE 
set, which is one-off event globally (when seccomp marks all its threads 
so due to seccomp filter change), and once all the TIF_SPEC_UPDATE tasks 
schedule at least once, we're in a consistent state again and don't need 
this, as every running task will then have its TIF consistent with MSR 
value.

Thanks,

-- 
Jiri Kosina
SUSE Labs