linux-kernel - Re: [PATCH] x86/speculation: Use Indirect Branch Prediction Barrier in context switch

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Date:   Tue, 30 Jan 2018 16:25:26 -0800
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     David Woodhouse <dwmw@...zon.co.uk>, arjan@...ux.intel.com,
        tglx@...utronix.de, karahmed@...zon.de, x86@...nel.org,
        linux-kernel@...r.kernel.org, peterz@...radead.org,
        pbonzini@...hat.com, ak@...ux.intel.com,
        torvalds@...ux-foundation.org, gregkh@...ux-foundation.org,
        mingo@...nel.org, luto@...nel.org, linux@...inikbrodowski.net
Subject: Re: [PATCH] x86/speculation: Use Indirect Branch Prediction Barrier
 in context switch

On 01/30/2018 02:43 PM, Borislav Petkov wrote:
> On Tue, Jan 30, 2018 at 02:26:53PM -0800, Tim Chen wrote:
>> If the process has multiple threads running on different cpus,
> 
> I'm talking about issuing the barrier in set_dumpable(). What threads on
> multiple CPUs?
> 

As dumpable is a property in mm->flags, it affects all threads running on other cpus sharing
the same mm.  If you issue IBPB only on the cpu that perform the set_dumpable(),
the theoretical hole you are trying to close still exist on threads running on
other cpu.

        time ----->
(cpu A)                set_dumpable victim (thread1), issue IBPB 
(cpu B) attacker -> victim (thread2), missed IBPB                 -> attacker -> victim (IBPB issued)                      


That said, I think the risk is minuscule and is not worth the cost to set IBPB on the other cpus.

Tim