linux-kernel - Re: [PATCH 30/35] x86/speculation: Use Indirect Branch Prediction Barrier in context switch

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Message-ID: <510876ab-97cd-9ec9-cc5e-0a2db038704b@linux.intel.com>
Date:   Fri, 19 Jan 2018 12:26:46 -0800
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Kevin Easton <kevin@...rana.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        linux-kernel@...r.kernel.org, Dave Hansen <dave.hansen@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Jason Baron <jbaron@...mai.com>
Subject: Re: [PATCH 30/35] x86/speculation: Use Indirect Branch Prediction
 Barrier in context switch

On 01/18/2018 08:03 PM, Kevin Easton wrote:
> On Thu, Jan 18, 2018 at 04:38:32PM -0800, Tim Chen wrote:
>> On 01/18/2018 05:48 AM, Peter Zijlstra wrote:
>>>
>>> +		/*
>>> +		 * Avoid user/user BTB poisoning by flushing the branch predictor
>>> +		 * when switching between processes. This stops one process from
>>> +		 * doing spectre-v2 attacks on another process's data.
>>> +		 */
>>> +		indirect_branch_prediction_barrier();
>>> +
>>
>> Some optimizations can be done here to avoid overhead in barrier call.
>>
>> For example, don't do the barrier if prev and next mm are
>> the same.  If the two process trust each other, or the new process
>> already have rights to look into the previous process,
>> the barrier could be skipped.
> 
> Isn't it the other way around with the BTB poisoning? previous is
> potentially attacking next, so the barrier can be avoided only if previous
> is allowed to ptrace next?
> 

Yes, if the next process don't trust the previous process, then
doing a prediction barrier before the context switch makes sense.

Tim