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

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Date:   Thu, 18 Jan 2018 23:03:29 -0500
From:   Kevin Easton <kevin@...rana.org>
To:     Tim Chen <tim.c.chen@...ux.intel.com>
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 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?

    - Kevin