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

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Date:   Thu, 01 Feb 2018 08:31:53 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Dominik Brodowski <linux@...inikbrodowski.net>, mingo@...nel.org,
        hpa@...or.com, tim.c.chen@...ux.intel.com,
        linux-kernel@...r.kernel.org, tglx@...utronix.de
Cc:     jpoimboe@...hat.com, "Wieczorkiewicz, Pawel" <wipawel@...zon.de>
Subject: Re: [tip:x86/pti] x86/speculation: Use Indirect Branch Prediction
 Barrier in context switch

On Wed, 2018-01-31 at 08:03 +0100, Dominik Brodowski wrote:
> Whether a process needs protection by IBPB on context switches is a
> different question to whether a process should be allowed to be dumped,
> though the former may be a superset of the latter. Enable IBPB on all
> context switches to a different userspace process, until we have a clear
> mitigation strategy for userspace against Spectre-v2 designed and
> implemented.
>
> ...
>                 if (tsk && tsk->mm &&
> -                   tsk->mm->context.ctx_id != last_ctx_id &&
> -                   get_dumpable(tsk->mm) != SUID_DUMP_USER)
> +                   tsk->mm->context.ctx_id != last_ctx_id)
>                         indirect_branch_prediction_barrier();


I understand your argument and I sympathise.

But that's going to hurt a *lot*, and we don't even have a viable
proof-of-concept for a user←→user Spectre v2 attack, do we? It's only
theoretical?

Show a working PoC and it makes the argument somewhat more
convincing...
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