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

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Date:   Thu, 1 Feb 2018 09:40:52 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     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,
        "Wieczorkiewicz, Pawel" <wipawel@...zon.de>
Subject: Re: [tip:x86/pti] x86/speculation: Use Indirect Branch Prediction
 Barrier in context switch

On Thu, Feb 01, 2018 at 08:31:53AM +0000, David Woodhouse wrote:
> 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...

Fair point.  From what I can gather, user space ASLR seems to be the
only layer of protection before a POC would be feasible.  So, unless I'm
mistaken, which is very possible, it seems to be a question of how much
we trust ASLR.

-- 
Josh