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Date:   Wed, 24 Jan 2018 07:29:53 +0100
From:   Martin Schwidefsky <schwidefsky@de.ibm.com>
To:     Dominik Brodowski <linux@dominikbrodowski.net>
Cc:     linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org,
        kvm@vger.kernel.org, Heiko Carstens <heiko.carstens@de.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Cornelia Huck <cohuck@redhat.com>,
        David Hildenbrand <david@redhat.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jon Masters <jcm@redhat.com>,
        Marcus Meissner <meissner@suse.de>,
        Jiri Kosina <jkosina@suse.cz>, w@1wt.eu, keescook@chromium.org
Subject: Re: [PATCH 1/5] prctl: add PR_ISOLATE_BP process control
In-Reply-To: <20180123170719.GA4154@isilmar-4.linta.de>
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        <1516712825-2917-2-git-send-email-schwidefsky@de.ibm.com>
        <20180123170719.GA4154@isilmar-4.linta.de>
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On Tue, 23 Jan 2018 18:07:19 +0100
Dominik Brodowski <linux@dominikbrodowski.net> wrote:

> On Tue, Jan 23, 2018 at 02:07:01PM +0100, Martin Schwidefsky wrote:
> > Add the PR_ISOLATE_BP operation to prctl. The effect of the process
> > control is to make all branch prediction entries created by the execution
> > of the user space code of this task not applicable to kernel code or the
> > code of any other task.  
> 
> What is the rationale for requiring a per-process *opt-in* for this added
> protection?
> 
> For KPTI on x86, the exact opposite approach is being discussed (see, e.g.
> http://lkml.kernel.org/r/1515612500-14505-1-git-send-email-w@1wt.eu ): By
> default, play it safe, with KPTI enabled. But for "trusted" processes, one
> may opt out using prctrl.

The rationale is that there are cases where you got code from *somewhere*
and want to run it in an isolated context. Think: a docker container that
runs under KVM. But with spectre this is still not really safe. So you
include a wrapper program in the docker container to use the trap door
prctl to start the potential malicious program. Now you should be good, no?

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.