Subject: Re: [PATCH 00/18] prevent bounds-check bypass via speculative execution

From: Alan Cox <[email-masked]>
Date: Mon, 8 Jan 2018 11:43:42 +0000

On Mon, 8 Jan 2018 11:08:36 +0100
Peter Zijlstra <[email-masked]> wrote:

> On Fri, Jan 05, 2018 at 10:30:16PM -0800, Dan Williams wrote:
> > On Fri, Jan 5, 2018 at 6:22 PM, Eric W. Biederman <[email-masked]> wrote:  
> > > In at least one place (mpls) you are patching a fast path.  Compile out
> > > or don't load mpls by all means.  But it is not acceptable to change the
> > > fast path without even considering performance.  
> > 
> > Performance matters greatly, but I need help to identify a workload
> > that is representative for this fast path to see what, if any, impact
> > is incurred. Even better is a review that says "nope, 'index' is not
> > subject to arbitrary userspace control at this point, drop the patch."  
> 
> I think we're focussing a little too much on pure userspace. That is, we
> should be saying under the attackers control. Inbound network packets
> could equally be under the attackers control.

Inbound network packets don't come with a facility to read back and do
cache timimg. For the more general case, timing attacks on network
activity are not exactly new, and you have to mitigate them in user space
because most of them are about how many instructions you execute on each
path. The ancient classic being telling if a user exists by seeing if the
password was actually checked.

Alan