On Mon, 8 Jan 2018 11:08:36 +0100 Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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 <ebiederm@xxxxxxxxxxxx> 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
