On 01/25/2018 12:20 AM, David Woodhouse wrote: > On Wed, 2018-01-24 at 16:36 -0800, Tim Chen wrote: >> It is possible that the last uesr mm that we recorded for a cpu was >> released, and a new mm with identical address was allocated when we >> check it again. We could skip IBPB flush here for the process with >> the new mm. >> >> It is a difficult to exploit case as we have to exit() a process on a >> cpu, free the mm, and fork() the victim to use the mm pointer on that >> cpu. The exploiter needs the old mm to get recycled to the >> newly forked process and no other processes run on the target cpu. > > That's what it takes to have the victim process leak information into > the cache. In order to *harvest* that information, the attacker must > then get run on the same CPU again? And since her first process had to > exits, as described above, she needs a new process for that? > > I confess, with all the other wildly theoretical loopholes that exist, > I wasn't losing much sleep over this one. > >> Nevertheless, the patch below is one way to close this hole by >> adding a ref count to prevent the last user mm from being released. >> It does add ref counting overhead, and extra memory cost of keeping an mm >> (though not the VMAs and most of page tables) around longer than we will >> otherwise need to. Any better solutions are welcomed. > > This has no upper bound on the amount of time the user mm gets held, > right? If a given CPU remains idle for ever (and what happens if it's > taken offline?) we'll never do that mmdrop() ? >
The downside with this approach is we do hold on to the mm longer than we needs to. Yes, the offline path does need to be fixed up. Tim
