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() ?
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