> On 22 Nov 2017, at 23:37, Pavel Machek <pa...@ucw.cz> wrote: > > Hi! > >>>>> If I'm willing to do timing attacks to defeat KASLR... what prevents >>>>> me from using CPU caches to do that? >>>>> >>>> >>>> Because it is impossible to get a cache hit on an access to an >>>> unmapped address? >>> >>> Um, no, I don't need to be able to directly access kernel addresses. I >>> just put some data in _same place in cache where kernel data would >>> go_, then do syscall and look if my data are still cached. Caches >>> don't have infinite associativity. >>> >> >> Ah ok. Interesting. >> >> But how does that leak address bits that are covered by the tag? > > Same as leaking any other address bits? Caches are "virtually > indexed",
Not on arm64, although I don’t see how that is relevant if you are trying to defeat kaslr. > and tag does not come into play... > Well, I must be missing something then, because I don’t see how knowledge about which userland address shares a cache way with a kernel address can leak anything beyond the bits that make up the index (i.e., which cache way is being shared) > Maybe this explains it? > No not really. It explains how cache timing can be used as a side channel, not how it defeats kaslr. Thanks, Ard.
