On Thu 2017-11-23 09:23:02, Ard Biesheuvel wrote: > On 23 November 2017 at 09:07, Pavel Machek <pa...@ucw.cz> wrote: > > Hi! > > > >> > 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) > >> > > > > Well, KASLR is about keeping bits of kernel virtual address secret > > from userland. Leaking them through cache sidechannel means KASLR is > > defeated. > > > > Yes, that is what you claim. But you are not explaining how any of the > bits that we do want to keep secret can be discovered by making > inferences from which lines in a primed cache were evicted during a > syscall. > > The cache index maps to low order bits. You can use this, e.g., to > attack table based AES, because there is only ~4 KB worth of tables, > and you are interested in finding out which exact entries of the table > were read by the process under attack. > > You are saying the same approach will help you discover 30 high order > bits of a virtual kernel address, by observing the cache evictions in > a physically indexed physically tagged cache. How?
I assumed high bits are hashed into cache index. I might have been wrong. Anyway, page tables are about same size as AES tables. So...: http://cve.circl.lu/cve/CVE-2017-5927 Pavel -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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