Unpatchable vulnerability in Apple chip leaks secret encryption keys
Prefetchers are crazy.
Prefetchers usually look at addresses of accessed data (ignoring values of accessed data) and try to guess future addresses that might be useful. The [Data Memory-dependent Prefetcher in M chips] is different in this sense as in addition to addresses it also uses the data values in order to make predictions (predict addresses to go to and prefetch). In particular, if a data value “looks like” a pointer, it will be treated as an “address” (where in fact it’s actually not!) and the data from this “address” will be brought to the cache. The arrival of this address into the cache is visible, leaking over cache side channels. Our attack exploits this fact. We cannot leak encryption keys directly, but what we can do is manipulate intermediate data inside the encryption algorithm to look like a pointer via a chosen input attack. The DMP then sees that the data value “looks like” an address, and brings the data from this “address” into the cache, which leaks the “address.” We don’t care about the data value being prefetched, but the fact that the intermediate data looked like an address is visible via a cache channel and is sufficient to reveal the secret key over time.
(via Mike)(tags: via:mike prefetchers dmp apple encryption side-channel-attacks cache)