Assume the implementation of AES is vulnerable to cache-side channel attacks. Have any attacks against this XEX mode of AES been demonstrated?
Actually, XEX wouldn't appear to make a side channel attack that much more difficult.
AES uses the first subkey as an XOR to the plaintext; it uses the last subkey as a final XOR to generate the ciphertext. Hence, AES-XEX can be viewed as normal AES, except that the first and last subkeys are set to arbitrary values.
What this does mean is (for example) your side channel attack manages to recover the first subkey for AES-128, that does not immediately give you the entire key. However, you could still logically peel off the AES internal operations until you get to the second AddRoundKey and attack that (using the same side channel attack); getting that will get you (for AES-128) everything...