In aes-neonbs, instead of going through the crypto API for the parts
that the bit-sliced AES code doesn't handle, namely AES-CBC encryption
and single-block AES, just call the ARM scalar AES cipher directly.
This basically goes back to the original approach that was used before
commit b56f5cbc7e ("crypto: arm/aes-neonbs - resolve fallback cipher
at runtime"). Calling the ARM scalar AES cipher directly is faster,
simpler, and avoids any chance of bugs specific to the use of fallback
ciphers such as module loading deadlocks which have happened twice. The
deadlocks turned out to be fixable in other ways, but there's no need to
rely on anything so fragile in the first place.
The rationale for the above-mentioned commit was to allow people to
choose to use a time-invariant AES implementation for the fallback
cipher. There are a couple problems with that rationale, though:
- In practice the ARM scalar AES cipher (aes-arm) was used anyway, since
it has a higher priority than aes-fixed-time. Users *could* go out of
their way to disable or blacklist aes-arm, or to lower its priority
using NETLINK_CRYPTO, but very few users customize the crypto API to
this extent. Systems with the ARMv8 Crypto Extensions used aes-ce,
but the bit-sliced algorithms are irrelevant on such systems anyway.
- Since commit 913a3aa07d ("crypto: arm/aes - add some hardening
against cache-timing attacks"), the ARM scalar AES cipher is partially
hardened against cache-timing attacks. It actually works like
aes-fixed-time, in that it disables interrupts and prefetches its
lookup table. It does use a larger table than aes-fixed-time, but
even so, it is not clear that aes-fixed-time is meaningfully more
time-invariant than aes-arm. And of course, the real solution for
time-invariant AES is to use a CPU that supports AES instructions.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
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