Remove aes-fixed-time, i.e. CONFIG_CRYPTO_AES_TI. This was a wrapper
around the 256-byte-table-based AES implementation in lib/crypto/aes.c,
with extra code to enable and disable IRQs for constant-time hardening.
While nice in theory, in practice this had the following issues:
- For bulk en/decryption it was 2-4 times slower than aes-generic. This
resulted in aes-generic still being needed, creating fragmentation.
- Having both aes-generic and aes-fixed-time punted an AES
implementation decision to distros and users who are generally
unprepared to handle it. In practice, whether aes-fixed-time gets
used tends to be incidental and not match an explicit distro or user
intent. (While aes-fixed-time has a higher priority than aes-generic,
whether it actually gets enabled, loaded, and used depends on the
kconfig and whether a modprobe of "aes" happens to be done. It also
has a lower priority than aes-arm and aes-arm64.)
- My changes to the generic AES code (in other commits) significantly
close the gap with aes-fixed-time anyway. The table size is reduced
from 8192 bytes to 1024 bytes, and prefetching is added.
- While AES code *should* be constant-time, the real solutions for that
are AES instructions (which most CPUs have now) or bit-slicing. arm
and arm64 already have bit-sliced AES code for many modes; generic
bit-sliced code could be written but would be very slow for single
blocks. Overall, I suggest that trying to write constant-time
table-based AES code is a bit futile anyway, and in the rare cases
where a proper AES implementation is still unavailable it's reasonable
to compromise with an implementation that simply prefetches the table.
Thus, this commit removes aes-fixed-time and CONFIG_CRYPTO_AES_TI. The
replacement is just the existing CONFIG_CRYPTO_AES, which for now maps
to the existing aes-generic code, but I'll soon be changing to use the
improved AES library code instead.
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20260112192035.10427-9-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
641e70563a