commit b41c1d8d07 upstream.
Make fscrypt no longer use Crypto API drivers for non-inline crypto
engines, even when the Crypto API prioritizes them over CPU-based code
(which unfortunately it often does). These drivers tend to be really
problematic, especially for fscrypt's workload. This commit has no
effect on inline crypto engines, which are different and do work well.
Specifically, exclude drivers that have CRYPTO_ALG_KERN_DRIVER_ONLY or
CRYPTO_ALG_ALLOCATES_MEMORY set. (Later, CRYPTO_ALG_ASYNC should be
excluded too. That's omitted for now to keep this commit backportable,
since until recently some CPU-based code had CRYPTO_ALG_ASYNC set.)
There are two major issues with these drivers: bugs and performance.
First, these drivers tend to be buggy. They're fundamentally much more
error-prone and harder to test than the CPU-based code. They often
don't get tested before kernel releases, and even if they do, the crypto
self-tests don't properly test these drivers. Released drivers have
en/decrypted or hashed data incorrectly. These bugs cause issues for
fscrypt users who often didn't even want to use these drivers, e.g.:
- https://github.com/google/fscryptctl/issues/32
- https://github.com/google/fscryptctl/issues/9
- https://lore.kernel.org/r/PH0PR02MB731916ECDB6C613665863B6CFFAA2@PH0PR02MB7319.namprd02.prod.outlook.com
These drivers have also similarly caused issues for dm-crypt users,
including data corruption and deadlocks. Since Linux v5.10, dm-crypt
has disabled most of them by excluding CRYPTO_ALG_ALLOCATES_MEMORY.
Second, these drivers tend to be *much* slower than the CPU-based code.
This may seem counterintuitive, but benchmarks clearly show it. There's
a *lot* of overhead associated with going to a hardware driver, off the
CPU, and back again. To prove this, I gathered as many systems with
this type of crypto engine as I could, and I measured synchronous
encryption of 4096-byte messages (which matches fscrypt's workload):
Intel Emerald Rapids server:
AES-256-XTS:
xts-aes-vaes-avx512 16171 MB/s [CPU-based, Vector AES]
qat_aes_xts 289 MB/s [Offload, Intel QuickAssist]
Qualcomm SM8650 HDK:
AES-256-XTS:
xts-aes-ce 4301 MB/s [CPU-based, ARMv8 Crypto Extensions]
xts-aes-qce 73 MB/s [Offload, Qualcomm Crypto Engine]
i.MX 8M Nano LPDDR4 EVK:
AES-256-XTS:
xts-aes-ce 647 MB/s [CPU-based, ARMv8 Crypto Extensions]
xts(ecb-aes-caam) 20 MB/s [Offload, CAAM]
AES-128-CBC-ESSIV:
essiv(cbc-aes-caam,sha256-lib) 23 MB/s [Offload, CAAM]
STM32MP157F-DK2:
AES-256-XTS:
xts-aes-neonbs 13.2 MB/s [CPU-based, ARM NEON]
xts(stm32-ecb-aes) 3.1 MB/s [Offload, STM32 crypto engine]
AES-128-CBC-ESSIV:
essiv(cbc-aes-neonbs,sha256-lib)
14.7 MB/s [CPU-based, ARM NEON]
essiv(stm32-cbc-aes,sha256-lib)
3.2 MB/s [Offload, STM32 crypto engine]
Adiantum:
adiantum(xchacha12-arm,aes-arm,nhpoly1305-neon)
52.8 MB/s [CPU-based, ARM scalar + NEON]
So, there was no case in which the crypto engine was even *close* to
being faster. On the first three, which have AES instructions in the
CPU, the CPU was 30 to 55 times faster (!). Even on STM32MP157F-DK2
which has a Cortex-A7 CPU that doesn't have AES instructions, AES was
over 4 times faster on the CPU. And Adiantum encryption, which is what
actually should be used on CPUs like that, was over 17 times faster.
Other justifications that have been given for these non-inline crypto
engines (almost always coming from the hardware vendors, not actual
users) don't seem very plausible either:
- The crypto engine throughput could be improved by processing
multiple requests concurrently. Currently irrelevant to fscrypt,
since it doesn't do that. This would also be complex, and unhelpful
in many cases. 2 of the 4 engines I tested even had only one queue.
- Some of the engines, e.g. STM32, support hardware keys. Also
currently irrelevant to fscrypt, since it doesn't support these.
Interestingly, the STM32 driver itself doesn't support this either.
- Free up CPU for other tasks and/or reduce energy usage. Not very
plausible considering the "short" message length, driver overhead,
and scheduling overhead. There's just very little time for the CPU
to do something else like run another task or enter low-power state,
before the message finishes and it's time to process the next one.
- Some of these engines resist power analysis and electromagnetic
attacks, while the CPU-based crypto generally does not. In theory,
this sounds great. In practice, if this benefit requires the use of
an off-CPU offload that massively regresses performance and has a
low-quality, buggy driver, the price for this hardening (which is
not relevant to most fscrypt users, and tends to be incomplete) is
just too high. Inline crypto engines are much more promising here,
as are on-CPU solutions like RISC-V High Assurance Cryptography.
Fixes: b30ab0e034 ("ext4 crypto: add ext4 encryption facilities")
Cc: stable@vger.kernel.org
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20250704070322.20692-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
cfe27f8aff