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lib/crypto: aescfb: Don't disable IRQs during AES block encryption 

aes_encrypt() now uses AES instructions when available instead of always
using table-based code.  AES instructions are constant-time and don't
benefit from disabling IRQs as a constant-time hardening measure.

In fact, on two architectures (arm and riscv) disabling IRQs is
counterproductive because it prevents the AES instructions from being
used.  (See the may_use_simd() implementation on those architectures.)

Therefore, let's remove the IRQ disabling/enabling and leave the choice
of constant-time hardening measures to the AES library code.

Note that currently the arm table-based AES code (which runs on arm
kernels that don't have ARMv8 CE) disables IRQs, while the generic
table-based AES code does not.  So this does technically regress in
constant-time hardening when that generic code is used.  But as
discussed in commit a22fd0e3c4 ("lib/crypto: aes: Introduce improved
AES library") I think just leaving IRQs enabled is the right choice.
Disabling them is slow and can cause problems, and AES instructions
(which modern CPUs have) solve the problem in a much better way anyway.

Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20260331024414.51545-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
master
Eric Biggers 2026-03-30 19:44:14 -07:00
parent d2a68aba85
commit 1aa82df3eb
1 changed files with 3 additions and 22 deletions
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25
lib/crypto/aescfb.c
View File

@ -9,25 +9,6 @@
#include <crypto/algapi.h>
#include <linux/export.h>
#include <linux/module.h>
#include <asm/irqflags.h>
static void aescfb_encrypt_block(const struct aes_enckey *key, void *dst,
const void *src)
{
unsigned long flags;
/*
* In AES-CFB, the AES encryption operates on known 'plaintext' (the IV
* and ciphertext), making it susceptible to timing attacks on the
* encryption key. The AES library already mitigates this risk to some
* extent by pulling the entire S-box into the caches before doing any
* substitutions, but this strategy is more effective when running with
* interrupts disabled.
*/
local_irq_save(flags);
aes_encrypt(key, dst, src);
local_irq_restore(flags);
}
/**
* aescfb_encrypt - Perform AES-CFB encryption on a block of data
@ -45,7 +26,7 @@ void aescfb_encrypt(const struct aes_enckey *key, u8 *dst, const u8 *src,
const u8 *v = iv;
while (len > 0) {
aescfb_encrypt_block(key, ks, v);
aes_encrypt(key, ks, v);
crypto_xor_cpy(dst, src, ks, min(len, AES_BLOCK_SIZE));
v = dst;
@ -72,7 +53,7 @@ void aescfb_decrypt(const struct aes_enckey *key, u8 *dst, const u8 *src,
{
u8 ks[2][AES_BLOCK_SIZE];
aescfb_encrypt_block(key, ks[0], iv);
aes_encrypt(key, ks[0], iv);
for (int i = 0; len > 0; i ^= 1) {
if (len > AES_BLOCK_SIZE)
@ -81,7 +62,7 @@ void aescfb_decrypt(const struct aes_enckey *key, u8 *dst, const u8 *src,
* performing the XOR, as that may update in place and
* overwrite the ciphertext.
*/
aescfb_encrypt_block(key, ks[!i], src);
aes_encrypt(key, ks[!i], src);
crypto_xor_cpy(dst, src, ks[i], min(len, AES_BLOCK_SIZE));