lib/crypto: aescfb: Don't disable IRQs during AES block encryption
Subsystems (2)
Eric Biggers
maintainerPATCH 1/1 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 a22fd0e3c495 ("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.
--- lib/crypto/aescfb.c | 25 +++---------------------- 1 file changed, 3 insertions(+), 22 deletions(-)
lib/crypto/aescfb.c +3 -22
@@ -7,29 +7,10 @@#include <crypto/aes.h> #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 * * @key: The AES-CFB key schedule
@@ -43,11 +24,11 @@ void aescfb_encrypt(const struct aes_enckey *key, u8 *dst, const u8 *src, {u8 ks[AES_BLOCK_SIZE ];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; dst +=AES_BLOCK_SIZE ; src +=AES_BLOCK_SIZE ;
@@ -70,20 +51,20 @@ EXPORT_SYMBOL(aescfb_encrypt);void aescfb_decrypt (const struct aes_enckey *key,u8 *dst,const u8 *src,int len,const u8 iv[AES_BLOCK_SIZE ]) {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 )/* * Generate the keystream for the next block before * 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 )); dst +=AES_BLOCK_SIZE ; src +=AES_BLOCK_SIZE ;
base-commit: d2a68aba8505ce88b39c34ecb3b707c776af79d4
Signature · 1 line
2.53.0
Quoted 20 lines · Eric Biggers
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
a22fd0e3c495("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.
Quoted 5 lines
Signed-off-by Eric Biggers <[email protected]>--- lib/crypto/aescfb.c | 25 +++---------------------- 1 file changed, 3 insertions(+), 22 deletions(-)
Quoted 20 lines · Eric Biggers
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
a22fd0e3c495("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.
Quoted 4 lines
Signed-off-by Eric Biggers <[email protected]>--- lib/crypto/aescfb.c | 25 +++---------------------- 1 file changed, 3 insertions(+), 22 deletions(-)
Applied to https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/linux.git/log/?h=libcrypto-next
- Eric