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authorEric Biggers <ebiggers@google.com>2018-10-17 21:37:58 -0700
committerHerbert Xu <herbert@gondor.apana.org.au>2018-11-09 17:36:48 +0800
commit0a6a40c2a8c184a2fb467efacfb1cd338d719e0b (patch)
tree83c30f4e2775779925bd03c9d3f5f5f4f06887ae /crypto
parent9f4debe38415583086ce814798eeb864aeb39551 (diff)
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crypto: aes_ti - disable interrupts while accessing S-box
In the "aes-fixed-time" AES implementation, disable interrupts while accessing the S-box, in order to make cache-timing attacks more difficult. Previously it was possible for the CPU to be interrupted while the S-box was loaded into L1 cache, potentially evicting the cachelines and causing later table lookups to be time-variant. In tests I did on x86 and ARM, this doesn't affect performance significantly. Responsiveness is potentially a concern, but interrupts are only disabled for a single AES block. Note that even after this change, the implementation still isn't necessarily guaranteed to be constant-time; see https://cr.yp.to/antiforgery/cachetiming-20050414.pdf for a discussion of the many difficulties involved in writing truly constant-time AES software. But it's valuable to make such attacks more difficult. Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> Signed-off-by: Eric Biggers <ebiggers@google.com> Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Diffstat (limited to 'crypto')
-rw-r--r--crypto/Kconfig3
-rw-r--r--crypto/aes_ti.c18
2 files changed, 20 insertions, 1 deletions
diff --git a/crypto/Kconfig b/crypto/Kconfig
index f7a235db56aa..752005201013 100644
--- a/crypto/Kconfig
+++ b/crypto/Kconfig
@@ -1006,7 +1006,8 @@ config CRYPTO_AES_TI
8 for decryption), this implementation only uses just two S-boxes of
256 bytes each, and attempts to eliminate data dependent latencies by
prefetching the entire table into the cache at the start of each
- block.
+ block. Interrupts are also disabled to avoid races where cachelines
+ are evicted when the CPU is interrupted to do something else.
config CRYPTO_AES_586
tristate "AES cipher algorithms (i586)"
diff --git a/crypto/aes_ti.c b/crypto/aes_ti.c
index 03023b2290e8..1ff9785b30f5 100644
--- a/crypto/aes_ti.c
+++ b/crypto/aes_ti.c
@@ -269,6 +269,7 @@ static void aesti_encrypt(struct crypto_tfm *tfm, u8 *out, const u8 *in)
const u32 *rkp = ctx->key_enc + 4;
int rounds = 6 + ctx->key_length / 4;
u32 st0[4], st1[4];
+ unsigned long flags;
int round;
st0[0] = ctx->key_enc[0] ^ get_unaligned_le32(in);
@@ -276,6 +277,12 @@ static void aesti_encrypt(struct crypto_tfm *tfm, u8 *out, const u8 *in)
st0[2] = ctx->key_enc[2] ^ get_unaligned_le32(in + 8);
st0[3] = ctx->key_enc[3] ^ get_unaligned_le32(in + 12);
+ /*
+ * Temporarily disable interrupts to avoid races where cachelines are
+ * evicted when the CPU is interrupted to do something else.
+ */
+ local_irq_save(flags);
+
st0[0] ^= __aesti_sbox[ 0] ^ __aesti_sbox[128];
st0[1] ^= __aesti_sbox[32] ^ __aesti_sbox[160];
st0[2] ^= __aesti_sbox[64] ^ __aesti_sbox[192];
@@ -300,6 +307,8 @@ static void aesti_encrypt(struct crypto_tfm *tfm, u8 *out, const u8 *in)
put_unaligned_le32(subshift(st1, 1) ^ rkp[5], out + 4);
put_unaligned_le32(subshift(st1, 2) ^ rkp[6], out + 8);
put_unaligned_le32(subshift(st1, 3) ^ rkp[7], out + 12);
+
+ local_irq_restore(flags);
}
static void aesti_decrypt(struct crypto_tfm *tfm, u8 *out, const u8 *in)
@@ -308,6 +317,7 @@ static void aesti_decrypt(struct crypto_tfm *tfm, u8 *out, const u8 *in)
const u32 *rkp = ctx->key_dec + 4;
int rounds = 6 + ctx->key_length / 4;
u32 st0[4], st1[4];
+ unsigned long flags;
int round;
st0[0] = ctx->key_dec[0] ^ get_unaligned_le32(in);
@@ -315,6 +325,12 @@ static void aesti_decrypt(struct crypto_tfm *tfm, u8 *out, const u8 *in)
st0[2] = ctx->key_dec[2] ^ get_unaligned_le32(in + 8);
st0[3] = ctx->key_dec[3] ^ get_unaligned_le32(in + 12);
+ /*
+ * Temporarily disable interrupts to avoid races where cachelines are
+ * evicted when the CPU is interrupted to do something else.
+ */
+ local_irq_save(flags);
+
st0[0] ^= __aesti_inv_sbox[ 0] ^ __aesti_inv_sbox[128];
st0[1] ^= __aesti_inv_sbox[32] ^ __aesti_inv_sbox[160];
st0[2] ^= __aesti_inv_sbox[64] ^ __aesti_inv_sbox[192];
@@ -339,6 +355,8 @@ static void aesti_decrypt(struct crypto_tfm *tfm, u8 *out, const u8 *in)
put_unaligned_le32(inv_subshift(st1, 1) ^ rkp[5], out + 4);
put_unaligned_le32(inv_subshift(st1, 2) ^ rkp[6], out + 8);
put_unaligned_le32(inv_subshift(st1, 3) ^ rkp[7], out + 12);
+
+ local_irq_restore(flags);
}
static struct crypto_alg aes_alg = {