From: Eric Biggers <ebiggers@xxxxxxxxxx> 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. Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx> --- crypto/aes_ti.c | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) 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 = { -- 2.19.1
