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| author | Eric Biggers <ebiggers@google.com> | 2018-10-17 21:37:58 -0700 |
|---|---|---|
| committer | Herbert Xu <herbert@gondor.apana.org.au> | 2018-11-09 17:36:48 +0800 |
| commit | 0a6a40c2a8c184a2fb467efacfb1cd338d719e0b (patch) | |
| tree | 83c30f4e2775779925bd03c9d3f5f5f4f06887ae /crypto/aes_ti.c | |
| parent | 9f4debe38415583086ce814798eeb864aeb39551 (diff) | |
| download | lwn-0a6a40c2a8c184a2fb467efacfb1cd338d719e0b.tar.gz lwn-0a6a40c2a8c184a2fb467efacfb1cd338d719e0b.zip | |
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/aes_ti.c')
| -rw-r--r-- | crypto/aes_ti.c | 18 |
1 files changed, 18 insertions, 0 deletions
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 = { |