Hi Eric,
On 17 October 2018 at 14:18, Eric Biggers wrote:
> From: Eric Biggers
>
> 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
Thanks for taking a look. Could we add something to the Kconfig blurb
that mentions that it runs the algorithm witn interrupts disabled? In
any case,
Reviewed-by: Ard Biesheuvel
> ---
> 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
>
