Re: [PATCH 5/5] crypto: chacha20 - Fix keystream alignment for chacha20_block()

[Ard Biesheuvel]

On 22 November 2017 at 19:51, Eric Biggers <ebiggers3@xxxxxxxxx> wrote:
> From: Eric Biggers <ebiggers@xxxxxxxxxx>
>
> When chacha20_block() outputs the keystream block, it uses 'u32' stores
> directly.  However, the callers (crypto/chacha20_generic.c and
> drivers/char/random.c) declare the keystream buffer as a 'u8' array,
> which is not guaranteed to have the needed alignment.
>
> Fix it by having both callers declare the keystream as a 'u32' array.
> For now this is preferable to switching over to the unaligned access
> macros because chacha20_block() is only being used in cases where we can
> easily control the alignment (stack buffers).
>

Given this paragraph, I think we agree the correct way to fix this
would be to make chacha20_block() adhere to its prototype, so if we
deviate from that, there should be a good reason. On which
architecture that cares about alignment is this expected to result in
a measurable performance benefit?

> Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
> ---
>  crypto/chacha20_generic.c |  6 +++---
>  drivers/char/random.c     | 24 ++++++++++++------------
>  include/crypto/chacha20.h |  3 ++-
>  lib/chacha20.c            |  2 +-
>  4 files changed, 18 insertions(+), 17 deletions(-)
>
> diff --git a/crypto/chacha20_generic.c b/crypto/chacha20_generic.c
> index bb4affbd591c..e451c3cb6a56 100644
> --- a/crypto/chacha20_generic.c
> +++ b/crypto/chacha20_generic.c
> @@ -18,20 +18,20 @@
>  static void chacha20_docrypt(u32 *state, u8 *dst, const u8 *src,
>                              unsigned int bytes)
>  {
> -       u8 stream[CHACHA20_BLOCK_SIZE];
> +       u32 stream[CHACHA20_BLOCK_WORDS];
>
>         if (dst != src)
>                 memcpy(dst, src, bytes);
>
>         while (bytes >= CHACHA20_BLOCK_SIZE) {
>                 chacha20_block(state, stream);
> -               crypto_xor(dst, stream, CHACHA20_BLOCK_SIZE);
> +               crypto_xor(dst, (const u8 *)stream, CHACHA20_BLOCK_SIZE);
>                 bytes -= CHACHA20_BLOCK_SIZE;
>                 dst += CHACHA20_BLOCK_SIZE;
>         }
>         if (bytes) {
>                 chacha20_block(state, stream);
> -               crypto_xor(dst, stream, bytes);
> +               crypto_xor(dst, (const u8 *)stream, bytes);
>         }
>  }
>
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index ec42c8bb9b0d..11304bbc78cc 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -431,9 +431,9 @@ static int crng_init = 0;
>  static int crng_init_cnt = 0;
>  #define CRNG_INIT_CNT_THRESH (2*CHACHA20_KEY_SIZE)
>  static void _extract_crng(struct crng_state *crng,
> -                         __u8 out[CHACHA20_BLOCK_SIZE]);
> +                         __u32 out[CHACHA20_BLOCK_WORDS]);
>  static void _crng_backtrack_protect(struct crng_state *crng,
> -                                   __u8 tmp[CHACHA20_BLOCK_SIZE], int used);
> +                                   __u32 tmp[CHACHA20_BLOCK_WORDS], int used);
>  static void process_random_ready_list(void);
>  static void _get_random_bytes(void *buf, int nbytes);
>
> @@ -817,7 +817,7 @@ static void crng_reseed(struct crng_state *crng, struct entropy_store *r)
>         unsigned long   flags;
>         int             i, num;
>         union {
> -               __u8    block[CHACHA20_BLOCK_SIZE];
> +               __u32   block[CHACHA20_BLOCK_WORDS];
>                 __u32   key[8];
>         } buf;
>
> @@ -851,7 +851,7 @@ static void crng_reseed(struct crng_state *crng, struct entropy_store *r)
>  }
>
>  static void _extract_crng(struct crng_state *crng,
> -                         __u8 out[CHACHA20_BLOCK_SIZE])
> +                         __u32 out[CHACHA20_BLOCK_WORDS])
>  {
>         unsigned long v, flags;
>
> @@ -867,7 +867,7 @@ static void _extract_crng(struct crng_state *crng,
>         spin_unlock_irqrestore(&crng->lock, flags);
>  }
>
> -static void extract_crng(__u8 out[CHACHA20_BLOCK_SIZE])
> +static void extract_crng(__u32 out[CHACHA20_BLOCK_WORDS])
>  {
>         struct crng_state *crng = NULL;
>
> @@ -885,7 +885,7 @@ static void extract_crng(__u8 out[CHACHA20_BLOCK_SIZE])
>   * enough) to mutate the CRNG key to provide backtracking protection.
>   */
>  static void _crng_backtrack_protect(struct crng_state *crng,
> -                                   __u8 tmp[CHACHA20_BLOCK_SIZE], int used)
> +                                   __u32 tmp[CHACHA20_BLOCK_WORDS], int used)
>  {
>         unsigned long   flags;
>         __u32           *s, *d;
> @@ -897,14 +897,14 @@ static void _crng_backtrack_protect(struct crng_state *crng,
>                 used = 0;
>         }
>         spin_lock_irqsave(&crng->lock, flags);
> -       s = (__u32 *) &tmp[used];
> +       s = &tmp[used / sizeof(__u32)];
>         d = &crng->state[4];
>         for (i=0; i < 8; i++)
>                 *d++ ^= *s++;
>         spin_unlock_irqrestore(&crng->lock, flags);
>  }
>
> -static void crng_backtrack_protect(__u8 tmp[CHACHA20_BLOCK_SIZE], int used)
> +static void crng_backtrack_protect(__u32 tmp[CHACHA20_BLOCK_WORDS], int used)
>  {
>         struct crng_state *crng = NULL;
>
> @@ -920,7 +920,7 @@ static void crng_backtrack_protect(__u8 tmp[CHACHA20_BLOCK_SIZE], int used)
>  static ssize_t extract_crng_user(void __user *buf, size_t nbytes)
>  {
>         ssize_t ret = 0, i = CHACHA20_BLOCK_SIZE;
> -       __u8 tmp[CHACHA20_BLOCK_SIZE];
> +       __u32 tmp[CHACHA20_BLOCK_WORDS];
>         int large_request = (nbytes > 256);
>
>         while (nbytes) {
> @@ -1507,7 +1507,7 @@ static void _warn_unseeded_randomness(const char *func_name, void *caller,
>   */
>  static void _get_random_bytes(void *buf, int nbytes)
>  {
> -       __u8 tmp[CHACHA20_BLOCK_SIZE];
> +       __u32 tmp[CHACHA20_BLOCK_WORDS];
>
>         trace_get_random_bytes(nbytes, _RET_IP_);
>
> @@ -2114,7 +2114,7 @@ u64 get_random_u64(void)
>         if (use_lock)
>                 read_lock_irqsave(&batched_entropy_reset_lock, flags);
>         if (batch->position % ARRAY_SIZE(batch->entropy_u64) == 0) {
> -               extract_crng((u8 *)batch->entropy_u64);
> +               extract_crng((__u32 *)batch->entropy_u64);
>                 batch->position = 0;
>         }
>         ret = batch->entropy_u64[batch->position++];
> @@ -2144,7 +2144,7 @@ u32 get_random_u32(void)
>         if (use_lock)
>                 read_lock_irqsave(&batched_entropy_reset_lock, flags);
>         if (batch->position % ARRAY_SIZE(batch->entropy_u32) == 0) {
> -               extract_crng((u8 *)batch->entropy_u32);
> +               extract_crng(batch->entropy_u32);
>                 batch->position = 0;
>         }
>         ret = batch->entropy_u32[batch->position++];
> diff --git a/include/crypto/chacha20.h b/include/crypto/chacha20.h
> index caaa470389e0..b83d66073db0 100644
> --- a/include/crypto/chacha20.h
> +++ b/include/crypto/chacha20.h
> @@ -13,12 +13,13 @@
>  #define CHACHA20_IV_SIZE       16
>  #define CHACHA20_KEY_SIZE      32
>  #define CHACHA20_BLOCK_SIZE    64
> +#define CHACHA20_BLOCK_WORDS   (CHACHA20_BLOCK_SIZE / sizeof(u32))
>
>  struct chacha20_ctx {
>         u32 key[8];
>  };
>
> -void chacha20_block(u32 *state, void *stream);
> +void chacha20_block(u32 *state, u32 *stream);
>  void crypto_chacha20_init(u32 *state, struct chacha20_ctx *ctx, u8 *iv);
>  int crypto_chacha20_setkey(struct crypto_skcipher *tfm, const u8 *key,
>                            unsigned int keysize);
> diff --git a/lib/chacha20.c b/lib/chacha20.c
> index 250ceed9ec9a..29d3801dee24 100644
> --- a/lib/chacha20.c
> +++ b/lib/chacha20.c
> @@ -21,7 +21,7 @@ static inline u32 rotl32(u32 v, u8 n)
>         return (v << n) | (v >> (sizeof(v) * 8 - n));
>  }
>
> -extern void chacha20_block(u32 *state, void *stream)
> +void chacha20_block(u32 *state, u32 *stream)
>  {
>         u32 x[16], *out = stream;
>         int i;
> --
> 2.15.0.448.gf294e3d99a-goog
>