Re: [PATCH v5 2/5] random: Add and use arch_get_rng_seed

[Andy Lutomirski]

On Wed, Jul 23, 2014 at 9:57 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> Currently, init_std_data contains its own logic for using arch
> random sources.  This replaces that logic with a generic function
> arch_get_rng_seed that allows arch code to supply its own logic.
> The default implementation tries arch_get_random_seed_long and
> arch_get_random_long individually.
>
> The only functional change here is that random_get_entropy() is used
> unconditionally instead of being used only when the arch sources
> fail.  This may add a tiny amount of security.

tytso, are you okay with this approach?  I'd be happy to rework this
if you prefer some other way of doing it.

--Andy

>
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> ---
>  drivers/char/random.c  | 14 +++++++++++---
>  include/linux/random.h | 40 ++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 51 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index 0a7ac0a..be7a94e 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -1236,6 +1236,10 @@ void get_random_bytes_arch(void *buf, int nbytes)
>  }
>  EXPORT_SYMBOL(get_random_bytes_arch);
>
> +static void seed_entropy_store(void *ctx, u32 data)
> +{
> +       mix_pool_bytes((struct entropy_store *)ctx, &data, sizeof(data), NULL);
> +}
>
>  /*
>   * init_std_data - initialize pool with system data
> @@ -1251,15 +1255,19 @@ static void init_std_data(struct entropy_store *r)
>         int i;
>         ktime_t now = ktime_get_real();
>         unsigned long rv;
> +       char log_prefix[128];
>
>         r->last_pulled = jiffies;
>         mix_pool_bytes(r, &now, sizeof(now), NULL);
>         for (i = r->poolinfo->poolbytes; i > 0; i -= sizeof(rv)) {
> -               if (!arch_get_random_seed_long(&rv) &&
> -                   !arch_get_random_long(&rv))
> -                       rv = random_get_entropy();
> +               rv = random_get_entropy();
>                 mix_pool_bytes(r, &rv, sizeof(rv), NULL);
>         }
> +
> +       sprintf(log_prefix, "random: seeded %s pool", r->name);
> +       arch_get_rng_seed(r, seed_entropy_store, 8 * r->poolinfo->poolbytes,
> +                         log_prefix);
> +
>         mix_pool_bytes(r, utsname(), sizeof(*(utsname())), NULL);
>  }
>
> diff --git a/include/linux/random.h b/include/linux/random.h
> index 57fbbff..81a6145 100644
> --- a/include/linux/random.h
> +++ b/include/linux/random.h
> @@ -106,6 +106,46 @@ static inline int arch_has_random_seed(void)
>  }
>  #endif
>
> +#ifndef __HAVE_ARCH_GET_RNG_SEED
> +
> +/**
> + * arch_get_rng_seed() - get architectural rng seed data
> + * @ctx: context for the seed function
> + * @seed: function to call for each u32 obtained
> + * @bits_per_source: number of bits from each source to try to use
> + * @log_prefix: beginning of log output (may be NULL)
> + *
> + * Synchronously load some architectural entropy or other best-effort
> + * random seed data.  An arch-specific implementation should be no worse
> + * than this generic implementation.  If the arch code does something
> + * interesting, it may log something of the form "log_prefix with
> + * 8 bits of stuff".
> + *
> + * No arch-specific implementation should be any worse than the generic
> + * implementation.
> + */
> +static inline void arch_get_rng_seed(void *ctx,
> +                                    void (*seed)(void *ctx, u32 data),
> +                                    int bits_per_source,
> +                                    const char *log_prefix)
> +{
> +       int i;
> +
> +       for (i = 0; i < bits_per_source; i += 8 * sizeof(long)) {
> +               unsigned long rv;
> +
> +               if (arch_get_random_seed_long(&rv) ||
> +                   arch_get_random_long(&rv)) {
> +                       seed(ctx, (u32)rv);
> +#if BITS_PER_LONG > 32
> +                       seed(ctx, (u32)(rv >> 32));
> +#endif
> +               }
> +       }
> +}
> +
> +#endif /* __HAVE_ARCH_GET_RNG_SEED */
> +
>  /* Pseudo random number generator from numerical recipes. */
>  static inline u32 next_pseudo_random32(u32 seed)
>  {
> --
> 1.9.3
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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