linux-kernel - Re: [RESEND PATCH] Allow hwrng to initialize crng.

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Date:   Thu, 13 Dec 2018 16:51:02 +0800
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     Louis Collard <louiscollard@...omium.org>
Cc:     linux-crypto@...r.kernel.org, Matt Mackall <mpm@...enic.com>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Gary R Hook <gary.hook@....com>, Michael Buesch <m@...s.ch>,
        PrasannaKumar Muralidharan <prasannatsmkumar@...il.com>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        linux-kernel@...r.kernel.org, apronin@...omium.org,
        jarkko.sakkinen@...ux.intel.com, linux@...ewoehner.de,
        david.bild@...tum.com, Theodore Ts'o <tytso@....edu>
Subject: Re: [RESEND PATCH] Allow hwrng to initialize crng.

On Thu, Dec 13, 2018 at 04:40:17PM +0800, Louis Collard wrote:
> Some systems, for example embedded systems, do not generate
> enough entropy on boot through interrupts, and boot may be blocked for
> several minutes waiting for a call to getrandom to complete.
> 
> Currently, random data is read from a hwrng when it is registered,
> and is loaded into primary_crng. This data is treated in the same
> way as data that is device-specific but otherwise unchanging, and
> so primary_crng cannot become initialized with the data from the
> hwrng.
> 
> This change causes the data initially read from the hwrng to be
> treated the same as subsequent data that is read from the hwrng if
> it's quality score is non-zero.
> 
> The implications of this are:
> 
> The data read from hwrng can cause primary_crng to become
> initialized, therefore avoiding problems of getrandom blocking
> on boot.
> 
> Calls to getrandom (with GRND_RANDOM) may be using entropy
> exclusively (or in practise, almost exclusively) from the hwrng.
> 
> Regarding the latter point; this behavior is the same as if a
> user specified a quality score of 1 (bit of entropy per 1024 bits)
> so hopefully this is not too scary a change to make.
> 
> This change is the result of the discussion here:
> https://patchwork.kernel.org/patch/10453893/
> 
> Signed-off-by: Louis Collard <louiscollard@...omium.org>
> Acked-by: Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
> ---
>  drivers/char/hw_random/core.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
> index 95be7228f327..99c3e4127949 100644
> --- a/drivers/char/hw_random/core.c
> +++ b/drivers/char/hw_random/core.c
> @@ -24,6 +24,7 @@
>  #include <linux/sched.h>
>  #include <linux/slab.h>
>  #include <linux/uaccess.h>
> +#include <crypto/chacha20.h>
>  
>  #define RNG_MODULE_NAME		"hw_random"
>  
> @@ -64,13 +65,17 @@ static size_t rng_buffer_size(void)
>  static void add_early_randomness(struct hwrng *rng)
>  {
>  	int bytes_read;
> -	size_t size = min_t(size_t, 16, rng_buffer_size());
> +	/* Read enough to initialize crng. */
> +	size_t size = 2*CHACHA20_KEY_SIZE;
>  
>  	mutex_lock(&reading_mutex);
>  	bytes_read = rng_get_data(rng, rng_buffer, size, 1);
>  	mutex_unlock(&reading_mutex);
>  	if (bytes_read > 0)
> -		add_device_randomness(rng_buffer, bytes_read);
> +		/* Allow crng to become initialized, but do not add
> +		 * entropy to the pool.
> +		 */
> +		add_hwgenerator_randomness(rng_buffer, bytes_read, 0);
>  }
>  
>  static inline void cleanup_rng(struct kref *kref)
> -- 
> 2.13.5
> 

Adding Ted Ts'o to the cc list.
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt