linux-kernel - Re: [PATCH][RESEND 3] hwrng: add randomness to system from rng sources

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Message-ID: <20140304153841.GN1872@titan.lakedaemon.net>
Date:	Tue, 4 Mar 2014 10:38:42 -0500
From:	Jason Cooper <jason@...edaemon.net>
To:	Kees Cook <keescook@...omium.org>, Theodore Ts'o <tytso@....edu>
Cc:	linux-kernel@...r.kernel.org, Matt Mackall <mpm@...enic.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Satoru Takeuchi <satoru.takeuchi@...il.com>,
	linux-crypto@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH][RESEND 3] hwrng: add randomness to system from rng
 sources

Kees, Ted,

On Mon, Mar 03, 2014 at 03:51:48PM -0800, Kees Cook wrote:
> When bringing a new RNG source online, it seems like it would make sense
> to use some of its bytes to make the system entropy pool more random,
> as done with all sorts of other devices that contain per-device or
> per-boot differences.

Why is this necessary?  init_std_data() already calls
arch_get_random_long() while stirring each of the pools.

I'm a little concerned here because this gives potentially untrusted
hwrngs more influence over the entropy pools initial state than most
users of random.c expect.  Many of the drivers in hw_random/ are
platform drivers and are initialized before random.c.

I'm comfortable with the design decisions Ted has made wrt random.c and
hwrngs.  However, I think that this changes that trust relationship in a
fundamental way.  I'm ok with building support into my kernels for
hwrngs as long as random.c's internal use of them is limited to the
mixing in extract_buf() and init_std_data().

By adding this patch, even without crediting entropy to the pool, a
rogue hwrng now has significantly more influence over the initial state
of the entropy pools.  Or, am I missing something?

thx,

Jason.

> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>  drivers/char/hw_random/core.c |    7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
> index a0f7724852eb..6e5bb68a708c 100644
> --- a/drivers/char/hw_random/core.c
> +++ b/drivers/char/hw_random/core.c
> @@ -41,6 +41,7 @@
>  #include <linux/miscdevice.h>
>  #include <linux/delay.h>
>  #include <linux/slab.h>
> +#include <linux/random.h>
>  #include <asm/uaccess.h>
>  
>  
> @@ -305,6 +306,8 @@ int hwrng_register(struct hwrng *rng)
>  	int must_register_misc;
>  	int err = -EINVAL;
>  	struct hwrng *old_rng, *tmp;
> +	unsigned char bytes[16];
> +	int bytes_read;
>  
>  	if (rng->name == NULL ||
>  	    (rng->data_read == NULL && rng->read == NULL))
> @@ -348,6 +351,10 @@ int hwrng_register(struct hwrng *rng)
>  	}
>  	INIT_LIST_HEAD(&rng->list);
>  	list_add_tail(&rng->list, &rng_list);
> +
> +	bytes_read = rng_get_data(rng, bytes, sizeof(bytes), 1);
> +	if (bytes_read > 0)
> +		add_device_randomness(bytes, bytes_read);
>  out_unlock:
>  	mutex_unlock(&rng_mutex);
>  out:
> -- 
> 1.7.9.5
> 
> 
> -- 
> Kees Cook
> Chrome OS Security
> --
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