Re: [PATCH v25 09/12] LRNG - add Jitter RNG fast noise source

[Neil Horman]

On Sat, Nov 16, 2019 at 10:36:52AM +0100, Stephan Müller wrote:
> The Jitter RNG fast noise source implemented as part of the kernel
> crypto API is queried for 256 bits of entropy at the time the seed
> buffer managed by the LRNG is about to be filled.
> 
> CC: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> CC: "Alexander E. Patrakov" <patrakov@xxxxxxxxx>
> CC: "Ahmed S. Darwish" <darwish.07@xxxxxxxxx>
> CC: "Theodore Y. Ts'o" <tytso@xxxxxxx>
> CC: Willy Tarreau <w@xxxxxx>
> CC: Matthew Garrett <mjg59@xxxxxxxxxxxxx>
> CC: Vito Caputo <vcaputo@xxxxxxxxxxx>
> CC: Andreas Dilger <adilger.kernel@xxxxxxxxx>
> CC: Jan Kara <jack@xxxxxxx>
> CC: Ray Strode <rstrode@xxxxxxxxxx>
> CC: William Jon McCann <mccann@xxxxxxx>
> CC: zhangjs <zachary@xxxxxxxxxxxxxxxx>
> CC: Andy Lutomirski <luto@xxxxxxxxxx>
> CC: Florian Weimer <fweimer@xxxxxxxxxx>
> CC: Lennart Poettering <mzxreary@xxxxxxxxxxx>
> CC: Nicolai Stange <nstange@xxxxxxx>
> Reviewed-by: Marcelo Henrique Cerri <marcelo.cerri@xxxxxxxxxxxxx>
> Reviewed-by: Roman Drahtmueller <draht@xxxxxxxxxxxxxx>
> Tested-by: Roman Drahtmüller <draht@xxxxxxxxxxxxxx>
> Tested-by: Marcelo Henrique Cerri <marcelo.cerri@xxxxxxxxxxxxx>
> Tested-by: Neil Horman <nhorman@xxxxxxxxxx>
> Signed-off-by: Stephan Mueller <smueller@xxxxxxxxxx>
> ---
>  drivers/char/lrng/Kconfig     | 11 +++++
>  drivers/char/lrng/Makefile    |  1 +
>  drivers/char/lrng/lrng_jent.c | 88 +++++++++++++++++++++++++++++++++++
>  3 files changed, 100 insertions(+)
>  create mode 100644 drivers/char/lrng/lrng_jent.c
> 
> diff --git a/drivers/char/lrng/Kconfig b/drivers/char/lrng/Kconfig
> index 03e6e2ec356b..80fc723c67d2 100644
> --- a/drivers/char/lrng/Kconfig
> +++ b/drivers/char/lrng/Kconfig
> @@ -80,4 +80,15 @@ config LRNG_KCAPI
>  	  provided by the selected kernel crypto API RNG.
>  endif # LRNG_DRNG_SWITCH
>  
> +config LRNG_JENT
> +	bool "Enable Jitter RNG as LRNG Seed Source"
> +	select CRYPTO_JITTERENTROPY
> +	help
> +	  The Linux RNG may use the Jitter RNG as noise source. Enabling
> +	  this option enables the use of the Jitter RNG. Its default
> +	  entropy level is 16 bits of entropy per 256 data bits delivered
> +	  by the Jitter RNG. This entropy level can be changed at boot
> +	  time or at runtime with the lrng_base.jitterrng configuration
> +	  variable.
> +
>  endif # LRNG
> diff --git a/drivers/char/lrng/Makefile b/drivers/char/lrng/Makefile
> index 027b6ea51c20..a87d800c9aae 100644
> --- a/drivers/char/lrng/Makefile
> +++ b/drivers/char/lrng/Makefile
> @@ -13,3 +13,4 @@ obj-$(CONFIG_SYSCTL)		+= lrng_proc.o
>  obj-$(CONFIG_LRNG_DRNG_SWITCH)	+= lrng_switch.o
>  obj-$(CONFIG_LRNG_DRBG)		+= lrng_drbg.o
>  obj-$(CONFIG_LRNG_KCAPI)	+= lrng_kcapi.o
> +obj-$(CONFIG_LRNG_JENT)		+= lrng_jent.o
> diff --git a/drivers/char/lrng/lrng_jent.c b/drivers/char/lrng/lrng_jent.c
> new file mode 100644
> index 000000000000..43114a44b8f5
> --- /dev/null
> +++ b/drivers/char/lrng/lrng_jent.c
> @@ -0,0 +1,88 @@
> +// SPDX-License-Identifier: GPL-2.0 OR BSD-2-Clause
> +/*
> + * LRNG Fast Noise Source: Jitter RNG
> + *
> + * Copyright (C) 2016 - 2019, Stephan Mueller <smueller@xxxxxxxxxx>
> + */
> +
> +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
> +
> +#include "lrng_internal.h"
> +
> +/*
> + * Estimated entropy of data is a 16th of LRNG_DRNG_SECURITY_STRENGTH_BITS.
> + * Albeit a full entropy assessment is provided for the noise source indicating
> + * that it provides high entropy rates and considering that it deactivates
> + * when it detects insufficient hardware, the chosen under estimation of
> + * entropy is considered to be acceptable to all reviewers.
> + */
> +static u32 jitterrng = LRNG_DRNG_SECURITY_STRENGTH_BITS>>4;
> +module_param(jitterrng, uint, 0644);
> +MODULE_PARM_DESC(jitterrng, "Entropy in bits of 256 data bits from Jitter "
> +			    "RNG noise source");
> +
> +/**
> + * Get Jitter RNG entropy
> + *
> + * @outbuf buffer to store entropy
> + * @outbuflen length of buffer
> + * @return > 0 on success where value provides the added entropy in bits
> + *	   0 if no fast source was available
> + */
> +struct rand_data;
> +struct rand_data *jent_lrng_entropy_collector(void);
> +int jent_read_entropy(struct rand_data *ec, unsigned char *data,
> +		      unsigned int len);
> +static struct rand_data *lrng_jent_state;
> +
> +u32 lrng_get_jent(u8 *outbuf, unsigned int outbuflen)
> +{
> +	int ret;
> +	u32 ent_bits = jitterrng;
> +	unsigned long flags;
> +	static DEFINE_SPINLOCK(lrng_jent_lock);
> +	static int lrng_jent_initialized = 0;
> +
> +	spin_lock_irqsave(&lrng_jent_lock, flags);
> +
> +	if (!ent_bits || (lrng_jent_initialized == -1)) {
> +		spin_unlock_irqrestore(&lrng_jent_lock, flags);
> +		return 0;
> +	}
> +
this works, but I think you can avoid the use of the spin lock on the read calls
here.  If you assign a global pointer to the value of &lrng_jent_state on init,
you can just take the spinlock on assignment, and assume its stable after that
(which it should be given that its only ever going to point to a static data
structure).

Neil

> +	if (!lrng_jent_initialized) {
> +		lrng_jent_state = jent_lrng_entropy_collector();
> +		if (!lrng_jent_state) {
> +			jitterrng = 0;
> +			lrng_jent_initialized = -1;
> +			spin_unlock_irqrestore(&lrng_jent_lock, flags);
> +			pr_info("Jitter RNG unusable on current system\n");
> +			return 0;
> +		}
> +		lrng_jent_initialized = 1;
> +		pr_debug("Jitter RNG working on current system\n");
> +	}
> +	ret = jent_read_entropy(lrng_jent_state, outbuf, outbuflen);
> +	spin_unlock_irqrestore(&lrng_jent_lock, flags);
> +
> +	if (ret) {
> +		pr_debug("Jitter RNG failed with %d\n", ret);
> +		return 0;
> +	}
> +
> +	/* Obtain entropy statement */
> +	if (outbuflen != LRNG_DRNG_SECURITY_STRENGTH_BYTES)
> +		ent_bits = (ent_bits * outbuflen<<3) /
> +			   LRNG_DRNG_SECURITY_STRENGTH_BITS;
> +	/* Cap entropy to buffer size in bits */
> +	ent_bits = min_t(u32, ent_bits, outbuflen<<3);
> +	pr_debug("obtained %u bits of entropy from Jitter RNG noise source\n",
> +		 ent_bits);
> +
> +	return ent_bits;
> +}
> +
> +u32 lrng_jent_entropylevel(void)
> +{
> +	return min_t(u32, jitterrng, LRNG_DRNG_SECURITY_STRENGTH_BITS);
> +}
> -- 
> 2.23.0
> 
> 
> 
>