Subject: Re: [PATCH v9 2/3] fdt: add support for rng-seed

From: Stephen Boyd <[email-masked]>
Date: Thu, 22 Aug 2019 10:39:51 -0700

Quoting Hsin-Yi Wang (2019-08-22 00:15:22)
> Introducing a chosen node, rng-seed, which is an entropy that can be
> passed to kernel called very early to increase initial device
> randomness. Bootloader should provide this entropy and the value is
> read from /chosen/rng-seed in DT.
> 
> Obtain of_fdt_crc32 for CRC check after early_init_dt_scan_nodes(),
> since early_init_dt_scan_chosen() would modify fdt to erase rng-seed.
> 
> Add a new interface add_bootloader_randomness() for rng-seed use case.
> Depends on whether the seed is trustworthy, rng seed would be passed to
> add_hwgenerator_randomness(). Otherwise it would be passed to
> add_device_randomness(). Decision is controlled by kernel config
> RANDOM_TRUST_BOOTLOADER.
> 
> Signed-off-by: Hsin-Yi Wang <[email-masked]>
> Reviewed-by: Stephen Boyd <[email-masked]>
> Reviewed-by: Rob Herring <[email-masked]>
> ---
> Change from v8:
> * Add a new interface add_bootloader_randomness
> * Add a new kernel config
> ---
>  drivers/char/Kconfig   | 10 ++++++++++
>  drivers/char/random.c  | 15 +++++++++++++++
>  drivers/of/fdt.c       | 14 ++++++++++++--
>  include/linux/random.h |  1 +
>  4 files changed, 38 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/char/Kconfig b/drivers/char/Kconfig
> index 96156c729a31..5974a5906fd0 100644
> --- a/drivers/char/Kconfig
> +++ b/drivers/char/Kconfig
> @@ -551,3 +551,13 @@ config RANDOM_TRUST_CPU
>         has not installed a hidden back door to compromise the CPU's
>         random number generation facilities. This can also be configured
>         at boot with "random.trust_cpu=on/off".
> +
> +config RANDOM_TRUST_BOOTLOADER
> +       bool "Trust the bootloader to initialize Linux's CRNG"
> +       default n

You can drop the default.

> +       help
> +       Bootloader could provide rng-seed set in /chosen/rng-seed in DT to help
> +       increase initial device randomness. Assume the entropy provided is
> +       trustworthy, it would be regarded as true hardware RNGs and update the
> +       entropy estimate. Otherwise it would be regarded as device input that
> +       could help mix the entropy pool, but won't be added to actual entropy.

Maybe reword this to something like:

	Some bootloaders can provide entropy to increase the kernel's
	initial device randomness. Say Y here to assume the entropy
	provided by the booloader is trustworthy so it will be added to
	the kernel's entropy pool. Otherwise, say N here so it will be
	regarded as device input that only mixes the entropy pool.

> \ No newline at end of file
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index 5d5ea4ce1442..29d3ff3de1e1 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -2445,3 +2445,18 @@ void add_hwgenerator_randomness(const char *buffer, size_t count,
>         credit_entropy_bits(poolp, entropy);
>  }
>  EXPORT_SYMBOL_GPL(add_hwgenerator_randomness);
> +
> +/* Handle random seed passed by bootloader.
> + * If the seed is trustworthy, it would be regarded as hardware RNGs. Otherwise
> + * it would be regarded as device data.
> + * The decision is controlled by CONFIG_RANDOM_TRUST_BOOTLOADER.
> + */
> +void add_bootloader_randomness(const void *buf, unsigned int size)
> +{
> +#ifdef CONFIG_RANDOM_TRUST_BOOTLOADER
> +       add_hwgenerator_randomness(buf, size, size * 8);
> +#else
> +       add_device_randomness(buf, size);
> +#endif

Maybe use

	if (IS_ENABLED(CONFIG_RANDOM_TRUST_BOOTLOADER))
		add_hwgenerator_randomness(buf, size, size * 8);
	else
		add_device_randomness(buf, size);
	
> +}
> +EXPORT_SYMBOL_GPL(add_bootloader_randomness);
> \ No newline at end of file

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