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

From: Kees Cook <[email-masked]>
Date: Sat, 24 Aug 2019 12:04:01 -0700

On Thu, Aug 22, 2019 at 10:39:51AM -0700, Stephen Boyd wrote:
> 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

Can this please be a boot param (with the default controlled by the
CONFIG)? See how CONFIG_RANDOM_TRUST_CPU is wired up...

-Kees

> > +       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

-- 
Kees Cook

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