Re: [DistroKit] [PATCH 11/11] v7a: barebox: enable HWRNG drivers and allow fallback to PRNG

[Roland Hieber <rhi@pengutronix.de>]

On Mon, May 26, 2025 at 12:20:36PM +0200, Ahmad Fatoum wrote:
> Hi Roland,
> 
> On 5/26/25 12:11, Roland Hieber wrote:
> > Without ALLOW_PRNG_FALLBACK, barebox prints an unsightly error message
> > on boot:
> > 
> >     ERROR: no HWRNG available!
> 
> Is this because of the stack protector? Which platforms lack the RNG?

No, it's directly from get_crypto_bytes() in lib/random.c:

    137         if (!IS_ENABLED(CONFIG_ALLOW_PRNG_FALLBACK)) {
    138                 pr_err("no HWRNG available!\n");
    139                 return err;
    140         }

That's why I enabled CONFIG_ALLOW_PRNG_FALLBACK.

In menuconfig there is:

    --- HWRNG Support
    [ ]   Timer IOMEM HW Random Number Generator support
    [*]   STM32 Random Number Generator
    [*]   OP-TEE based Random Number Generator support
    [*]   Broadcom BCM2835/BCM63xx Random Number Generator support
    [ ]   Broadcom iProc/STB RNG200 support
    [*]   OMAP Random Number Generator support

I'm not sure whether we want CONFIG_HW_RANDOM_TIMERIOMEM, its help text sounds
more like a workaround than a solution:

    This driver provides barebox support for a generic Random                                                                  │   
    Number Generator used by reading a 'dumb' iomem address that                                                               │   
    is to be read no faster than, for example, once a second. 

 - Roland

> Cheers,
> Ahmad
> 
> > 
> > Since we don't have HWRNG drivers for all available platforms, enable
> > those that we have (RPi and AM33xx). For the other platforms allow
> > fallback to the PRNG, which will at least only print a warning on boot
> > on platforms without HWRNG:
> > 
> >     WARNING: falling back to Pseudo RNG source!
> > 
> > … but in this case, this is expected.
> > 
> > Signed-off-by: Roland Hieber <rhi@pengutronix.de>
> > ---
> >  configs/platform-v7a/barebox.config | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/configs/platform-v7a/barebox.config b/configs/platform-v7a/barebox.config
> > index 4780b6493a7a..86cb40acbf17 100644
> > --- a/configs/platform-v7a/barebox.config
> > +++ b/configs/platform-v7a/barebox.config
> > @@ -1106,9 +1106,9 @@ CONFIG_HWRNG=y
> >  # CONFIG_HW_RANDOM_TIMERIOMEM is not set
> >  CONFIG_HWRNG_STM32=y
> >  CONFIG_HW_RANDOM_OPTEE=y
> > -# CONFIG_HW_RANDOM_BCM2835 is not set
> > +CONFIG_HW_RANDOM_BCM2835=y
> >  # CONFIG_HW_RANDOM_IPROC_RNG200 is not set
> > -# CONFIG_HW_RANDOM_OMAP is not set
> > +CONFIG_HW_RANDOM_OMAP=y
> >  
> >  #
> >  # DMA support
> > @@ -1380,7 +1380,7 @@ CONFIG_IMAGE_SPARSE=y
> >  CONFIG_STMP_DEVICE=y
> >  CONFIG_LIBFDT=y
> >  # CONFIG_RATP is not set
> > -# CONFIG_ALLOW_PRNG_FALLBACK is not set
> > +CONFIG_ALLOW_PRNG_FALLBACK=y
> >  # CONFIG_CRC_CCITT is not set
> >  CONFIG_CRC8=y
> >  
> 
> -- 
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> 
> 

-- 
Roland Hieber, Pengutronix e.K.          | rhi@pengutronix.de          |
Steuerwalder Str. 21                     | https://www.pengutronix.de/ |
31137 Hildesheim, Germany                | Phone: +49-5121-206917-0    |
Amtsgericht Hildesheim, HRA 2686         | Fax:   +49-5121-206917-5555 |