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

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

Hi Roland,

On 27.05.25 11:15, Roland Hieber wrote:
> On Mon, May 26, 2025 at 12:59:33PM +0200, Ahmad Fatoum wrote:
>> Hello Roland,
>>
>> On 5/26/25 12:39, Roland Hieber wrote:
>>> 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.
>>
>> Yes, but why is get_crypto_bytes called in your case? Things like
>> MAC address generation don't use a non-secure PRNG anyways.
> 
> Ah now I get what you mean. In that case, dmesg seems to suggest it's the stack
> protector. On riotboard I get:
> 
>     […]
>     register_driver: barebox-environment
>         probe-> chosen:environment.of
>     barebox-environment chosen:environment.of: Setting default environment path to /dev/mmc3.barebox-environment
>     initcall-> stackprot_randomize_guard+0x1/0x38
>     WARNING: falling back to Pseudo RNG source!
>     initcall-> eth_register_of_fixup+0x1/0x10
>     initcall-> dhcp_global_init+0x1/0xf8
>     […]

Ah, there is a warning in the stack protector code:

  WARNING: stackprot: proceeding without randomized stack protector

But it was only printed, when CONFIG_ALLOW_PRNG_FALLBACK was disabled.

I add some extra context to the warning message inside get_crypto_bytes
in [1], so it now looks like this:

  WARNING: stackprot_randomize_guard: falling back to Pseudo RNG source!

[1]: https://lore.barebox.org/barebox/20250527201359.889550-9-a.fatoum@pengutronix.de/T/#u

Thanks by the way for enabling the stack protector support!
I have been meaning to do this for some time now. :)

Cheers,
Ahmad

> 
>>> 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
>>
>> There is also the CAAM driver for i.MX, which is found separately in
>> a crypto submenu.
> 
> Thanks, that got rid of the PRNG warning on riotboard :-) I'll enable it in v2
> of this series too:
> 
>     --- a/configs/platform-v7a/barebox.config
>     +++ b/configs/platform-v7a/barebox.config
>     @@ -1239,7 +1239,9 @@ CONFIG_PHY_STM32_USBPHYC=y
>      #
>      # Hardware crypto devices
>      #
>     -# CONFIG_CRYPTO_DEV_FSL_CAAM is not set
>     +CONFIG_CRYPTO_DEV_FSL_CAAM=y
>     +CONFIG_CRYPTO_DEV_FSL_CAAM_RINGSIZE=9
>     +CONFIG_CRYPTO_DEV_FSL_CAAM_RNG=y
>      # end of Hardware crypto devices
> 
>  - Roland
> 
>>> 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. 
>>
>> I don't think we have hardware in DistroKit that this is useful on.
>>
>> Cheers,
>> Ahmad
>>
>>>
>>>  - 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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