rngd taking all CPU cycles for a long time on reboot imx6ul Yocto kernel 5.4

[RayC]

On imx6ul reboot, I notice rngd is taking up virtually 100% CPU cycles for a few minutes.

Since we care about security rngd is beneficial.

I just would like to know if this is normal? What is the  /dev/hwrng device? Is it the CAAM crypto? Why is it so slow? 

My yocto built kernel is 5.4.70.

Thanks

[rsm4]

Hi,

Also I monitored the behaviour of 100% CPU usage at imx6ul with default settings of rngd.
So I have removed the jitterlibrary as mentioned with the -x jitter option.
As mentioned above, the CPU usage is no longer an issue BUT in general the boot time has incereased by around +10sec.
Can you explain this?
It seems as the entropy, which is only derived from hwrng grows to slow without the jitter entropy source.
To you have any idea?
Is the hwrng as single source for entropy generation that slow?

Thanks!

[igorpadykov]

Hi Ray

When system boot, especially the first boot up, we can see some RNG related process(like random: crng init) will take some minutes to finish. This it related to Linux RNG framework.

We exposed CAAM’s TRNG through the “hwrng” framework. Please note that we can use hwrng to get “entropy” instead of “randomness”, it is backend by CAAM, we reseed the DRBG every time to get entropy from TRNG inside of CAAM. so it is slower than the non-reseed case is expected.

Best regards
igor

[RayC]

The problem goes away if I exclude "jitter".

Could someone help me understand this?

Thanks

root@imx6ulevk:~# cat /etc/default/rng-tools
EXTRA_ARGS="-r /dev/hwrng -x jitter"

[igorpadykov]

>The problem goes away if I exclude "jitter"

answer from team:

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rng-tools is not from our NXP, and is from 3rd party which can download from Github, and also is integrated into our Yocto BSP.

By checking its code, we can see CPU jitter is a RNG entropy source which is based on running multi-threads on CPUs. and it is tagged as a slow source in rng-tools.

After customer add "-x jitter", it seems like that the jitter entropy source is disabled. So the customer's observation is reasonable.

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Best regards
igor