[Development] QRandomGenerator and boot times

[Sami Nurmenniemi]

On 14.09.2017 20:02, Thiago Macieira wrote:
> On Thursday, 14 September 2017 08:18:05 PDT Giuseppe D'Angelo wrote:
>> Il 14/09/2017 17:01, Thiago Macieira ha scritto:
>>>> 2. Add "rng-tools" to the image for inputting entropy to the kernel
>>>> - Speeds up initialization of nonblocking pool
>>>> - On devices with HW random generator, this adds real entropy to the
>>>> kernel
>>>> pool - On devices without HW random generator, this adds bad entropy from
>>>> /dev/urandom to the kernel pool
>>> Huh? Adding entropy from /dev/urandom back to /dev/urandom? This is worse
>>> than the "rob Peter to pay Paul" - you want to rob Peter to pay Peter.
>> It's a myth of some crazy tutorials and broken default configurations
>> found on the internet. Don't do that!
>>
>> By the way, it's strange that the entropy is emptied just after boot
>> (which is when there should be plenty of I/O to refill the entropy pool).
> The entropy pool starts empty at boot. How much data is required to fill it
> enough for the kernel's needs will vary from device to device. The faster your
> device and the more I/O it does, the faster that will happen. A HWRNG will
> also help, but it will clearly start with no entropy either, so how soon it
> will work is again dependent on the implementation.
>
> On my Skylake system, which does have a HWRNG, it takes 3.8 seconds after boot
> for the random system on Linux to be up to speed. But that's slow enough that
> several system services have already started:
>
> $ dmesg | grep random:
> [    3.475259] random: systemd: uninitialized urandom read (16 bytes read)
> [    3.475305] random: systemd: uninitialized urandom read (16 bytes read)
> [    3.475328] random: systemd: uninitialized urandom read (16 bytes read)
> [    3.475612] random: systemd: uninitialized urandom read (16 bytes read)
> [    3.475634] random: systemd: uninitialized urandom read (16 bytes read)
> [    3.475643] random: systemd: uninitialized urandom read (16 bytes read)
> [    3.475655] random: systemd: uninitialized urandom read (16 bytes read)
> [    3.475662] random: systemd: uninitialized urandom read (16 bytes read)
> [    3.475667] random: systemd: uninitialized urandom read (16 bytes read)
> [    3.475715] random: systemd: uninitialized urandom read (16 bytes read)
> [    3.837995] random: crng init done
>
Thank you all for your comments. We'll setup the rng-tools which will 
feed entropy to the kernel from /dev/hwrng as suggested in the kernel 
documentation https://www.kernel.org/doc/Documentation/hw_random.txt.

I think we'll just have to accept blocking for the devices without 
hwrng. I don't know if we really support any such devices. If we do and 
boot time is essential for those, we'll have to figure out some way 
(probably saving entropy over reboot).