[LEDE-DEV] [PATCH] base-files: seed /dev/urandom

[Etienne Champetier]

Hi David,

2016-06-14 20:21 GMT+03:00 David Lang <david at lang.hm>:
> On Tue, 14 Jun 2016, Etienne Champetier wrote:
>
>> 2016-06-14 9:08 GMT+02:00 Felix Fietkau <nbd at nbd.name>:
>>>
>>> On 2016-06-13 22:10, Etienne Champetier wrote:
>>>>
>>>> Hi John, Felix,
>>>>
>>>> 2016-06-13 13:55 GMT+02:00 John Crispin <john at phrozen.org>:
>>>>>
>>>>>
>>>>>
>>>>> On 13/06/2016 00:56, Etienne Champetier wrote:
>>>>>>
>>>>>> Hi Felix,
>>>>>>
>>>>>> 2016-06-12 12:45 GMT+02:00 Felix Fietkau <nbd at nbd.name>:
>>>>>>>
>>>>>>> On 2016-06-11 08:37, Etienne CHAMPETIER wrote:
>>>>>
>>>>>
>>>>> lets add a system.system.write_state_to_flash_on_boot=0/1 uci option
>>>>> and
>>>>> lock this and the dnssec time stuff with it and default it to 0
>>>>
>>>>
>>>> Security can't be opt in !
>>>> When you see "random: ubus urandom read with 4 bits of entropy
>>>> available" let's hope it's not security sensitive, because 2^4 will
>>>> not take a lot of time to bruteforce...
>>>
>>> First of all, the kernel entropy estimation is *really* pessimistic, so
>>> there will be a lot more random bits at this point than just 4.
>>>
>>>> Before we try to minimize writes, how much writes are we talking about?
>>>> my openwrt routers have multiple months of uptime, and even if we get
>>>> down to 1 week, that gets us to 53 writes a year.
>>>> How much writes can a flash handle these days?
>>>
>>> I'm more concerned about the worst case than the average case here.
>>> There are people that do a forced reboot every day (as a stability
>>> workaround), or only power up their devices during specific times of the
>>> day (multiple reboots per day). This can easily add up to bigger numbers.
>>>
>>> Also, adding something like this makes other people want to add even
>>> more stuff that writes to flash on every boot, as you've so clearly
>>> demonstrated by pointing out that this behavior are already done for
>>> dnssec/dnsmasq.
>>
>>
>> Ok, let's find a middle ground :)
>> What about saving a seed if there is none (on boot), and then using an
>> ntp hotplug (stratum event) and save a new seed if older than say 1
>> week?
>
>
> The worst thing that you can do is to use the same seed on multiple boots.

I agree that we should use a new seed each time,
but are you suggesting that using the same seed is worse than no seed?
Do you have some links to back your claim?

Seeding here is just writing 512 bytes from getrandom() back into
/dev/urandom, so this gets mixed with what's already available without
affecting entropy estimation

>
> David Lang