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> Because it looks at inter-IRQ timing, the risk is mainly (as per
> previous posting) the theoretical risk of being able to determine
> that inter-IRQ timing from observation of the network(s) connected.

So I looked at this a bit more. The stuff which increases entropy
is meant to be secure from non-root users.

However (standard debian install - headless machine), unpriveleged

$ cat /proc/interrupts
0: 1116302985 XT-PIC timer
1: 2 XT-PIC keyboard
2: 0 XT-PIC cascade
8: 1 XT-PIC rtc
9: 28980016 XT-PIC usb-uhci, eth0
14: 698146587 XT-PIC ide0
15: 5 XT-PIC ide1
NMI: 0
ERR: 0

Shock horror - I can continually poll this and spot
when an IRQ occurs.

So polling /proc/interrupts gives me a pretty good indication
of timing for ide0 interrupts (and, if I had one, keyboard
interrupts). The /proc reading routine is sufficiently fast
that by repeating reading (as a user) I should be able to
get the inter-IRQ timing down to a few tens of microseconds,
which I think is a few tens of possible values added to the
entropy pool. This tells me that actually keyboard and ide
interrupt timings are no less observable by non-root people than
network interrupts.

Now if you have an IR or radio keyboard, the situation is even worse.

So I don't think Robert's patch is any more flawed than using
k/b, mouse, ide IRQs.

Alex Bligh
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