Re: Real-World Impact of Hash DoS in Lua

[Florian Weimer <fw@...>]

* Roberto Ierusalimschy:

>> OK.  Cool.  This is a showstopper for the company I am working with for
>> rolling out embedded Lua with nginx.  Is there anything I can do to help?
>
> what it is still missing now is how to create the initial per-state
> random seed. Suggestions included some address and arc4random. I am
> afraid that, for the backup ANSI implementation, we cannot do much
> better than something like this:
>
>   seed = (unsigned int)time() + (unsigned int)L;

Addresses of a stack variable and a public Lua function should provide
a few bits of randomness, too.

Instead of addition, you should use one of Bob Jenkin's mixing
functions.  This means that it's not important where the randomness is
inside the initial values (sometimes, it's in the lower bits,
sometimes, somewhere in the middle).

Reading from /dev/urandom might be problematic because drains entropy
from the whole system.

> We can have better implementations for particular system. For
> instance, we can use arc4random if present, but how to detect it?
> Are there any other suggestions?

Have you tried using PATRICIA and similar trie structures for
interning strings?  I'm just curious how large the performance impact
would be.  I suspect it's so large that it's not worth it.