Messages in this thread |  | | | From | (Eric W. Biederman) | | Date | Tue, 05 May 2009 01:14:05 -0700 | | Subject | Re: [Security] [PATCH] proc: avoid information leaks to non-privileged processes |
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Ingo Molnar <mingo@elte.hu> writes:
> * Matt Mackall <mpm@selenic.com> wrote: > >> As to what's the appropriate sort of RNG for ASLR to use, finding >> a balance between too strong and too weak is tricky. [...] > > In exec-shield i mixed 'easily accessible and fast' semi-random > state to the get_random_int() result: xor-ed the cycle counter, the > pid and a kernel address to it. That strengthened the result in a > pretty practical way (without strengthening the theoretical > randomless - each of those items are considered guessable) and does > so without weakening the entropy of the random pool.
The trouble is, that thinking completely misses the problem, and I expect that is why we have a problem. Throwing a bunch of possibly truly random values into the pot for luck is nice. But you didn't throw in a pseudo random number generator. An unpredictable sequence that is guaranteed to change from one invocation to the next.
In a very practical sense a pseudo random generator is completely sufficient. Throwing in a few truly random numbers guards against attacks on the random number generator.
What we have now is a hash over an a value that changes every 5 minutes and some well known values.
Eric
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