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SubjectRe: [Security] [PATCH] proc: avoid information leaks to non-privileged processes
On Tue, May 05, 2009 at 09:52:46PM +0200, Ingo Molnar wrote:
>
> * Eric W. Biederman <ebiederm@xmission.com> wrote:
>
> > 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.
>
> Alas, i did - it got 'reviewed' out of existence ;)
>
> I still have the backups, here's the original exec-shield RNG:
>
> +/*
> + * Get a random word:
> + */
> +static inline unsigned int get_random_int(void)
> +{
> + unsigned int val = 0;
> +
> + if (!exec_shield_randomize)
> + return 0;
> +
> +#ifdef CONFIG_X86_HAS_TSC
> + rdtscl(val);
> +#endif
> + val += current->pid + jiffies + (int)&val;
> +
> + /*
> + * Use IP's RNG. It suits our purpose perfectly: it re-keys itself
> + * every second, from the entropy pool (and thus creates a limited
> + * drain on it), and uses halfMD4Transform within the second. We
> + * also spice it with the TSC (if available), jiffies, PID and the
> + * stack address:
> + */
> + return secure_ip_id(val);
> +}

Ingo, what are you on about? On every architecture but X86 with TSC
this is identical to the broken code.

TSC only helps matters slightly - the timescales involved in process
creation are very short and we can probably brute-force attack it with
a useful probability of success. ie:

a) record TSC
b) fork target process
c) record TSC
d) guess TSC value
e) attempt attack
f) repeat

--
Mathematics is the supreme nostalgia of our time.


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