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SubjectRe: [Security] [PATCH] proc: avoid information leaks to non-privileged processes

* Eric W. Biederman <ebiederm@xmission.com> wrote:

> Matt Mackall <mpm@selenic.com> writes:
>
> > 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.
>
> Well it has the val = (int)&val bit.
>
> However you are quite right the original get_random_int does not
> have any state that persists from one call to the next. Ingo you
> failed to copy that from the way ip uses secure_ip_id.
>
> Which ultimately means get_random_int has never had a pseudo
> random number generator in it.

Indeed, good point. But, practical randomness was still saved by the
other layers. Which what it is really about: adding different
sources of semi-random values really increases attack complexity,
and is cheap.

Ingo


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