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

* Matt Mackall <mpm@selenic.com> wrote:

> 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.

Note that this was the exec-shield arch/*86/mm/mmap.c code.
(Also, obviously "only" covering 95% of the Linux systems has its
use as well. Most other architectures have their own cycle counters
as well.)

> 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

Try that one day and see how much jitter there is in that sequence,
even on a completely quiescent system.

Ingo


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