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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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