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

    * 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);
    +}

    I thought that basing it on the networking PRNG is proper design:
    the networking folks have showed it time and again that they care
    about the PRNG not being brute-forceable easily, while still staying
    fast enough.

    I added the TSC and a few other pseudo-random details to increase
    complexity of any brute-force attack. (in the hope of rendering it
    less practical, at minimal cost)

    > 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.

    Yes.

    Ingo


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