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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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