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

Eric


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