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

[Eric W. Biederman]

Matt Mackall <mpm@xxxxxxxxxxx> writes:

> On Tue, May 05, 2009 at 09:52:46PM +0200, Ingo Molnar wrote:
>> 
>> * Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote:
>> 
>> > Ingo Molnar <mingo@xxxxxxx> writes:
>> > 
>> > > * Matt Mackall <mpm@xxxxxxxxxxx> 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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