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

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Message-ID: <20090506103316.GB25203@elte.hu>
Date:	Wed, 6 May 2009 12:33:16 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Matt Mackall <mpm@...enic.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Jake Edge <jake@....net>, security@...nel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	James Morris <jmorris@...ei.org>,
	linux-security-module@...r.kernel.org,
	Eric Paris <eparis@...hat.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Roland McGrath <roland@...hat.com>, mingo@...hat.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg KH <greg@...ah.com>, Dave Jones <davej@...hat.com>
Subject: Re: [Security] [PATCH] proc: avoid information leaks to
	non-privileged processes


* Eric W. Biederman <ebiederm@...ssion.com> wrote:

> Matt Mackall <mpm@...enic.com> writes:
> 
> > On Tue, May 05, 2009 at 09:52:46PM +0200, Ingo Molnar wrote:
> >> 
> >> * Eric W. Biederman <ebiederm@...ssion.com> wrote:
> >> 
> >> > Ingo Molnar <mingo@...e.hu> writes:
> >> > 
> >> > > * Matt Mackall <mpm@...enic.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.

Indeed, good point. But, practical randomness was still saved by the 
other layers. Which what it is really about: adding different 
sources of semi-random values really increases attack complexity, 
and is cheap.

	Ingo
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