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

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Date:	Tue, 5 May 2009 21:52:46 +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:

> 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);
+}

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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