Re: /dev/urandom uses uninit bytes, leaks user data

[Pavel Machek <pavel@ucw.cz>]

On Thu 2007-12-20 15:36:01, Theodore Tso wrote:
> On Wed, Dec 19, 2007 at 11:18:54PM -0500, Andrew Lutomirski wrote:
> > I understand that there's no way that /dev/random can provide good
> > output if there's insufficient entropy.  But it still shouldn't leak
> > arbitrary bits of user data that were never meant to be put into the
> > pool at all.
> 
> Let's be clear here.  It's not arbitrary bits of user data.  It's at
> most three bytes of kernel stack garbage from the process which is
> adding entropy to /dev/random from userspace.  And the attacker
> doesn't get to pick which 3 bytes they are, either!  That means they
> are not "arbitrary bits".
> 
> > (My hypothetical attack is a lot hypothetical than I thought at first.
> >  An attacker does not need to break into the kernel and steal the
> > state of the pool.  It may be as easy as this to trigger:
> > 
> > Step 1: Boot a system without a usable entropy source.
> > Step 2: add some (predictable) "entropy" from userspace which isn't a
> > multiple of 4, so up to three extra bytes get added.
> > Step 3: Read a few bytes of /dev/random and send them over the network.
> 
> So Step 1 assumes a system without a random seed file, or any usable
> entropy source.  As I've mentioned, this means that any cryptographic
> users of /dev/random --- like, say, openssh daemon --- are in deep,
> DEEP, trouble.  This is a very serious problem, and in my opinion, far
> more serious and far more REAL that your hypothetical attack.  So we
> need to address Step 1 anyway, and more urgently.
> 
> In step 2, what program are you expecting will be writing to
> /dev/random?  Very few programs do so today, and none that are
> commonly installed on most Linux distributions, other than the dd
> program run out of /etc/init.d/random to initialize /dev/random from
> the random seed file.  But, oh wait --- you're assuming that doesn't
> exist, even though all major distributions have it.
> 
> Why would a legitimate program read a few bytes of /dev/random and
> send them on the network in step 3?  Or are you assuming your
> hypothetical attacker has a shell account on the system?  In which
> case there is probably far more concerns about information leakage
> than 3 bytes of kernel stack garbage, which might or might not contain
> user data.
> 
> And note that this attack can only be done for 3 bytes.  After that,
> the work factors involved become intractable.
> 
> So yes, there is a theoretical hole given your assumptions, but the
> problem started the "system without any intial entropy", and there are
> far worse problems that result in that case.  So that is the problem
> we should be addressing.
> 
> That being said, write_pool() is hardly a fastpath, and the memset
> isn't going to make any difference.  So if it makes people feel
> better, I don't object to adding it.  I'm just pointing out that the
> assumption is either pretty silly (since all of the distributions do
> use the random seed file, and in practice *someone* has logged into
> the console to add some amount of entropy at some point in the
> machine's lifetime, even it if was when the machine was initially
> installed) and/or points out a more critical issue, which is we need
> to make sure that we do have a reasonable entropy source on all
> machines.

Lets memset. It is not a fastpath, and code will be more obvious that
way.

-- 
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(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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