linux-kernel - Re: [kernel-hardening] Re: [PATCH v3 04/13] crypto/rng: ensure that the RNG is ready before using

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Date:   Mon, 5 Jun 2017 21:44:04 -0700
From:   Eric Biggers <ebiggers3@...il.com>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Theodore Ts'o <tytso@....edu>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel-hardening@...ts.openwall.com,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        David Miller <davem@...emloft.net>,
        Herbert Xu <herbert@...dor.apana.org.au>
Subject: Re: [kernel-hardening] Re: [PATCH v3 04/13] crypto/rng: ensure that
 the RNG is ready before using

On Tue, Jun 06, 2017 at 05:56:20AM +0200, Jason A. Donenfeld wrote:
> Hey Ted,
> 
> On Tue, Jun 6, 2017 at 5:00 AM, Theodore Ts'o <tytso@....edu> wrote:
> > Note that crypto_rng_reset() is called by big_key_init() in
> > security/keys/big_key.c as a late_initcall().  So if we are on a
> > system where the crng doesn't get initialized until during the system
> > boot scripts, and big_key is compiled directly into the kernel, the
> > boot could end up deadlocking.
> >
> > There may be other instances of where crypto_rng_reset() is called by
> > an initcall, so big_key_init() may not be an exhaustive enumeration of
> > potential problems.  But this is an example of why the synchronous
> > API, although definitely much more convenient, can end up being a trap
> > for the unwary....
> 
> Thanks for pointing this out. I'll look more closely into it and see
> if I can figure out a good way of approaching this.

I don't think big_key even needs randomness at init time.  The 'big_key_rng'
could just be removed and big_key_gen_enckey() changed to call
get_random_bytes().  (Or get_random_bytes_wait(), I guess; it's only reachable
via the keyring syscalls.)

It's going to take a while to go through all 217 users of get_random_bytes()
like this, though...  It's really a shame there's no way to guarantee good
randomness at boot time.

Eric