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

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Date:   Mon, 5 Jun 2017 23:00:04 -0400
From:   Theodore Ts'o <tytso@....edu>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     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: [PATCH v3 04/13] crypto/rng: ensure that the RNG is ready before
 using

On Tue, Jun 06, 2017 at 02:50:59AM +0200, Jason A. Donenfeld wrote:
> Otherwise, we might be seeding the RNG using bad randomness, which is
> dangerous.
> 
> Cc: Herbert Xu <herbert@...dor.apana.org.au>
> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
> ---
>  crypto/rng.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/crypto/rng.c b/crypto/rng.c
> index f46dac5288b9..e042437e64b4 100644
> --- a/crypto/rng.c
> +++ b/crypto/rng.c
> @@ -48,12 +48,14 @@ int crypto_rng_reset(struct crypto_rng *tfm, const u8 *seed, unsigned int slen)
>  		if (!buf)
>  			return -ENOMEM;
>  
> -		get_random_bytes(buf, slen);
> +		err = get_random_bytes_wait(buf, slen);

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....

						- Ted