linux-kernel - Re: [kernel-hardening] Re: get_random_bytes returns bad randomness before seeding is complete

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Message-ID: <CAHmME9rSNdTYK2GiazG0y_9POnBf__=puWJwrtKVptZxBXNiaA@mail.gmail.com>
Date:   Fri, 2 Jun 2017 19:46:43 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Daniel Micay <danielmicay@...il.com>
Cc:     Stephan Mueller <smueller@...onox.de>,
        "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
Subject: Re: [kernel-hardening] Re: get_random_bytes returns bad randomness
 before seeding is complete

On Fri, Jun 2, 2017 at 7:41 PM, Daniel Micay <danielmicay@...il.com> wrote:
> One of the early uses is initializing the stack canary value for SSP in
> very early boot. If that blocks, it's going to be blocking nearly
> anything else from happening.
>
> On x86, that's only the initial canary since the per-task canaries end
> up being used, but elsewhere at least without SMP disabled or changes to
> GCC that's all there is so the entropy matters.

If this is the case, then we simply need a function called
get_random_bytes_but_potentially_crappy_ones_because_we_are_desperate_for_anything(),
which would respond with a weaker guarantee than that
get_random_bytes(), which the documentation says always returns
cryptographically secure numbers.