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Message-ID: <20170618154625.5qu3eduqjtgk5bal@thunk.org>
Date: Sun, 18 Jun 2017 11:46:25 -0400
From: Theodore Ts'o <tytso@....edu>
To: Stephan Müller <smueller@...onox.de>
Cc: kernel-hardening@...ts.openwall.com,
Michael Ellerman <mpe@...erman.id.au>,
"Jason A. Donenfeld" <Jason@...c4.com>,
Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
LKML <linux-kernel@...r.kernel.org>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
David Miller <davem@...emloft.net>,
Eric Biggers <ebiggers3@...il.com>
Subject: Re: Re: [PATCH v4 13/13] random: warn when kernel
uses unseeded randomness
On Thu, Jun 15, 2017 at 01:59:43PM +0200, Stephan Müller wrote:
> I would think that the issue regarding the logging is relevant for
> cryptographic use cases or use cases requiring strong random numbers only.
> Only those use cases should be fixed eventually to wait for a fully seeded
> DRNG.
>
> The logged messages you present here indicate use cases where no strong
> security is required. It looks like that the logs show ASLR related use of
> random numbers. Those do not require a fully seeded ChaCha20 DRNG.
I suspect there is a range of opinions aobut whether or not ASLR
requires strongly secure random numbers or not. It seems pretty clear
that if we proposed using prandom_u32 for ASLR, people would object
very strongly indeed, since that would make it trivially easy for
attackers to circumvent ASLR protections.
> IMHO, users using the get_random_u64 or get_random_u32 are use cases that do
> not require a fully seeded DRNG thus do not need a cryptographically strong
> random number. Hence, I would think that the logging should be removed from
> get_random_u32/u64.
You are effectively proposing that there ought to be a middle range of
security between prandom_32, get_random_u32/get_random_u64 and
get_random_bytes(). I think that's going to lead to all sorts of
complexity and bugs from people not understanding when they should use
get_random_u32 vs get_random_bytes versus prandom_u32. And then we'll
end up needing to audit all of the callsites for get_random_u32() so
they don't violate this new usage rule that you are proposing.
- Ted
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