linux-kernel - arch_random

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Date:	Mon, 12 May 2014 01:01:58 +0200
From:	Stephan Mueller <smueller@...onox.de>
To:	"H. Peter Anvin" <hpa@...ux.intel.com>
Cc:	Theodore Ts'o <tytso@....edu>, LKML <linux-kernel@...r.kernel.org>,
	linux-crypto@...r.kernel.org
Subject: arch_random_refill

Hi Peter,

some time back when the RDRAND instruction was debated, a patch was offered 
for driver/char/random.c that in essence turned /dev/random into a frontend 
for RDRAND in case that instruction was available. The patch kind of 
monopolized the noise sources such that if a user space "random number hog" 
pulled data from /dev/random endlessly, the (almost) only noise source was 
RDRAND. As that patch treated RDRAND to provide entropy, the blocking behavior 
went away for /dev/random.

That patch did not sit well with some developers and it got finally changed 
such that the output of RDRAND is now just XORed with the output of the 
"classic" /dev/random behavior -- /dev/random is still blocking. 

With the current development cycle for 3.15, the function arch_random_refill 
is added as presented in [1]. It now uses RDSEED instead of RDRAND. Yet, the 
way this function is called in random_read seems (as I have no system with an 
RDSEED, I cannot test) to show the very same behavior as the aforementioned 
RDRAND patch: the blocking behavior of /dev/random will be gone and RDSEED 
will monopolize the noise sources in case of a user space hog.

Note, I do not see an issue with the patch that adds RDSEED as part of 
add_interrupt_randomness outlined in [2]. The reason is that this patch does 
not monopolizes the noise sources.

I do not want to imply that Intel (or any other chip manufacturer that will 
hook into arch_random_refill) intentionally provides bad entropy (and this 
email shall not start a discussion about entropy again), but I would like to 
be able to only use noise sources that I can fully audit. As it is with 
hardware, I am not able to see what it is doing.

Thus, may I ask that arch_random_refill is revised such that it will not 
monopolize the noise sources? If somebody wants that, he can easily use rngd.

[1] https://lkml.org/lkml/2014/3/4/968

[2] https://lkml.org/lkml/2014/3/4/934

Thanks
Stephan
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