linux-kernel - Re: [PATCH] random: Add "initialized" variable to proc

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Date:	Wed, 30 Apr 2014 21:05:00 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"Theodore Ts'o" <tytso@....edu>,
	Andy Lutomirski <luto@...capital.net>
CC:	Florian Weimer <fweimer@...hat.com>, linux-kernel@...r.kernel.org,
	Kees Cook <kees@...flux.net>
Subject: Re: [PATCH] random: Add "initialized" variable to proc

Qemu is using /dev/random because there is no point in emptily feeding one prng from another.


Giving the guest a seed would be highly useful, though.  There are a number of ways to do that; changing the boot protocol is probably only useful if Qemu itself bouts the kernel as opposed to an in-VM bootloader.

That leaves several options other than virtio: I/O port, magic number in memory, CPUID, MSR, or even emulating RDRAND/RDSEED in the VMM.

On April 30, 2014 7:06:27 PM PDT, Theodore Ts'o <tytso@....edu> wrote:
>On Wed, Apr 30, 2014 at 01:52:35PM -0700, Andy Lutomirski wrote:
>> 
>> 1. It simply doesn't work on my system.  In particular, it never
>returns
>> entropy.  It just blocks forever.
>
>Why?  Is this a bug in qemu?  The host OS?  The guest OS?  It is qemu
>trying to use /dev/random instead of /dev/urandom?  Any thing else?
>
>> 3. There should be a way to provide some entropy-free
>cryptographically
>> secure data, too.  Regardless of the speed of the hosts's
>/dev/random,
>> the guest should start with at least 256 bits of cryptographically
>> secure seed material IMO.
>
>Well, the simplest way to do this is to pass it in via the command
>line, and then have the the kernel make sure it gets obscured so it's
>not exposed via /proc/cmdline.
>
>Otherwise we would have to define an extension where we pass 32 bytes
>or so after the boot command line.  But the downside of doing that is
>we would have to modify every single architecture to define where
>those 32 bytes could be found.
>
>Aside from passing it on the command line as being a bit grotty, the
>other big problem this is that some architectures only have 256 bytes
>of command line, and if we use a base 64 encoding, 256 bits will take
>43 characters.  Not a problem on x86, and it seems rather unlikely
>that people would want to virtualize a m68k or avr32 CPU.  It just
>feels really unclean.
>
>I've cc'ed Peter Anvin for his opinion about extending Linux boot
>parameter protocol.  I agree it would be a lot simpler and easier to
>enable things like Kernel ASLR with real randomness on guest OS's if
>we didn't have to erect the whole virtio-pci infrastructure during
>early boot.  :-)
>
>						 -Ted

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