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From: Willy Tarreau <w@1wt.eu>
To: Jann Horn <jannh@google.com>
Cc: Jason Donenfeld <Jason@zx2c4.com>, KVM list <kvm@vger.kernel.org>,
	"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
	ghammer@redhat.com, "Weiss, Radu" <raduweis@amazon.com>,
	Qemu Developers <qemu-devel@nongnu.org>,
	"open list:VIRTIO GPU DRIVER"
	<virtualization@lists.linux-foundation.org>,
	Pavel Machek <pavel@ucw.cz>,
	Colm MacCarthaigh <colmmacc@amazon.com>,
	Jonathan Corbet <corbet@lwn.net>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Eric Biggers <ebiggers@kernel.org>,
	"Singh, Balbir" <sblbir@amazon.com>,
	bonzini@gnu.org, "Graf \(AWS\), Alexander" <graf@amazon.de>,
	oridgar@gmail.com, "Catangiu, Adrian Costin" <acatan@amazon.com>,
	Andy Lutomirski <luto@kernel.org>,
	Michal Hocko <mhocko@kernel.org>,
	"Theodore Y. Ts'o" <tytso@mit.edu>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	kernel list <linux-kernel@vger.kernel.org>,
	Linux API <linux-api@vger.kernel.org>,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	"Woodhouse, David" <dwmw@amazon.co.uk>
Subject: Re: [PATCH] drivers/virt: vmgenid: add vm generation id driver
Date: Sat, 17 Oct 2020 07:37:12 +0200
Message-ID: <20201017053712.GA14105@1wt.eu> (raw)
In-Reply-To: <CAG48ez1ZtvjOs2CEq8-EMosPCd_o7WQ3Mz_+1mDe7OrH2arxFA@mail.gmail.com>

On Sat, Oct 17, 2020 at 07:01:31AM +0200, Jann Horn wrote:
> Microsoft's documentation
> (http://go.microsoft.com/fwlink/?LinkId=260709) says that the VM
> Generation ID that we get after a fork "is a 128-bit,
> cryptographically random integer value". If multiple people use the
> same image, it guarantees that each use of the image gets its own,
> fresh ID:

No. It cannot be more unique than the source that feeds that cryptographic
transformation. All it guarantees is that the entropy source is protected
from being guessed based on the output. Applying cryptography on a simple
counter provides apparently random numbers that will be unique for a long
period for the same source, but as soon as you duplicate that code between
users and they start from the same counter they'll get the same IDs.

This is why I think that using a counter is better if you really need something
unique. Randoms only reduce predictability which helps avoiding collisions.
And I'm saying this as someone who had on his external gateway the same SSH
host key as 89 other hosts on the net, each of them using randoms to provide
a universally unique one...

Willy


  parent reply index

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <AQHWo8lIfZnFKGe8nkGmhTCXwq5R3w==>
2020-10-16 14:33 ` Catangiu, Adrian Costin
2020-10-16 15:00   ` Catangiu, Adrian Costin
2020-10-16 15:14   ` gregkh
2020-10-17  1:40   ` Jann Horn
2020-10-17  3:36     ` Willy Tarreau
2020-10-17  4:02       ` Jann Horn
2020-10-17  4:34         ` Colm MacCarthaigh
2020-10-17  5:01           ` Jann Horn
2020-10-17  5:29             ` Colm MacCarthaigh
2020-10-17  5:37             ` Willy Tarreau [this message]
2020-10-17  5:52               ` Jann Horn
2020-10-17  6:44                 ` Willy Tarreau
2020-10-17  6:55                   ` Jann Horn
2020-10-17  7:17                     ` Willy Tarreau
2020-10-17 13:24                     ` Jason A. Donenfeld
2020-10-17 18:06                       ` Catangiu, Adrian Costin
2020-10-17 18:09                       ` Alexander Graf
2020-10-18  2:08                         ` Jann Horn
2020-10-20  9:35                         ` Christian Borntraeger
2020-10-20  9:54                           ` Alexander Graf
2020-10-20 16:54                         ` Catangiu, Adrian Costin
2020-10-18  3:14                       ` Colm MacCarthaigh
2020-10-18 15:52                       ` Michael S. Tsirkin
2020-10-18 15:54                         ` Andy Lutomirski
2020-10-18 15:59                           ` Michael S. Tsirkin
2020-10-18 16:14                             ` Andy Lutomirski
2020-10-19 15:00                               ` Michael S. Tsirkin
2020-10-17 18:10     ` Andy Lutomirski
2020-10-19 17:15       ` Mathieu Desnoyers
2020-10-20 10:00         ` Alexander Graf

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