linux-kernel - Re: propagating vmgenid outward and upward

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Date:   Wed, 2 Mar 2022 10:20:25 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Laszlo Ersek <lersek@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        KVM list <kvm@...r.kernel.org>,
        QEMU Developers <qemu-devel@...gnu.org>,
        linux-hyperv@...r.kernel.org,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        Alexander Graf <graf@...zon.com>,
        "Michael Kelley (LINUX)" <mikelley@...rosoft.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        adrian@...ity.io,
        Daniel P. Berrangé <berrange@...hat.com>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Jann Horn <jannh@...gle.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        "Brown, Len" <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
        Linux PM <linux-pm@...r.kernel.org>,
        Colm MacCarthaigh <colmmacc@...zon.com>,
        Theodore Ts'o <tytso@....edu>, Arnd Bergmann <arnd@...db.de>
Subject: Re: propagating vmgenid outward and upward

On Wed, Mar 02, 2022 at 04:14:56PM +0100, Jason A. Donenfeld wrote:
> Hi Michael,
> 
> On Wed, Mar 2, 2022 at 3:46 PM Michael S. Tsirkin <mst@...hat.com> wrote:
> > I just don't see how "value changed while it was read" is so different
> > from "value changed one clock after it was read".  Since we don't detect
> > the latter I don't see why we should worry about the former.
> 
> The "barrier" is at the point where the plaintext has been chosen AND
> the nonce for a given keypair has been selected. So, if you have
> plaintext in a buffer, and a key in a buffer, and the nonce for that
> encryption in a buffer, and then after those are all selected, you
> check to see if the vmgenid has changed since the birth of that key,
> then you're all set. If it changes _after_ that point of check (your
> "one clock after"), it doesn't matter: you'll just be
> double-transmitting the same ciphertext, which is something that flaky
> wifi sometimes does _anyway_ (and attackers can do intentionally), so
> network protocols already are resilient to replay. This is the same
> case you asked about earlier, and then answered yourself, when you
> were wondering about reaching down into qdiscs.
> 
> Jason

So writing some code:

1:
	put plaintext in a buffer
	put a key in a buffer
	put the nonce for that encryption in a buffer

	if vm gen id != stored vm gen id
		stored vm gen id = vm gen id
		goto 1

I think this is race free, but I don't see why does it matter whether we
read gen id atomically or not.

-- 
MST