linux-kernel - Re: propagating vmgenid outward and upward

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Message-ID: <Yh+cB5bWarl8CFN1@zx2c4.com>
Date:   Wed, 2 Mar 2022 17:32:07 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     "Michael S. Tsirkin" <mst@...hat.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

Hi Michael,

On Wed, Mar 02, 2022 at 11:22:46AM -0500, Michael S. Tsirkin wrote:
> > Because that 16 byte read of vmgenid is not atomic. Let's say you read
> > the first 8 bytes, and then the VM is forked.
> 
> But at this point when VM was forked plaintext key and nonce are all in
> buffer, and you previously indicated a fork at this point is harmless.
> You wrote "If it changes _after_ that point of check ... it doesn't
> matter:"

Ahhh, fair point. I think you're right.

Alright, so all we're talking about here is an ordinary 16-byte read,
and 16 bytes of storage per keypair, and a 16-byte comparison.

Still seems much worse than just having a single word...

Jason