linux-kernel - Re: propagating vmgenid outward and upward

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Date:   Wed, 2 Mar 2022 11:22:46 -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 04:36:49PM +0100, Jason A. Donenfeld wrote:
> Hi Michael,
> 
> On Wed, Mar 02, 2022 at 10:20:25AM -0500, Michael S. Tsirkin wrote:
> > 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.
> 
> 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:"

> In the forked VM, the next
> 8 bytes are the same as last time, but the first 8 bytes, which you
> already read, have changed. In that case, your != becomes a ==, and the
> test fails.

Yes I'm aware what an atomic read is. If the read is not atomic
a part of value can change ;)

-- 
MST