linux-kernel - Re: propagating vmgenid outward and upward

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Date:   Tue, 1 Mar 2022 19:37:06 +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 Tue, Mar 1, 2022 at 6:17 PM Michael S. Tsirkin <mst@...hat.com> wrote:
> Hmm okay, so it's a performance optimization... some batching then? Do
> you really need to worry about every packet? Every 64 packets not
> enough?  Packets are after all queued at NICs etc, and VM fork can
> happen after they leave wireguard ...

Unfortunately, yes, this is an "every packet" sort of thing -- if the
race is to be avoided in a meaningful way. It's really extra bad:
ChaCha20 and AES-CTR work by xoring a secret stream of bytes with
plaintext to produce a ciphertext. If you use that same secret stream
and xor it with a second plaintext and transmit that too, an attacker
can combine the two different ciphertexts to learn things about the
original plaintext.

But, anyway, it seems like the race is here to stay given what we have
_currently_ available with the virtual hardware. That's why I'm
focused on trying to get something going that's the least bad with
what we've currently got, which is racy by design. How vitally
important is it to have something that doesn't race in the far future?
I don't know, really. It seems plausible that that ACPI notifier
triggers so early that nothing else really even has a chance, so the
race concern is purely theoretical. But I haven't tried to measure
that so I'm not sure.

Jason