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Date:   Sat, 17 Oct 2020 11:10:26 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Jann Horn <jannh@...gle.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     "Catangiu, Adrian Costin" <acatan@...zon.com>,
        Andy Lutomirski <luto@...nel.org>,
        Jason Donenfeld <Jason@...c4.com>,
        "Theodore Y. Ts'o" <tytso@....edu>, Willy Tarreau <w@....eu>,
        Eric Biggers <ebiggers@...nel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "Graf (AWS), Alexander" <graf@...zon.de>,
        "MacCarthaigh, Colm" <colmmacc@...zon.com>,
        "Woodhouse, David" <dwmw@...zon.co.uk>,
        "bonzini@....org" <bonzini@....org>,
        "Singh, Balbir" <sblbir@...zon.com>,
        "Weiss, Radu" <raduweis@...zon.com>,
        "oridgar@...il.com" <oridgar@...il.com>,
        "ghammer@...hat.com" <ghammer@...hat.com>,
        "corbet@....net" <corbet@....net>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "mst@...hat.com" <mst@...hat.com>,
        "qemu-devel@...gnu.org" <qemu-devel@...gnu.org>,
        KVM list <kvm@...r.kernel.org>,
        Michal Hocko <mhocko@...nel.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Pavel Machek <pavel@....cz>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH] drivers/virt: vmgenid: add vm generation id driver

On Fri, Oct 16, 2020 at 6:40 PM Jann Horn <jannh@...gle.com> wrote:
>
> [adding some more people who are interested in RNG stuff: Andy, Jason,
> Theodore, Willy Tarreau, Eric Biggers. also linux-api@, because this
> concerns some pretty fundamental API stuff related to RNG usage]
>
> On Fri, Oct 16, 2020 at 4:33 PM Catangiu, Adrian Costin
> <acatan@...zon.com> wrote:
> > - Background
> >
> > The VM Generation ID is a feature defined by Microsoft (paper:
> > http://go.microsoft.com/fwlink/?LinkId=260709) and supported by
> > multiple hypervisor vendors.
> >
> > The feature is required in virtualized environments by apps that work
> > with local copies/caches of world-unique data such as random values,
> > uuids, monotonically increasing counters, etc.
> > Such apps can be negatively affected by VM snapshotting when the VM
> > is either cloned or returned to an earlier point in time.
> >
> > The VM Generation ID is a simple concept meant to alleviate the issue
> > by providing a unique ID that changes each time the VM is restored
> > from a snapshot. The hw provided UUID value can be used to
> > differentiate between VMs or different generations of the same VM.
> >
> > - Problem
> >
> > The VM Generation ID is exposed through an ACPI device by multiple
> > hypervisor vendors but neither the vendors or upstream Linux have no
> > default driver for it leaving users to fend for themselves.
> >
> > Furthermore, simply finding out about a VM generation change is only
> > the starting point of a process to renew internal states of possibly
> > multiple applications across the system. This process could benefit
> > from a driver that provides an interface through which orchestration
> > can be easily done.
> >
> > - Solution
> >
> > This patch is a driver which exposes the Virtual Machine Generation ID
> > via a char-dev FS interface that provides ID update sync and async
> > notification, retrieval and confirmation mechanisms:
> >
> > When the device is 'open()'ed a copy of the current vm UUID is
> > associated with the file handle. 'read()' operations block until the
> > associated UUID is no longer up to date - until HW vm gen id changes -
> > at which point the new UUID is provided/returned. Nonblocking 'read()'
> > uses EWOULDBLOCK to signal that there is no _new_ UUID available.
> >
> > 'poll()' is implemented to allow polling for UUID updates. Such
> > updates result in 'EPOLLIN' events.
> >
> > Subsequent read()s following a UUID update no longer block, but return
> > the updated UUID. The application needs to acknowledge the UUID update
> > by confirming it through a 'write()'.
> > Only on writing back to the driver the right/latest UUID, will the
> > driver mark this "watcher" as up to date and remove EPOLLIN status.
> >
> > 'mmap()' support allows mapping a single read-only shared page which
> > will always contain the latest UUID value at offset 0.
>
> It would be nicer if that page just contained an incrementing counter,
> instead of a UUID. It's not like the application cares *what* the UUID
> changed to, just that it *did* change and all RNGs state now needs to
> be reseeded from the kernel, right? And an application can't reliably
> read the entire UUID from the memory mapping anyway, because the VM
> might be forked in the middle.
>
> So I think your kernel driver should detect UUID changes and then turn
> those into a monotonically incrementing counter. (Probably 64 bits
> wide?) (That's probably also a little bit faster than comparing an
> entire UUID.)
>
> An option might be to put that counter into the vDSO, instead of a
> separate VMA; but I don't know how the other folks feel about that.
> Andy, do you have opinions on this? That way, normal userspace code
> that uses this infrastructure wouldn't have to mess around with a
> special device at all. And it'd be usable in seccomp sandboxes and so
> on without needing special plumbing. And libraries wouldn't have to
> call open() and mess with file descriptor numbers.

The vDSO might be annoyingly slow for this.  Something like the rseq
page might make sense.  It could be a generic indication of "system
went through some form of suspend".

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