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From: "Michael S. Tsirkin" <mst@redhat.com>
To: "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc: KVM list <kvm@vger.kernel.org>,
	"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
	ghammer@redhat.com, "Weiss, Radu" <raduweis@amazon.com>,
	Qemu Developers <qemu-devel@nongnu.org>,
	"open list:VIRTIO GPU DRIVER"
	<virtualization@lists.linux-foundation.org>,
	Pavel Machek <pavel@ucw.cz>,
	Colm MacCarthaigh <colmmacc@amazon.com>,
	Jonathan Corbet <corbet@lwn.net>,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	Eric Biggers <ebiggers@kernel.org>,
	"Singh, Balbir" <sblbir@amazon.com>,
	bonzini@gnu.org, "Graf \(AWS\), Alexander" <graf@amazon.de>,
	Jann Horn <jannh@google.com>,
	oridgar@gmail.com, "Catangiu, Adrian Costin" <acatan@amazon.com>,
	Andy Lutomirski <luto@kernel.org>,
	Michal Hocko <mhocko@kernel.org>,
	"Theodore Y. Ts'o" <tytso@mit.edu>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	kernel list <linux-kernel@vger.kernel.org>,
	Linux API <linux-api@vger.kernel.org>, Willy Tarreau <w@1wt.eu>,
	"Woodhouse, David" <dwmw@amazon.co.uk>
Subject: Re: [PATCH] drivers/virt: vmgenid: add vm generation id driver
Date: Sun, 18 Oct 2020 11:52:06 -0400
Message-ID: <20201018114625-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <CAHmME9qHGSF8w3DoyCP+ud_N0MAJ5_8zsUWx=rxQB1mFnGcu9w@mail.gmail.com>

On Sat, Oct 17, 2020 at 03:24:08PM +0200, Jason A. Donenfeld wrote:
> 4c. The guest kernel maintains an array of physical addresses that are
> MADV_WIPEONFORK. The hypervisor knows about this array and its
> location through whatever protocol, and before resuming a
> moved/snapshotted/duplicated VM, it takes the responsibility for
> memzeroing this memory. The huge pro here would be that this
> eliminates all races, and reduces complexity quite a bit, because the
> hypervisor can perfectly synchronize its bringup (and SMP bringup)
> with this, and it can even optimize things like on-disk memory
> snapshots to simply not write out those pages to disk.
> 
> A 4c-like approach seems like it'd be a lot of bang for the buck -- we
> reuse the existing mechanism (MADV_WIPEONFORK), so there's no new
> userspace API to deal with, and it'd be race free, and eliminate a lot
> of kernel complexity.

Clearly this has a chance to break applications, right?
If there's an app that uses this as a non-system-calls way
to find out whether there was a fork, it will break
when wipe triggers without a fork ...
For example, imagine:

MADV_WIPEONFORK
copy secret data to MADV_DONTFORK
fork


used to work, with this change it gets 0s instead of the secret data.


I am also not sure it's wise to expose each guest process
to the hypervisor like this. E.g. each process needs a
guest physical address of its own then. This is a finite resource.


The mmap interface proposed here is somewhat baroque, but it is
certainly simple to implement ...

-- 
MST



  parent reply index

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <AQHWo8lIfZnFKGe8nkGmhTCXwq5R3w==>
2020-10-16 14:33 ` Catangiu, Adrian Costin
2020-10-16 15:00   ` Catangiu, Adrian Costin
2020-10-16 15:14   ` gregkh
2020-10-17  1:40   ` Jann Horn
2020-10-17  3:36     ` Willy Tarreau
2020-10-17  4:02       ` Jann Horn
2020-10-17  4:34         ` Colm MacCarthaigh
2020-10-17  5:01           ` Jann Horn
2020-10-17  5:29             ` Colm MacCarthaigh
2020-10-17  5:37             ` Willy Tarreau
2020-10-17  5:52               ` Jann Horn
2020-10-17  6:44                 ` Willy Tarreau
2020-10-17  6:55                   ` Jann Horn
2020-10-17  7:17                     ` Willy Tarreau
2020-10-17 13:24                     ` Jason A. Donenfeld
2020-10-17 18:06                       ` Catangiu, Adrian Costin
2020-10-17 18:09                       ` Alexander Graf
2020-10-18  2:08                         ` Jann Horn
2020-10-20  9:35                         ` Christian Borntraeger
2020-10-20  9:54                           ` Alexander Graf
2020-10-20 16:54                         ` Catangiu, Adrian Costin
2020-10-18  3:14                       ` Colm MacCarthaigh
2020-10-18 15:52                       ` Michael S. Tsirkin [this message]
2020-10-18 15:54                         ` Andy Lutomirski
2020-10-18 15:59                           ` Michael S. Tsirkin
2020-10-18 16:14                             ` Andy Lutomirski
2020-10-19 15:00                               ` Michael S. Tsirkin
2020-10-17 18:10     ` Andy Lutomirski
2020-10-19 17:15       ` Mathieu Desnoyers
2020-10-20 10:00         ` Alexander Graf

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