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Re: [Qemu-devel] [PATCH V8 10/14] Encrypt state blobs using AES CBC encr

From: Stefan Berger
Subject: Re: [Qemu-devel] [PATCH V8 10/14] Encrypt state blobs using AES CBC encryption
Date: Thu, 08 Sep 2011 11:27:49 -0400
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On 09/08/2011 09:16 AM, Michael S. Tsirkin wrote:
On Thu, Sep 08, 2011 at 08:11:00AM -0400, Stefan Berger wrote:
On 09/08/2011 06:32 AM, Michael S. Tsirkin wrote:
On Wed, Sep 07, 2011 at 08:16:27PM -0400, Stefan Berger wrote:
On 09/07/2011 02:55 PM, Michael S. Tsirkin wrote:
On Thu, Sep 01, 2011 at 10:23:51PM -0400, Stefan Berger wrote:
An additional 'layer' for reading and writing the blobs to the underlying
block storage is added. This layer encrypts the blobs for writing if a key is
available. Similarly it decrypts the blobs after reading.
So a couple of further thoughts:
1. Raw storage should work too, and with e.g. NFS migration will be fine, right?
    So I'd say it's worth supporting.
NFS via shared storage, yes, but not migration via Qemu's block
migration mechanism. If snapshotting was supposed to be a feature to
support then that's only possible via block storage (QCoW2 in
As disk has the same limitation, that sounds fine.
Let the user decide whether snapshoting is needed,
same as disk.

Adding plain file support to the TPM code so it can store its 3
blobs into adds quite a bit of complexity to the code. The command
line parameter that previously pointed to QCoW2 image file would
probably have to point to a directory where files for the 3 blobs
can be written into. Besides that, snapshotting would actually have
to be prevented maybe through registering a (fake) file of other
than QCoW2 type since the plain TPM files won't handle snapshotting
correctly, either, and QEMU pretty much would have to be prevented
>from doing snapshotting at all. Maybe there's an API for this, but I
don't know. Though why create this additional complexity? I don't
mind relaxing the requirement of using a QCoW2 image and allowing
for example RAW images (that then automatically prevent the
snapshotting from happening) but the same code I now have would work
for writing the blobs into it the single file.
Right. Write all blobs into a single files at different
offsets, or something.
That's exactly what I am doing already. Just that I am doing this
with Qemu's BlockStorage (bdrv)  writing to sectors rather than
seek()ing in files. To avoid more complexity I'd rather not
introduce more code handling plain files but rely on all the image
formats that qemu already supports and that give features like
encryption (QCoW2 only), snapshotting (QCoW2 only) and block
migration (presumably all of them). Plain files offer none of that.
Devices that need to write their state to persistent storage really
have to aim for doing this through Qemu's bdrv since they will
otherwise be the ones killing the snapshot feature. TPM certainly
doesn't want to be one of them. If the user doesn't want
snapshotting to be supported since his VM image files are not QCoW2
type of files, just create a raw image file for the TPM's persistent
state and bdrv will automatically prevent snapshotting. The point is
that the TPM code now using the bdrv layer works with any image
format already.
Ah, that's fine then. I had an impression there was a qcow only
limitation, not sure what in code gave me that impression.
Hm, currently I force the image to be a QCoW2.

    bdrv_get_format(bs, buf, sizeof(buf));
    if (strcmp(buf, "qcow2")) {
        fprintf(stderr, "vTPM backing store must be of type qcow2\n");
        goto err_exit;

I can remove this and we should be fine.
2. File backed nvram is interesting outside tpm.
    For example,vpd and chassis number for pci, eeprom emulation for network 
    Using a file per device might be inconvenient though.
    So please think of a format and API that will allow sections
    for use by different devices.
Also here 'snapshotting' is the most 'demanding' feature of QEMU I
would say. Snapshotting isn't easily supported outside of the block
layer from what I understand. Once you are tied to the block layer
you end up having to use images and those don't grow quite well. So
other devices wanting to use those type of devices would need to
know what the worst case sizes are for writing their state into --
unless an image format is created that can grow.

As for the format: Ideally all devices could write into one file,
right? That would at least prevent too many files besides the VM's
image file from floating around which presumably makes image
management easier. Following the above, you add up all the worst
case sizes the individual devices may need for their blobs and
create an image with that capacity. Then you need some form of a
(primitive?) directory that lets you write blobs into that storage.
Assuming there were well defined names for those devices one could
say for example store this blobs under the name
'tpm-permanent-state' and later on load it under that name. The
possible size of the directory would have to be considered as
well... I do something like that for the TPM where I have up to 3
such blobs that I store.

The bad thing about the above is of course the need to know what the
sum of all the worst case sizes is.
A typical usecase I know about has prepared vpd/eeprom content.
We'll typically need a tool to get binary blobs and put that into the
file image.  That tool can do the necessary math.
We could also integrate this into qemu-img if we like.

So a growable image format would
be quite good to have. I haven't followed the conversations much,
but is that something QCoW3 would support?
I don't follow - does TPM need a growable image format? Why?
Hardware typically has fixed amount of memory :)
Ideally the user wouldn't have to worry about creating the single
file for persistent storage for all the devices at all but Qemu
could 'somehow' do this.
Assume the user starts the VM with a device having an EEPROM. Now
that device has the need for 10k of persistent storage. So somehow
with the limitations of images that don't grow you have to have
created an image of at least 10k a priori. Later the user adds
another device to the same VM that needs 40k of persistent storage.
What now? Dispose the old image with the EPPROM data and create a
new image with at least 50k to hold both their data? Or add another
image with just 40k to hold that device's persistent state? I'd
rather have the 10k image grow to 50k and accommodate both state
I see, yes, might be useful. But even without that,
simple users - without hotplug - will be able to have
a single file with all data, and advanced users will
be able to have a file per device. Not ideal
but I think manageable.

As long as the management software keeps everything in one place (dir with subdirs?) it should be manageable. User just need to remember to pass on several files then for a single VM.
3. Home-grown file formats give us enough trouble in migration.
    Could this use one of the variants of ASN.1?
    There are portable libraries to read/write that, even.

I am not sure what 'this' refers to. What I am doing with the TPM is
writing 3 independent blobs at certain offset into the QCoW2 block
file. A directory in the first sector holds the offsets, sizes and
crc32's of these (unencrypted) blobs.
By the way, why do we checksum data? Should be optional?

It lets one detect corruption.
In case encryption is being used for the individual blobs (that's NOT the QCoW2 native encryption but the one I added and that works for RAW images as well) I can use this crc32 after trying to decrypt the data. If the crc32 doesn't match the expected one it was either the wrong key or the data is corrupted. At least I can react to it and tell the user that the data could not be decrypted or are corrupted and terminate Qemu rather than having the device go into panic/shutdown mode on the bad state.

I am not that familiar with ASN.1 except that from what I have seen
it looks like a fairly terrible format needing an object language to
create a parser from etc. not to mention the problems I had with
snacc trying to compile the ASN.1 object language of an RFC...

Sorry about the confusion, we don't need the notation, I don't mean that.
I mean use a subset of the ASN.1 basic encoding

So we could have a set of sequences, with an ascii string (a tag)
followed by an octet string (content).

I think the data layout in the image should be in such format that
you don't have to re-write the whole content of the image if a blob
is stored.
With predefined blob size, we can use octet strings and not
have to rewrite anything, find the right octet and change it inplace.

I think a directory at the beginning could solve this.
It could, but it's not needed for that.

To make it simple one probably would need to know how big the
'directory' could be otherwise one has to get into allocation of
sectors so that once the directory was to grow beyond 512 bytes that
one would know where its next data are written into. The same is
true for the devices' data blobs. If one knows the sizes of all the
blobs one can lay them out to start and end at specific offsets in
the image. And knowing the size of all the blobs helps in creating
the image of correct size.
Well, all this is a work-around for not having a 'filesystem'.


Sounds like overkill. A sequence with tags in DER format is much easier.

What data should be encoded using DER?

- indication whether encryption was used on all blobs
- a sequence of
    - name of the blob
    - worst-case size of the blob
    - actual size of the blob
    - crc32 of the blob
    - the actual blob


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