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Re: [libvirt] passing secrets to block devices



On 10/20/2011 12:24 PM, Daniel P. Berrange wrote:
On Thu, Oct 20, 2011 at 11:30:42AM -0700, Josh Durgin wrote:
We're working on libvirt support for block device authentication [1]. To
authenticate, rbd needs a username and a secret. Normally, to
avoid putting the secret on the command line, you can store the secret
in a file and pass the file to qemu, but when this is automated,
there's no good way to know when the file can be removed. There are
a few ways to pass the secret to qemu that avoid this problem:

This is the same problem the iSCSI block driver currently faces,
and also if the Curl/HTTP block driver wanted todo authentication
we'd hit this. So it isn't unique to Ceph/RBD.

1) pass an fd to an unlinked file containing the secret

This is the simplest method, but it sounds like qemu developers don't
like fd passing from libvirt. [2]

That would be workable, but it means people trying to run the libvirt
QEMU command line themselves, would have to remove some args.

Isn't this already the case for chardevs? I can understand not wanting more things like that though.

2) start guests paused, without disks requiring authentication, then
    use the drive_add monitor command to attach them

This would make disks with authentication somewhat of a special case
in libvirt, but would be simple to implement, and require no qemu changes.

This makes it very hard for people to take the libvirt QEMU command line
and run themselves, since now an entire chunk of it is just missing.
So I really don't want to go down this route.

3) start guests paused, then send the secret via a new QMP/HMP
    command (block_set_conf<key>  <value>?)

This is a larger change, but it would be more generally useful for
changing configuration at runtime.

I don't think you need to try to solve the problem of a general
purpose 'set configuration' command here, not least because that
will likely get you drawn into a huge discussion about qemu device
configuration in general which will likely never end.

We already have a 'block_passwd' command for setting qcow2 decryption
keys. These aren't decryption passwords, rather they are authentication
passwords, so they're a little different, but I think this command could
still likely be leveraged for Ceph/iSCSI/etc auth passwords.

Ideally, we want to cope with having both a decryption&  auth password
for the same block device. eg, an encrypted qcow2 image accessed, over
HTTP would require both. In these case there are 2 block drivers involved,
the 'qcow2' driver and the 'http' driver. So perhaps an extra parameter
for the 'block_password' command to identify which driver the password
is intended for is the right approach. If omitted,we'd default to 'qcow2'
for back compat.

So eg, for a encrypted qcow2 disk accessed over http

    -drive  file=http://fred host/my.iso,format=qcow2,id=mydrive

the app would invoke

   { "execute": "block_password", "argument": { "device": "mydrive",
                                                "driver", "qcow2",
                                                "password", "12345" } }
   { "execute": "block_password", "argument": { "device": "mydrive",
                                                "driver", "curl",
                                                "password", "7890" } }

For Ceph/RBD with a plain file, you'd just do


   { "execute": "block_password", "argument": { "device": "mydrive",
                                                "driver", "rbd",
                                                "password", "7890" } }


This sounds good to me, although the same driver might use authentication and encryption. Adding another argument to specify 'auth' or 'encryption' would fix this, i.e.:

  { "execute": "block_password", "argument": { "device": "mydrive",
                                               "driver": "qcow2",
                                               "use": "encryption"
                                               "password": "12345" } }

I'll prepare a patch if there are no objections to this approach.


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