Re: [Qemu-devel] passing secrets to block devices

[Daniel P. Berrange]

On Fri, Oct 21, 2011 at 10:30:39AM +0200, Kevin Wolf wrote:
> Am 20.10.2011 23:48, schrieb Josh Durgin:
> > 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://address@hidden/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.
> 
> This proposed interface solves a problem that is currently purely
> theoretical. With blockdev-add and friends, we'll get all of this for
> free, so I'm not excited about adding something preliminary now even
> though there's no practical need.

Ok.

> For the rbd driver, please use the existing interface that qcow2 uses
> for encrypted images.

If you're ok with just reusing 'block_passwd' as it is today, with
no arg parameters, then that's fine with me too.

Daniel
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