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

[Daniel P. Berrange]

On Thu, Oct 20, 2011 at 02:48:15PM -0700, Josh Durgin wrote:
> 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.

In absence of other suggestions, it sounds workable to me.

Daniel
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