Re: [Qemu-devel] [PATCH v8 1/2] block/vxhs.c: Add support for a new block device type called "vxhs"

[Ketan Nilangekar]

> On Mar 8, 2017, at 1:13 AM, Daniel P. Berrange <address@hidden> wrote:
> 
>> On Tue, Mar 07, 2017 at 05:27:55PM -0800, ashish mittal wrote:
>> Thanks! There is one more input I need some help with!
>> 
>> VxHS network library opens a fixed number of connection channels to a
>> given host, and all the vdisks (that connect to the same host) share
>> these connection channels.
>> 
>> Therefore, we need to open secure channels to a specific target host
>> only once for the first vdisk that connects to that host. All the
>> other vdisks that connect to the same target host will share the same
>> set of secure communication channels.
>> 
>> I hope the above scheme is acceptable?
> 
> I don't think I'm in favour of such an approach, as it forces a single
> QEMU process to use the same privileges for all disks it uses.
> 
> Consider an example where a QEMU process has two disks, one shared
> readonly disk and one exclusive writable disk, both on the same host.
> 

This is not a use case for VxHS as a solution. We do not support sharing of 
vdisks across QEMU instances.

Vxhs library was thus not designed to open different connections for individual 
vdisks within a QEMU instance.

Implementing this will involve rewrite of significant parts of libvxhs client 
and server. Is this a new requirement for acceptance into QEMU?


> It is reasonable as an administrator to want to use different credentials
> for each of these. ie, they might have a set of well known credentials to
> authenticate to get access to the read-only disk, but have a different set
> of strictly limited access credentials to get access to the writable disk
> 
> Trying to re-use the same connection for multiple cause prevents QEMU from
> authenticating with different credentials per disk, so I don't think that
> is a good approach - each disk should have totally independant state.
> 
>> 
>> If yes, then we have a couple of options to implement this:
>> 
>> (1) Accept the TLS credentials per vdisk using the previously
>> discussed --object tls-creds-x509 mechanism. In this case, if more
>> than one vdisk have the same host info, then we will use only the
>> first one's creds to set up the secure connection, and ignore the
>> others. vdisks that connect to different target hosts will use their
>> individual tls-creds-x509 to set up the secure channels. This is, of
>> course, not relevant for qemu-img/qemu-io as they can open only one
>> vdisk at a time.
>> 
>> (2) Instead of having a per-vdisk --object tls-creds-x509, have a
>> single such argument on the command line for vxhs block device code to
>> consume - if that is possible! One way to achieve this could be the
>> user/password authentication we discussed earlier, which we could use
>> to pass the directory where cert/keys are kept.
>> 
>> (3) Use the instance UUID, when available, to lookup the cert files
>> per instance (i.e. for qemu-kvm), and use the --object tls-creds-x509
>> mechanism, when instance UUID is NULL (i.e. qemu-io, qemu-img etc).
>> The cert/key files are anyway protected by file permissions in either
>> case, so I guess there is no additional security provided by either
>> method.
> 
> Regards,
> Daniel
> -- 
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