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Re: [libvirt] [PATCH v3] Added new attribute mount_security to filesystem element

On 10/13/2010 8:21 AM, Daniel P. Berrange wrote:
> On Mon, Oct 11, 2010 at 12:21:19PM +0530, Harsh Prateek Bora wrote:
>> This patch introduces new attribute to filesystem element
>> to support customizable security for mount type.
>> Valid mount_security are: passthrough and mapped.
>> Usage:
>> 	<filesystem type='mount' mount_security='passthrough'>
>> 	<source dir='/export/to/guest'/>
>> 	<target dir='mount_tag'/>
>> 	</filesystem>
>> Here is the detailed explanation on these security models:
>> Security model: mapped
>> ----------------------
>> Fileserver intercepts and maps all the file object create requests.
>> Files on the fileserver will be created with Fileserver's user credentials
>> and the
>> client-user's credentials are stored in extended attributes.
>> During getattr() server extracts the client-user's credentials from extended
>> attributes and sends to the client.
>> This adds a great deal of security in the cloud environments where the
>> guest's(client) user space is kept completely isolated from host's user
>> space.
>> Security model : passthrough
>> ----------------------------
>> In this security model, Fileserver passes down all requests to the
>> underlying filesystem. File system objects on the fileserver will be created
>> with client-user's credentials. This is done by setting setuid()/setgid()
>> during creation or chmod/chown after file creation. At the end of create
>> protocol
>> request, files on the fileserver will be owned by cleint-user's uid/gid.
>> This model mimic's current NFSv3 level of security.
> In your first patch you had a 3rd option 'none', whose semantics I
> had asked about, because they appeared to be the same as passthrough.
> Looking at the QEMU virtio-9p.c file comments though, it appears
> that there is in fact a difference. 
>  - In 'passthrough' the user/group ownership is preserved from the guest
>    requests. 
>  - In 'mapped' the guest user/group ownership is stored in xtended attrs
>    with files on host keeping same uid/gid as the QEMU process
>  - In 'none' the user/group ownership from guest is completely ignored,
>    and all files just have uid/gid matching the QEMU process.
> If this interpretation of QEMU code is correct, then I think we should
> support all 3 options in libvirt after all. 
> The 'passthrough' option should be the default in libvirt, because that
> matches the semantics of the <filesystem> element usage in LXC / OpenVZ
> drivers in libvirt.
> I think I'd give 'none' a different name in the XML , perhaps call it 
> 'squash', inspired by NFS root squash, which squashes uid/gid onto a 
> single user.
> Finally I'm thinking that the security attribute should be called
> 'accessmode' rather than 'mount_security', mostly because I don't
> like  underscores in XML attribute/element names.

I am complete in agreement with all the observations above.
Harsh can quickly make these changes and repost the patch.


> Regards,
> Daniel

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