Re: [libvirt] [PATCH v3] Added new attribute mount_security to filesystem element
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.
Regards,
Daniel
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