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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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