Re: [PATCH 5/5] NFSv3: Add server namespace support for XATTR protocol implementation

Casey Schaufler
2010-03-01T00:56:05Z

Stephen Smalley wrote:
> On Fri, 2010-02-26 at 15:37 +1100, James Morris wrote:
>
>> Add support for a server namespace for storing and retrieving
>> client-supplied xattrs. All xattrs are now stored on the server under
>> the user._nfsd namespace, and are not interpreted by the server at all.
>> This allows clients to utilize arbitrary xattr namespaces and for the
>> server to act only as a storage mechanism for the xattrs.
>>
>> The user._nfsd namespace was chosen because filesystems generally have
>> a user. xattr handler implemented. Using something like system. would
>> require implementing a generalized handler for each backing fs on the
>> server, as well as requiring privilege to access.
>>
>
> I'm wondering whether the user. namespace is best for storage on the
> server given that:
> - the user. namespace is restricted to regular files or directories (see
> xattr_permission()), and
> - the user. namespace applies its own set of permission checking logic
> that we don't necessarily want applied in the case of trusted. or
> security. attributes (e.g. special rules on sticky directories and read
> or write access to the file, see xattr_permission()), and
> - storing the attributes in the user. namespace would allow local users
> on the server to manipulate them directly. I know that you stated an
> assumption that end users do not have local access to the exported
> filesystem, but it would be better if the system itself would restrict
> the ability to set these attributes to privileged processes on the
> server.
>
> Using the security. or trusted. namespace would avoid the restriction on
> file type and require CAP_SYS_ADMIN on setxattr. That would prevent
> local modification by non-admins and should have no effect on clients
> (since nfsd doesn't drop CAP_SYS_ADMIN).
>
>

I'm mostly in agreement with Stephen. I wouldn't object to a separate
"nfsd." namespace, as opposed to "security." or "trusted." because I
think that in reality you're not going to get very far without treating
is a special case in any event. May as well acknowledge it up front.

>> Currently, only the user and trusted namespaces are supported by
>> the client, as system and security namespaces require further analysis,
>> and some special-case handling will be required (for security.selinux,
>> at least).
>>
>> NFS ACLs continue to work as expected, because they implement a
>> hard-coded system xattr namespace which is invoked before the
>> NFS layer. e.g. any access to a system.posix_acl_access xattr
>> is invokes the NFS_ACL protocol.
>>
>
>