Re: [NFS] [PATCH] nfs4, special files, and set/listxattr asymmetry

["J. Bruce Fields" <bfields@fieldses.org>]

On Tue, Jan 08, 2008 at 04:47:15PM -0500, Erez Zadok wrote:
> A few months ago I looked into the issue of xattrs and copyup in more
> detail, when I was tracking a problem for a user using an SE-linux enabled
> livecd with unionfs.  I didn't realize before then that selinux made such a
> heavy use of xattrs.  After tracking down the code maze I found out that the
> list of xattrs being defined/used depends on your overall security mode and
> global security ops.
> 
> Anyway, xattrs can be used for security reasons, and really any semantics
> could be attached to them.  So in Unionfs I take the conservative approach:
> if unionfs is compiled with xattr support, then during copyup I try to copy
> the xattrs too (if any exist).  If unionfs fails to copyup the xattrs, then
> I abort the copyup.  I figured it's safer to abort the copyup (which is
> typically initiated when trying to modify a file on a readonly
> branch/media), than to potentially open up a security hole by giving a
> copied-up file more permissions than its source file may have had.  This
> policy has so far worked for unionfs, at least for those users who use
> xattrs/selinux, and in my limited testing.  But maybe I've got to rethink
> it?
> 
> NFS in some sense shares some common traits with a stackable file system, in
> that it has three "layers": client -> server -> backing-store f/s.  I'm
> curious how does nfs/d handle xattrs (and selinux's use of them)?  Does the
> client depend on having xattr support of any sort?  Does the server depend
> on having xattr support in the export f/s?  How do you handle mixes of those
> when one of the three layers has xattr support and another doesn't?

We ignore xattr's entirely for now.  The labeled nfs people (mailing
list at labeled-nfs@linux-nfs.org) have plans to address the selinux
case, but I haven't really followed that.

--b.

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