Why can not user_t link var_lib_t files?

[Stephen Smalley]

On Sun, 2009-05-17 at 18:44 +0200, Göran Uddeborg wrote:
> Is there some reason user_t is denied to link a file with type
> var_lib_t (among others)?  Or did it just happen that way?  I don't
> see any security advantage.

In a least privilege scheme, the question is not why should it be denied
but rather what legitimate purpose does user_t have in creating hard
links to random files under /var/lib.  Generally none; in your case, you
ought to have a distinct type for those files (and if they are in fact
served via NFS, then I don't see why they would be in var_lib_t unless
you mounted the NFS filesystem with
context=system_u:object_r:var_lib_t).

user_t is supposed to be an unprivileged user account, and creating hard
links to files to which you have no create/write permissions is usually
a sign of something wrong (hence a wide variety of Linux security
patches prohibit link'ing to files you don't own).

> (It doesn't matter for the question, but I suspect somebody will ask
> why I want this.  The particular use case where we were hit by this is
> non-standard.  We have a digital TV receiver box that saves recordings
> via NFS under /var/lib/TV on a server.  A user wanted to edit out the
> commercials from one recording using the m2vmp2cut tool.  The tool is
> most easy to use when the original recording is in the working
> directory.  She could copy the file from /var/lib/TV/... to her home
> directory, but to save a lot of time and space she tried to make a
> (hard) link instead.  SELinux denied her that.  Obviously
> non-standard, and the regular policy doesn't know anything about these
> files.  And I know various ways to work around it, including adding a
> module.  But I was a bit surprised over the denial.  I would have
> expected user_t to be allowed to do this.  Thus my question, is this
> by design or by mistake?)
> 
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-- 
Stephen Smalley
National Security Agency