selinux: how to query if selinux is enabled

[Stephen Smalley]

On Fri, Oct 9, 2020 at 12:36 PM Olga Kornievskaia <aglo at umich.edu> wrote:
>
> On Fri, Oct 9, 2020 at 10:08 AM Chuck Lever <chucklever at gmail.com> wrote:
> >
> >
> >
> > > On Oct 9, 2020, at 7:49 AM, Olga Kornievskaia <aglo at umich.edu> wrote:
> > >
> > > On Thu, Oct 8, 2020 at 9:03 PM Paul Moore <paul at paul-moore.com> wrote:
> > >>
> > >> ->On Thu, Oct 8, 2020 at 9:50 AM Olga Kornievskaia <aglo at umich.edu> wrote:
> > >>> On Wed, Oct 7, 2020 at 9:07 PM Paul Moore <paul at paul-moore.com> wrote:
> > >>>> On Wed, Oct 7, 2020 at 8:41 PM Olga Kornievskaia <aglo at umich.edu> wrote:
> > >>>>> Hi folks,
> > >>>>>
> > >>>>> From some linux kernel module, is it possible to query and find out
> > >>>>> whether or not selinux is currently enabled or not?
> > >>>>>
> > >>>>> Thank you.
> > >>>>
> > >>>> [NOTE: CC'ing the SELinux list as it's probably a bit more relevant
> > >>>> that the LSM list]
> > >>>>
> > >>>> In general most parts of the kernel shouldn't need to worry about what
> > >>>> LSMs are active and/or enabled; the simply interact with the LSM(s)
> > >>>> via the interfaces defined in include/linux/security.h (there are some
> > >>>> helpful comments in include/linux/lsm_hooks.h).  Can you elaborate a
> > >>>> bit more on what you are trying to accomplish?
> > >>>
> > >>> Hi Paul,
> > >>>
> > >>> Thank you for the response. What I'm trying to accomplish is the
> > >>> following. Within a file system (NFS), typically any queries for
> > >>> security labels are triggered by the SElinux (or I guess an LSM in
> > >>> general) (thru the xattr_handler hooks). However, when the VFS is
> > >>> calling to get directory entries NFS will always get the labels
> > >>> (baring server not supporting it). However this is useless and affects
> > >>> performance (ie., this makes servers do extra work  and adds to the
> > >>> network traffic) when selinux is disabled. It would be useful if NFS
> > >>> can check if there is anything that requires those labels, if SElinux
> > >>> is enabled or disabled.
> > >>
> > >> [Adding Chuck Lever to the CC line as I believe he has the most recent
> > >> LSM experience from the NFS side - sorry Chuck :)]
> > >>
> > >> I'll need to ask your patience on this as I am far from a NFS expert.
> > >>
> > >> Looking through the NFS readdir/getdents code this evening, I was
> > >> wondering if the solution in the readdir case is to simply tell the
> > >> server you are not interested in the security label by masking out
> > >> FATTR4_WORD2_SECURITY_LABEL in the nfs4_readdir_arg->bitmask in
> > >> _nfs4_proc_readdir()?  Of course this assumes that the security label
> > >> genuinely isn't needed in this case (and not requesting it doesn't
> > >> bypass access controls or break something on the server side), and we
> > >> don't screw up some NFS client side cache by *not* fetching the
> > >> security label attribute.
> > >>
> > >> Is this remotely close to workable, or am I missing something fundamental?
> > >>
> > >
> > > No this is not going to work, as NFS requires labels when labels are
> > > indeed needed by the LSM. What I'm looking for is an optimization.
> > > What we have is functionality correct but performance might suffer for
> > > the standard case of NFSv4.2 seclabel enabled server and clients that
> > > don't care about seclabels.
> >
> > Initial thought: We should ask linux-nfs for help with this.
> > I've added them to the Cc: list.
> >
> > Olga, are you asking if the kernel NFS client module can somehow find
> > out whether the rest of the kernel is configured to care about security
> > labels before it forms an NFSv4 READDIR or LOOKUP request?
>
> Yes exactly, but I'm having a hard time trying to figure out how to
> use security_ismaclabel() function as has been suggested by Casey.

I would suggest either introducing a new hook for your purpose, or
altering the existing one to support a form of query that isn't based
on a particular xattr name but rather just checking whether the module
supports/uses MAC labels at all.  Options: 1) NULL argument to the
existing hook indicates a general query (could hide a bug in the
caller, so not optimal), 2) Add a new bool argument to the existing
hook to indicate whether the name should be used, or 3) Add a new hook
that doesn't take any arguments.

>
> > I would certainly like to take the security label query out of every
> > LOOKUP operation if that is feasible!
>
> A LOOKUP doesn't add the seclabel query (by default) like READDIR does
> (it's hard-coded in the xdr code). LOOKUP uses server's bitmask and
> chooses the version without the seclabel bitmask because no label is
> passed into it. It looks like LOOKUP just allocates a label in
> nfs_lookup_revalidate_dentry().  So it's not driven by the something
> that I see used by the xattr_handle example in the NFS code.