Re: [RFC PATCH] selinux: add a fallback to defcontext for native labeling

[Paul Moore]

On Tue, Sep 25, 2018 at 1:45 AM Taras Kondratiuk <takondra@xxxxxxxxx> wrote:
> Quoting Paul Moore (2018-09-24 20:46:57)
> > On Fri, Sep 21, 2018 at 10:39 AM Stephen Smalley <sds@xxxxxxxxxxxxx> wrote:
> > > On 09/20/2018 06:59 PM, Taras Kondratiuk wrote:
> > > > Quoting Stephen Smalley (2018-09-20 07:49:12)
> > > >> On 09/19/2018 10:41 PM, Taras Kondratiuk wrote:
> > > >>> Quoting Stephen Smalley (2018-09-19 12:00:33)
> > > >>>> On 09/19/2018 12:52 PM, Taras Kondratiuk wrote:
> >
> > ...
> >
> > > > IMO it would be more consistent if defcontext cover all "unlabeled"
> > > > groups. It seems unlikely to me that somebody who currently uses
> > > > defcontext can somehow rely on mapping invalid labels to unlabeled
> > > > instead of default context.
> > >
> > > Yes, and that seems more consistent with the current documentation in
> > > the mount man page for defcontext=.
> > >
> > > I'd be inclined to change selinux_inode_notifysecctx() to call
> > > security_context_to_sid_default() directly instead of using
> > > selinux_inode_setsecurity() and change security_context_to_sid_core()
> > > and sidtab_search_core() as suggested above to save and use the def_sid
> > > instead of SECINITSID_UNLABELED always (initializing the context def_sid
> > > to SECINITSID_UNLABELED as the default).  selinux_inode_setsecurity() we
> > > should leave unchanged, or if we change it at all, it should be more
> > > like the handling in selinux_inode_setxattr().  The notifysecctx hook is
> > > invoked by the filesystem to notify the security module of the file's
> > > existing security context, so in that case we always want the _default
> > > behavior, whereas the setsecurity hook is invoked by the vfs or the
> > > filesystem to set the security context of a file to a new value, so in
> > > that case we would only use the _force interface if the caller had
> > > CAP_MAC_ADMIN.
> > >
> > > Paul, what say you?  NB This would be a user-visible behavior change for
> > > mounts specifying defcontext= on xattr filesystems; files with invalid
> > > contexts will then show up with the defcontext value instead of the
> > > unlabeled context.  If that's too risky, then we'd need a flag or
> > > something to security_context_to_sid_default() to distinguish the
> > > behaviors and only set it when called from selinux_inode_notifysecctx().
> >
> > Visible changes like this are always worrisome, even though I think it
> > is safe to assume that the defcontext option is not widely used.  I'd
> > feel much better if this change was opt-in.
> >
> > Which brings about it's own problems.  We have the policy capability
> > functionality, but that is likely a poor fit for this as the policy
> > capabilities are usually controlled by the Linux distribution while
> > the mount options are set by the system's administrator when the
> > filesystem is mounted.  We could add a toggle somewhere in selinuxfs,
> > but I really dislike that idea, and would prefer to find a different
> > solution if possible.  I'm not sure how much flak we would get for
> > introducing a new mount option, but perhaps that is the best way to
> > handle this: defcontext would continue to behave as it does now, but
> > new option X would behave as mentioned in this thread.
> >
> > Thoughts?
>
> The new option X will also mean "default context", so it will be pretty
> hard to come up with a different but a sensible name. I'm afraid that
> having two options with hardly distinguishable semantics will be very
> confusing.
>
> What about a kernel config option that modifies the behavior of
> defcontext?

A kernel config option would be going in the wrong direction; that
would put it almost completely under the control of the distribution.

-- 
paul moore
www.paul-moore.com