On Monday, October 28, 2013 03:14:48 PM Stephen Smalley wrote:
> On 10/28/2013 03:03 PM, Paul Moore wrote:
> > On Monday, October 28, 2013 02:24:35 PM Daniel J Walsh wrote:
> >> On 10/28/2013 02:10 PM, Paul Moore wrote:
> >>> On Monday, October 28, 2013 12:58:55 PM Stephen Smalley wrote:
> >>>> On 10/28/2013 11:56 AM, Eric Paris wrote:
> >>>>> On Mon, 2013-10-28 at 10:46 -0400, Daniel J Walsh wrote:
> >>>>>> Maybe the solution here is to add logging messages to the function.
> >>>>>>
> >>>>>> My opionion is that if something is wrong with SELinux, IE The labels
> >>>>>> are wrong, the policy is wrong or the app is wrong, we should not
> >>>>>> block in permissive mode.
> >>>>>>
> >>>>>> Having the tool write "foobar_t is not a valid source context" would
> >>>>>> be better then what we have now, which is a silent denial even in
> >>>>>> permissive mode.>
> >>>>>
> >>>>> I understand Stephen's argument. But agree with dwalsh/bigon that
> >>>>> hiding this in the library is a lot better than moving the logic to
> >>>>> userspace programs. So this might not be so super simple to do. How
> >>>>> about the idea of a new interface which always returns 0 in
> >>>>> permissive? But it does a couple of extra things. These are just
> >>>>> rough
> >>>>> early thoughts....
> >>>>>
> >>>>> 0) new interface just like avc_has_perm() but which always returns 0
> >>>>> in permissive.
> >>>>>
> >>>>> 1) a new SELINX_USER_ERR audit message. On EINVAL we check if the
> >>>>> scontext/tcontext are valid and print the equivalent of a SELINUX_ERR
> >>>>> message into the audit log if not.
> >>>>>
> >>>>> 2) a new /sys/fs/selinux/context like mechanism, which will both
> >>>>> validate the context and will force it into the sid cache. So
> >>>>> subsequent broken calls to avc_has_perm() will not generate a second
> >>>>> SELINX_USER_ERR message, since the second call to 'access' will find a
> >>>>> valid type and will give a denial for that unlabeled_t type?
> >>>>>
> >>>>> maybe /sys/fs/selinux/access should be changed/new interface added to
> >>>>> do all of this in kernel? generating a real SELINUX_ERR in kernel and
> >>>>> forcing the invalid label into the sid cache?
> >>>>>
> >>>>> I really do think that userspace object managers should be allowed to
> >>>>> call avc_has_perm() and either get an error that should be handled as
> >>>>> a hard failure or a 0... checking permissive in userspace object
> >>>>> managers just seems prone to breakage...
> >>>>
> >>>> I'm ok with changing avc_has_perm as long as: a) Something gets
> >>>> logged/audited so you'll see that something went wrong in permissive
> >>>> mode
> >>>> and not just get silent failures in enforcing mode,
> >>>>
> >>>> b) We are careful about what error conditions are remapped to 0 in
> >>>> permissive mode. If we just hit a memory allocation failure, we
> >>>> shouldn't hide that from the caller. It should only affect things
> >>>> relating to policy.
> >>>
> >>> I'm far from an expert on the SELinux userland libraries, but as far as
> >>> my
> >>> two cents are concerned I like the idea of changing avc_has_perm() to
> >>> incorporate the permissive/enforcing logic. I think asking applications
> >>> to worry about things like that is a step in the wrong direction.
> >>>
> >>> I also agree with Stephen's comments above: logging is important and the
> >>> only failures that should be ignored by avc_has_perm() are those
> >>> relating
> >>> to access denials from policy, error conditions should propagate to the
> >>> caller.
> >>
> >> The only functions that can fail are the following:
> >> rc = avc_lookup(ssid, tsid, tclass, requested, aeref);
> >> if (rc) {
> >>
> >> rc = security_compute_av_flags_raw(ssid->ctx, tsid->ctx,
> >>
> >> tclass, requested,
> >> &entry.avd);
> >>
> >> if (rc)
> >>
> >> goto out;
> >>
> >> rc = avc_insert(ssid, tsid, tclass, &entry, aeref);
> >>
> >> Are we stating that we should check for EINVAL and return 0 if
> >> permissive,
> >> or is there some more complicated thing we should look up.
> >
> > Perhaps I'm mistaken, but I believe the concern has to do with what the
> > kernel returns via security_compute_av_flags_raw(). I think the idea is
> > to ensure that non-policy related error codes returned from the kernel
> > are not papered over when running in permissive mode.
>
> I think the kernel side is fine as is; it will only return EINVAL if one
> of the security contexts is invalid.
My mistake, I should have been more clear ... Yes, I agree the kernel is fine,
what I meant to get across was what you already said (far better than I): when
in permissive mode the library should return 0 when performing access control
checks and a policy related denial is returned from the kernel.
> Looking again, I see that invalid class is just handled like any other
> denial in the kernel these days, so EINVAL from writing to
> /sys/fs/selinux/access is unambiguously an invalid security context string
> (could be the source or target).
>
> I think we just need the userspace AVC to handle it cleanly and we'll be
> fine. I think my patch will work, but don't have a test case offhand;
> I think we'd essentially need to launch dbusd, go permissive, remove its
> domain from policy, and then trigger a dbus check?
Dan is probably the most knowledgeable here ... if it was left to me I'd
probably just cobble up some little test program, but that's probably
overkill.
--
paul moore
www.paul-moore.com
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