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.
--
paul moore
www.paul-moore.com
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