On Wed, Apr 08, 2009 at 09:11:00AM -0700, Casey Schaufler wrote: > The 1990's MLS solutions used labels that described sensitivity > levels and category sets. There was variation on exactly how > these were represented, some used fixed size bit maps, others > arrays of category numbers, but the information they contain is > a complete description of the information required to make an > access control decision. Unfortunately to the task at hand, MLS > is a technology that has seen its last development efforts.
But has MLS seen its last days for deployment? I think that for generic non-governmental agency deployment you're quite right that MLS is barely sufficient. DTE is needed. Incidentally, I should clarify that in the context of labeled NFS I only care to solve the sub-problem of how the client and server can determine what policy or subset thereof they share. I currently think that's a simpler problem than how to represent policies (and equivalencies), but only if we also assume a solution that. > The SELinux label, whose textual representation is called a "context", > contains enough information for the system on which it was generated > to interpret it relative to the system policy. It does not contain > sufficient information for a system that does understand that policy > to make decisions based on that context. For a context generated on > Stephen's machine to be meaningful on David's David's machine would > need a full understanding of both policies. Last I looked the > SELinux reference policy was approaching a million lines. Based on > the many interrelationships between policy elements, I do not > think that subsetting is going to be viable, although I'm willing > to be educated on that. I'm still learning about DTE. If I understand correctly though we could constrain ourselves to administrator defined policies built on system- and vendor-defined types/roles. If so then DTE policy subset agreement seems within reach. > In some ways Smack is even worse. The Smack label contains no > actual information, it is just a character string and the access > control is left completely up to the access control rules specified > on the system. A Smack label from Etienne's system has no intrinsic > value on Casey's and give no hint as to how it should be interpreted > or enforced. Ouch. > Summary: Old MLS systems passed sufficient information in their > labels for any number of mechanisms including SPIF, CIPSO, TSIX, > and IPSEC to be useful. 21st century MAC systems, including > SELinux and Smack, have labels that do not contain sufficient > information for another system to make an access control decision. Jarret's point was that this is true even for MLS labels because a node might not know what the meaning of a given sensitivity and compartment are. This is not a problem for CALIPSO because middle boxes need only determine label dominance, but Jarret thinks that this is a problem for NFS. > > IIUC it should be possible to generate SELinux policies from generic > > ones, but not the reverse. If so then that provides a path to > > interoperable deployment, though it would mean sacrificing some > > flexibility. Can you clarify my understanding? > > An SELinux policy can be created from scratch, and we're seeing some > people in the embedded space trying to do just that. Sure, you can > limit the problem by constraining the variation of the policies, but > as we type the reference policy continues to be refined and expanded. > I'm not going to tell Joshua that he has to slow policy development > in support of NFS. I see. This is important knowledge. I'm not sure what can be done about that. > Let's see if I can be clear for a change. (smiley here) :) > The assumption that policy agreement can be assumed is inappropriate. I think you're very likely right about that w.r.t. DTE. But I still harbor some hope. > > So you're saying that a) "a good number of people" think a solution is > > needed that allows the client and server to each know with certainty > > that the other will understand their labels, b) you're among those > > people (?), but c) no solution is feasible. Correct? > > > > a) Else there's no point to labeled NFS. > b) Yup. > c) It is not feasible to meet "a" using just the information contained > in either an SELinux context or a Smack label, regardless of how > the information is transmitted between the client and server. You > can meet "a" for 20th century MLS systems using just the information > contained in the MLS label. Crystal clear. Thanks. > I hope I've clarified the difference between the problem of MLS systems > and the problem of our current set of technologies. You have. Thanks! I'll need to think about the DTE issues, and SELinux vs. Solaris FMAC. Nico --
