On Thu, Jul 13, 2017 at 02:13:40PM -0400, Stephen Smalley wrote: > On Thu, 2017-07-13 at 18:55 +0200, Dominick Grift wrote: > > On Thu, Jul 13, 2017 at 11:59:55AM -0400, Stephen Smalley wrote: > > > On Thu, 2017-07-13 at 11:48 -0400, Stephen Smalley wrote: > > > > On Thu, 2017-07-13 at 09:25 -0400, Paul Moore wrote: > > > > > On Thu, Jul 13, 2017 at 8:44 AM, Stephen Smalley <sds@xxxxxxxxx > > > > > .gov > > > > > > > > > > > > > > > > wrote: > > > > > > On Wed, 2017-07-12 at 20:27 -0400, Chris PeBenito wrote: > > > > > > > On 07/12/2017 05:38 PM, Paul Moore wrote: > > > > > > > > On Wed, Jul 12, 2017 at 9:26 AM, Stephen Smalley <sds@tyc > > > > > > > > ho.n > > > > > > > > sa > > > > > > > > .gov > > > > > > > > > wrote: > > > > > > > > > On Tue, 2017-07-11 at 17:00 -0400, Paul Moore wrote: > > > > > > > > > > On Mon, Jul 10, 2017 at 4:25 PM, Stephen Smalley <sds > > > > > > > > > > @tyc > > > > > > > > > > ho > > > > > > > > > > .nsa > > > > > > > > > > .gov> > > > > > > > > > > wrote: > > > > > > > > > > > > > > > > While I think splitting the NNP/nosuid transition > > > > > > > > restrictions > > > > > > > > might > > > > > > > > be a good idea under the new policy capability, I'm not > > > > > > > > sure > > > > > > > > it > > > > > > > > is > > > > > > > > worth the cost of a "process2" object class. > > > > > > > > > > > > > > > > With that in mind, let's do two things with this patch: > > > > > > > > > > > > > > > > * Mention the nosuid restrictions in the patch > > > > > > > > description. It > > > > > > > > doesn't need much text, but something would be good so we > > > > > > > > have > > > > > > > > documentation in the git log. > > > > > > > > > > > > > > > > * Let's pick a new permission name that is not specific > > > > > > > > to > > > > > > > > NNP > > > > > > > > or > > > > > > > > nosuid. IMHO, nnpnosuid_transition is ... less than > > > > > > > > good. > > > > > > > > Unfortunately, I'm not sure I'm much better at picking > > > > > > > > names; > > > > > > > > how > > > > > > > > about "protected_transition"? "restricted_transition"? > > > > > > > > "enable_transition"? "override_transition"? > > > > > > > > > > > > > > I vote for nnp_transition anyway. "No New Privileges" > > > > > > > encompasses > > > > > > > nosuid in my mind. If the two perms had been separated I > > > > > > > would > > > > > > > have > > > > > > > been inclined to allow both every time one of them was > > > > > > > needed, > > > > > > > to > > > > > > > make > > > > > > > sure no one was surprised by the behavior difference. > > > > > > > > > > > > I agree; I'll keep it as nnp_transition and just document it > > > > > > in > > > > > > the > > > > > > patch description. > > > > > > > > > > Sorry to be a stubborn about this, but nnp_transition makes > > > > > little > > > > > sense for the nosuid restriction. Like it or not, NNP has a > > > > > concrete > > > > > meaning which is distinct from nosuid mounts. We don't have to > > > > > pick > > > > > any of the permission names I tossed out, none of those were > > > > > very > > > > > good, but I would like to see something that takes both NNP and > > > > > nosuid > > > > > into account, or preferably something that doesn't use either > > > > > name > > > > > explicitly but still conveys the meaning. > > > > > > > > NNP is essentially a superset of nosuid; it applies to all > > > > execve() > > > > calls by the process and its descendants rather than only to > > > > execve() > > > > calls on specially marked filesystems. So I viewed it as the > > > > more > > > > general term. > > > > > > > > If that's not viable, I can't think of anything clearer or better > > > > than > > > > nnp_nosuid_transition. That clearly links it to what it means > > > > (allow > > > > a > > > > SELinux domain transition under NNP or nosuid). It is somewhat > > > > ugly > > > > and verbose but it is clear in what it means, which I think is > > > > more > > > > important. All of your suggestions IMHO were less clear since > > > > they > > > > had > > > > no clear linkage to either NNP or nosuid, and I couldn't tell > > > > from > > > > reading the permission name what it meant. The SELinux domain > > > > transition isn't protected, it isn't restricted, it isn't clear > > > > what > > > > enable_transition means versus the regular transition or > > > > dyntransition > > > > permissions, and we aren't overriding a transition but rather > > > > allowing > > > > one under NNP/nosuid. > > > > > > > > The only other alternative I see is to introduce a process2 class > > > > and > > > > use separate nnp_transition and nosuid_transition permissions > > > > (but in > > > > practice I expect them to be both allowed or denied > > > > together). Note > > > > that this will require two avtab and AVC entries for every domain > > > > pair > > > > (if we allow whichever one ends up going in the process2 class), > > > > so > > > > that has an impact on policy and AVC size. Don't really see that > > > > as > > > > worthwhile. > > > > > > > > Other approach would be to make the nosuid transition checks > > > > file- > > > > based > > > > instead so that it would go in the file class (while the nnp one > > > > would > > > > remain in the process class), but I don't think that's really > > > > what we > > > > want either. Difference between "Can domain D1 transition under > > > > nosuid > > > > to D2?" and "Can domain D1 transition under nosuid when > > > > executing > > > > file > > > > with type T1?". > > > > > > Just to be clear, we would also be separately checking regular > > > transition permission between the old and new contexts on these > > > transitions, so you would need both: > > > allow D1 D2:process transition; > > > allow D1 T1:file nosuid_transition; > > > if we took the latter approach. > > > > > > So we wouldn't lose entirely on a domain-to-domain check, but it > > > would > > > no longer be domain-to-domain for the nosuid part. > > > > > > Whereas with original approach, we end up requiring: > > > allow D1 D2:process transition; > > > allow D1 D2:process nnp_nosuid_transition; # or whatever permission > > > name is used > > > > I don't know if i understand all the ins-and-outs of the matter but i > > think i do like the idea of differentiating between nosuid_transition > > and nnp_transition > > It provides more flexibility because i might not want to allow one or > > the other automatically. > > > > I do not think it its a good idea to allow a process to transiton on > > nosuid mounted filesystems just because i am forced to allow it nnp. > > Can you provide a real use case where you would need to distinguish > them? Nope I cannot, but its easy to merge the two permissions in policy, and thereby preserving flexibility. It will be hard to deal with a case that might arise that was not foreseen if we cover both scenarios with a single permission. > > Currently we handle them the same way (i.e. we don't allow transitions > unless bounded for both). The current patch preserves that > consistency, and just provides a way to allow transitions without > bounds for both. Of course, you still have to be allowed the usual > permissions in order to perform the transition at all (Can the caller > execute the file? Can the callee be entered (entrypoint) by the file? > Can the caller transition to the callee?) in addition to the new > permission check (Can the caller transition under NNP/nosuid to the > callee?). > > We can't distinguish them on a domain-to-domain basis without > introducing a new process2 class, and so far no one has been excited > about that. Why is that? Eventually that is going to happen anyway. Besides we also have a capability2 and its not like that's a big deal is it? But again, its not that big of a deal for me. > > > So since the stuff is ugly one way or another might as well make it > > consistent with SELinux flexibility goals > > > > Just my preference but I dont have a really strong opinion on the > > matter > > > > > > > > > > > > > On a separate note, I plan to cc luto on the next version of the > > > > patch > > > > as I suspect he will have concerns about relaxing this constraint > > > > on > > > > NNP and this likely requires updating > > > > Documentation/prctl/no_new_privs* > > > > and the man pages that describe NNP behavior. > > > > > > > > The other model would be to figure out a way to make the > > > > typebounds > > > > logic work cleanly in a manner that preserves the desired > > > > NNP/nosuid > > > > invariant _and_ doesn't require leaking unnecessary accesses into > > > > the > > > > ancestor domains that make them less secure, plus CIL support for > > > > automatically propagating permissions in the desired way. But I > > > > haven't yet come up with a way to do that. We can do it in some > > > > cases > > > > by creating typebounds between the object types, e.g.: > > > > typebounds parent_t child_t; > > > > allow child_t self:process execmem; > > > > allow child_t child_exec_t:file entrypoint; > > > > allow child_t child_tmp_t:file create; > > > > can be allowed via: > > > > allow parent_t child_t:process execmem; # an otherwise > > > > nonsensical > > > > rule > > > > typebounds parent_exec_t child_exec_t; > > > > typebounds parent_tmp_t child_tmp_t; > > > > but this breaks down when there isn't an equivalent type and > > > > permission > > > > set already allowed to the parent for every type allowed to the > > > > child. > > > > -- Key fingerprint = 5F4D 3CDB D3F8 3652 FBD8 02D5 3B6C 5F1D 2C7B 6B02 https://sks-keyservers.net/pks/lookup?op=get&search=0x3B6C5F1D2C7B6B02 Dominick Grift
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