Adding support for MAC_TASK_CONTEXTS and MAC_OBJ_CONTEXTS to userspace.

[Steve Grubb]

On Tuesday, June 15, 2021 1:01:18 PM EDT Casey Schaufler wrote:
> On 6/14/2021 2:13 PM, Steve Grubb wrote:
> > Hello,
> > 
> > On Monday, June 14, 2021 3:34:33 PM EDT Casey Schaufler wrote:
> >> I'm looking at the audit userspace implications of adding two
> >> new kernel audit records. AUDIT_MAC_TASK_CONTEXTS and
> >> AUDIT_MAC_OBJ_CONTEXTS are used when there are multiple security
> >> modules with a "security context" active on the system. This
> >> design has been discussed here at length. The records will look
> >> 
> >> like:
> >> 	AUDIT_MAC_TASK_CONTEXTS
> >> 	subj_<lsmname>=value
> >> 	subj_<lsmname>=value
> >> 	...
> >> 
> >> Looking at the audit user-space code I see several things
> >> that have me concerned. The first is the use of WITH_APPARMOR.
> >> Going forward what behavior would we want if subj_apparmor=something
> >> shows up on a system that has not got WITH_APPARMOR defined?
> > 
> > I think it should be ignored.
> > 
> >> The code is inconsistent in that it does not use WITH_SELINUX,
> >> but that's hardly a surprise given its origins. There is also no
> >> WITH_SMACK, but that's unlikely to be an issue since Smack's use
> >> of audit is very much like SELinux's.
> > 
> > We can add those WITH_* if you like.
> > 
> >> The question is what to
> >> do about filtering when subj=foo is specified. I suggest that if
> >> any of subj_selinux, subj_smack or subj_something is "foo", it is
> >> a match.
> > 
> > I think that's how we already treat things. There is a linked list for
> > AVC's and we match on any of.
> > 
> >> But the SELinux components of a label (level, user, ...)
> >> are also available for filtering. If someone wrote a simple Bell &
> >> LaPadula LSM filtering by some of those fields could be useful
> >> there, too.
> >> 
> >> I would like guidance on whether I ought to go the route of
> >> more extensive use of WITH_APPARMOR (and WITH_SMACK, WITH_MUMBLE)
> >> or take the path of greater generalization. Or, whether I should
> >> treat each case individually and give it my best whack.
> > 
> > To be honest, I have no idea how well the audit system works with any MAC
> > system except SE Linux.
> 
> Understood. Part of what I'm looking at is ensuring that as multiple
> concurrent LSMs come in that the audit user-space isn't mucked up.
> ausearch has these options:
> 
> 	-o,--object <SE Linux Object context>
> 	-se,--context <SE Linux context>
> 	-su,--subject <SE Linux context>
> 
> Without multiple LSMs we can easily ignore "SE Linux" in these
> options and use whatever kind of "context" is available. If I
> have SELinux and AppArmor, the implication is that you can't
> search on AppArmor information. Should we be adding
> 
> 	-aa,--apparmorcontext <AppArmor context>
> 	-as,--apparmorsubject <AppArmor subject context>
> 
> or should we change -se to look at all "contexts", and change
> the description to reflect that? Basicaly, I'm asking whether you'd
> rather add options for other LSMs or remove descriptions that
> specify SELinux.

I'd say any/all contexts available by default. Then we can maybe make a 
restriction to specific LSM's later.

-Steve

> >  I don't really know if its doing the right thing.
> > 
> > Ausearch and report share a parser. It is time sensitive. I usually test
> > it on 4 or 5 Gb of logs. We also have the ausearch-test program which
> > can be used to test any changes to the parser.
> > 
> > http://people.redhat.com/sgrubb/audit/ausearch-test-0.6.tar.gz
> > 
> > Once that is squared away, there is the auparse library. It has a table
> > that classifies a field name into what it is for interpretation
> > purposes. You will find a #ifdef WITH_APPARMOR. I don't know if that
> > table is complete or if it needs to be extended for any other MAC
> > system.
> > 
> > That then leads to the auparse normalizer. I don't know if we need to
> > make
> > any changes there. You can trigger its code with ausearch --format csv or
> > -- format text.
> > 
> > Also, we have some size limits in user space. How big can an event record
> > be if the file is MAX_PATH name length and it has a space in its name or
> > directory and each context is it's maximum size? We may need to think
> > about how this might change the whole userspace ecosystem's size
> > definition, MAX_AUDIT_MESSAGE_LENGTH, since this is part of the ABI. And
> > the kernel also has AUDIT_MESSAGE_TEXT_MAX. What would you get with:
> > 
> > # /usr/sbin/auditctl -m `perl -e 'print "A"x8880'`
> > 
> > And last...what about auditctl? Is the syscall filter going to allow
> > filtering on these other subject/object components?
> > 
> > -Steve