Re: Adding support for MAC_TASK_CONTEXTS and MAC_OBJ_CONTEXTS to userspace.

[Steve Grubb <sgrubb@redhat.com>]

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. 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


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