Re: Adding support for MAC_TASK_CONTEXTS and MAC_OBJ_CONTEXTS to userspace.
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