On May 27, 2007, at 03:25:27, Toshiharu Harada wrote:
2007/5/27, Kyle Moffett <[email protected]>:
On May 26, 2007, at 19:08:56, Toshiharu Harada wrote:
2007/5/27, James Morris <[email protected]>:
On Sat, 26 May 2007, Kyle Moffett wrote:
AppArmor). On the other hand, if you actually want to protect
the _data_, then tagging the _name_ is flawed; tag the *DATA*
instead.
Bingo.
(This is how traditional Unix DAC has always functioned, and is
what SELinux does: object labeling).
Object labeling (or labeled security) looks simple and straight
forward way, but it's not.
(1) Object labeling has a assumption that labels are always
properly defined and maintained. This can not be easily achieved.
That's a circular argument, and a fairly trivial one at that.
Sorry Kyle, I don't think it's a trivial one. The opposite.
How is that argument not trivially circular? "Foo has an assumption
that foo-property is always properly defined and maintained." That
could be said about *anything*:
* Unix permissions have an assumption that mode bits are always
properly defined and maintained
* Apache .htaccess security has an assumtion that .htaccess files
are always properly defined and maintained.
* Functional email communication has an assumption that the email
servers are always properly defined and maintained
If you can't properly manage your labels, then how do you expect
any security at all?
Please read my message again. I didn't say, "This can never be
achieved". I said, "This can not be easily achieved".
So you said "(data labels) can not be easily achieved". My question
for you is: How do you manage secure UNIX systems without standard
UNIX permission bits? Also: If you have problems with data labels
then what makes pathname based labels "easier"? If there is
something that could be done to improve SELinux and make it more
readily configurable then it should probably be done.
If you can't achieve the first with reasonable security, then you
probably can't achieve the second either. Also, if you can't
manage correct object labeling then I'm very interested in how you
are maintaining secure Linux systems without standard DAC.
I'm very interested in how you can know that you have the correct
object labeling (this is my point). Could you tell?
I know that I have the correct object labeling because:
1) I rewrote/modified the default policy to be extremely strict on
the system where I wanted the extra security and hassle.
2) I ensured that the type transitions were in place for almost
everything that needed to be done to administer the system.
3) I wrote a file-contexts file and relabeled *once*
4) I loaded the customized policy plus policy for restorecon and
relabeled for the last time
5) I reloaded the customized policy without restorecon privileges
and without the ability to reload the policy again.
6) I never reboot the system without enforcing mode.
7) If there are unexpected errors or files have incorrect labels,
I have to get the security auditor to log in on the affected system
and relabel the problematic files manually (rare occurrence which
requires excessive amounts of paperwork).
(2) Also, assigning a label is something like inventing and
assigning a *new* name (label name) to objects which can cause
flaws.
I don't understand how assigning new attributes to objects "can
cause flaws", nor what flaws those might be; could you elaborate
further? In particular, I don't see how this is really all that
more complicated than defining additional access control in
apache .htaccess files. The principle is the same: by stacking
multiple independent security-verification mechanisms (Classical
UNIX DAC and Apache permissions) you can increase security, albeit
at an increased management cost. You might also note that
".htaccess" files are yet another form of successful "label-based"
security; the security context for a directory depends on
the .htaccess "label" file found within. The *exact* same
principles apply to SELinux: you add additional attributes backed
by a simple and powerful state-machine. The cross-checks are
lower-level than those from .htaccess files, but the principles
are the same.
I don't deny DAC at all. If we deny DAC, we can't live with Linux
it's the base. MAC can be used to cover the shortages of DAC and
Linux's simple user model, that's it.
From security point of view, simplicity is always the virtue and
the way to go. Inode combined label is guaranteed to be a single
at any point time. This is the most noticeable advantage of label-
based security.
I would argue that pathname-based security breaks the "simplicity is
the best virtue (of a security system)" paradigm, because it
attributes multiple potentially-conflicting labels to the same piece
of data. It also cannot protect the secrecy of specific *data* as
well as SELinux can. For example: In SELinux MLS a system could
mark customer credit-card data as the "cust_private_info" category
and it would be completely impossible for any program without the
"cust_private_info" category to read that data, and even then it
could only be written to files which also have "cust_private_info"
set. While a few privileged programs may have "mlsread" or
"mlswrite" attributes allowing them to override such restrictions,
it's a much stronger security guarantee than pathname-based security
could ever provide.
But writing policy with labels are somewhat indirect way (I mean,
we need "ls -Z" or "ps -Z"). Indirect way can cause flaw so we
need a lot of work that is what I wanted to tell.
I don't really use "ls -Z" or "ps -Z" when writing SELinux policy; I
do that only when I actually think I mislabeled files. Typically the
SELinux-policy-development cycle is:
1) Modify and reload the policy
2) Relabel the affected files (either by hand or with some
automated tool like restorecon)
3) Rerun the problem program or daemon
4) Examine the errors in the audit logs. If there are no errors
and it works then you're finished.
5) Go back to step 1 and fix your policy
Cheers,
Kyle Moffett
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