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SubjectRe: [TOMOYO #7 30/30] Hooks for SAKURA and TOMOYO.
Serge E. Hallyn wrote:
> Quoting Matthew Wilcox (
>> On Fri, Apr 11, 2008 at 11:12:27PM +0900, Tetsuo Handa wrote:
>>> Matthew Wilcox wrote:
>>>> When the rule is put in place, say "No modifications to /etc/passwd",
>>>> look up the inode and major:minor of /etc/passwd. If there's a rename,
>>>> look up the new inode number. If it's mounted elsewhere, it doesn't
>>>> matter, they still can't modify it because it has the same
>>>> major:minor:inode.
>>> If write access is denied because of a rule "No modifications to /etc/passwd",
>>> a rule "Allow modifications to /tmp/passwd" can no longer be enforced after
>>> "mount --bind /etc/ /tmp/" or "mount --bind /etc/passwd /tmp/passwd" or
>>> "mv /etc/passwd /tmp/passwd" or "ln /etc/passwd /tmp/passwd" is done.
>> That's a fundamental limitation of pathname-based security though.
>> If the same file exists in two places, you have to resolve the question
>> of which rule overrides the other.
> In the past, Crispin has given clear, concise explanations of a few of
> the things pathname based access control in fact excels at. Crispin,
> can you recite those again so we can think constructively about which
> (if any) of the currently considered options are or are not sufficient?
> I.e. what would be a motivation for a rule like 'no modifications to
> /etc/passwd', and what precisely would and would not be accepted ways to
> get around it (and why)?
As I just posted, a rule of "no mods to /some/pathname" is broken, which
is why AppArmor has no such construct.

Things that pathname-based access control is good at:

* *System Integrity:* Many of the vital components of a UNIX system
are stored in files with Well Known Names such as /etc/shadow,
/var/www/htdocs/index.html and /home/crispin/.ssh/known_hosts. The
contents of the actual data blocks is less important than the
integrity of what some random process gets when it asks for these
resources by name. Preserving the integrity of what responds to
the Well Known Name is thus easier if you restrict access based on
the name.
* *Dynamic Access Control:* A special case of the above pertains to
files that may or may not exist. If you don't *have* a /etc/hosts
file, it is still important to have a rule that controls whether
it can be created. This is hard to do in label-based systems,
because the file does not exist to put a label on, so you have to
kludge it by either creating the file with zero length and
labeling it, or by creating more complex policy for the parent
/etc directory, and that's hard given the number of uses for /etc
such as /etc/motd. In a name based scheme, you simply don't
provide write permission to "/etc/hosts" unless you mean it, and
it can be enforced even if such a file does not exist.
* *Ad Hoc Generalization:* Label-based access control generalizes
policy in that the policy treats all files and resources that
share a label the same. If you want to make a new generalization
that encompasses *part* of the files that share a label, but *not
all* of those files, then you have to "split the label", relabel
the file system, and revise the policy accordingly. Name-based
access control lets you create ad hoc generalizations, so that
*my* policy can grant access to "/var/www/*.pl" without regard to
whatever labels or rules some other policy has applied to the same
directory tree.
* *Familiar Interface:* Administrators are accustomed to
administering the box in terms of the names of the files that
matter. A name-based policy is therefore somewhat more familiar
than a policy with label abstractions, where they then have to go
look at the restorecon file to discover what files will/should
have what labels.
* *Practical Concerns:* Not all file systems support labels, and so
label-based schemes become coarse grained when they run into them.
Name based schemes remain granular when specifying access down
into the internals of such file systems. Legacy Linux file systems
like ext2 don't matter much any more, but persistent file systems
that will never have label support include NFSv3 file systems
(nearly every Network Appliance NAS server out there) and FAT32
file system (most every USB storage device).

A lot of what's going on is the duality of the one:many relationships
between labels and names:

* Labels: one label represents many files
o this property is why ad hoc generalization fails in label
based systems
* Names: many names can represent a single file
o This property is why deny rules fail in name based systems

Therefore, I am not claiming name-based access controls to be the
ultimate. Rather, there are duals for all of the above that induce
circumstances where label-based systems are superior. Each class of
system has strengths and weaknesses. These are name-based strengths.


Crispin Cowan, Ph.D.
The Olympic Games: Symbolizing oppression and corruption for over a hundred years

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