Re: [AppArmor 39/45] AppArmor: Profile loading and manipulation,pathname matching

From: david
Date: Sat Jun 09 2007 - 16:44:50 EST

On Sat, 9 Jun 2007, Kyle Moffett wrote:

On Jun 09, 2007, at 13:32:05, david@xxxxxxx wrote:
On Sat, 9 Jun 2007, Kyle Moffett wrote:
> On Jun 09, 2007, at 12:46:40, david@xxxxxxx wrote:
> > so as I understand this with SELinux you will have lots of labels > > around your system (more as you lock down the system more) you need to > > define policy so that your unrestricted users must have access to every > > label, and every time you create a new label you need to go back to all > > your policies to see if the new label needs to be allowed from that > > policy
> > Actually, it's easier than that. There are type attributes which may be > assigned to an arbitrary set of types, and each "type" field in an access > rule may use either a type or an attribute. So you don't actually need > to modify existing rules when adding new types, you just add the > appropriate existing attributes to your new type. For example, you could > set up a "logfile" attribute which allows logrotate to archive old > versions and allows audit-admin users to modify/delete them, then > whenever you need to add a new logfile you just declare the > "my_foo_log_t" type to have the "logfile" attribute.

isn't this just the flip side of the same problem?

every time you define a new attribute you need to go through all the files and decide if the new attribute needs to be given to that file.

No you don't, you can add attributes to a type after-the-fact. In concept this problem is very similar to programming: You have various documented interfaces used by different policy files to interact with each other. As long as your policy files conform to the documented interfaces then you *DONT* have to manually inspect each file because you can make basic assumptions. On the other hand, when you break that interface "contract" you will get very unexpected results. For the above example:

My syslog policy file would create a "logfile" attribute and types for "/var/log/auth/auth.log", "/var/log/kern/kern.log", and "/var/log/messages". It would also create a "logdaemon" attribute which has automatic type transitions to create files in different "/var/log/*" directories Finally, it would allow the syslogd type to create and append to its specific file types for "auth.log", "kern.log", and "messages".

My logrotate policy file would depend on the syslog policy and would declare the logrotate daemon type as a "logdaemon", and additionally allow logrotate to read, rename, append, and delete "logfile" types. Since logrotate is a "logdaemon", it already has the appropriate type transitions for new types.

My samba policy file would depend on the syslog policy and would declare the samba daemon type as a "logdaemon" and the "/var/log/samba/*" type as a "logfile". Then it would add a type transition rule so when "logdaemon" creates new files in "samba_log_dir_t", they have the appropriate "samba_log_t" label. Finally, samba would allow itself to append to "samba_log_t" files.

if you have your policy figured out and then go and apply it to applications then you are correct.

I'm talking about the situation where you start off by defining a policy for Samba, and then afterwords decide that you want to have seperate "logfile" and "logdaemon" type then you would need to go back and re-examine all the files to tell what needs to be labeled as what.

if you can do all the policy design in advance (and get it right) then SELinux is a great solution. if you don't have that time and have to do the policy incrementaly you will end up revisiting and revising your entire policy each time you go to add something.

Note that now when "logrotate" runs and rotates files in /var/log/samba, it will automatically create the new files with type "samba_log_t", even though there are no *direct* associations between those types. If the syslog policy file was poorly written it could seriously adversely affect the security of the system, but hopefully that's obvious :-D. Policy development is _hard_, it's a whole separate state-machine and pseudo-programming-language that should mostly be left to security professionals or very experienced developers/sysadmins.

I _am_ a security professional. I've been doing security sysadmin work on Linux for over a decade now. From your description I'm exactly the type of person you are saying should be figuring this out. So please back off of the 'this is hard, you should leave it to the professionals' line a little bit ;-)

if the AA policies can be compiled into SELinux policies that would work (currently they can't and many SELinux people oppose the features that would need to be added to make it possible) but the compile process leaves room for more bugs in an areas that's going to be hard to investigate. (This isn't a fault of SELinux, it's a common issue with compilers, the compiled 'thing' is designed to be machine friendly, not user friendly. it doesn't matte if it's compiling C into machine code or high-level firewall rules into iptables commands or AA policies into SELinux labels and policies). adding a complex intermediate layer does not nessasarily add security, but it definantly adds complication.

once SELinux has a way for a trusted user to edit a security sensitive file (via the process of creating a file and renameing it into place) without needing SELinux aware tools and have the result accepted as valid by the rest of the system then it becomes possible to implement an AA to SELinux policy compiler, but saying that AA should not be accepted becouse it's possible to modify SELinux and write such a compile with sufficant effort is not reasonable. While there are many cases where multiple ways of doing something are not allowed into the kernel (suspend/hibernate is an example) there are also cases where multiple ways of doing things are allowed, and the LSM hooks are in place explicitly to permit different types of security modules to use them.

David Lang
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