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Crispin Cowan wrote:
> Of *course* AppArmor protects the integrity of /etc/shadow, and
> unauthorized parties are not permitted to feed data into that file
> unless explicit access is granted. The difference is in how it is done:
> * SELinux marks the inode with a label, and only processes with the
> right permissions can mess with the label.
> o Residual problem: someone could rename the inode and drop a
> new inode into place named "/etc/shadow". SELinux addresses
> this with access control on the parent directory.
<small> I have actually hacked a system by renaming /etc/passwd in this way. /etc was owned by user "bin", and I had a login as "bin" due to a misfeature in some program. So I substituted another /etc/passwd, and gave myself a root shell. </small>
The trouble with access control on the parent directory is that occasionally some human accidentally forgets how important that is, thinking that permissions on the /etc/shadow file are important.
Also *programs* care about a file with that name. They reference it by name, apply security decisions based on a process which starts with that name. So the name is the most relevant point of communication between the policy setter and programs which need to be affected.
So I think AppArmor's approach is good here.
> * AppArmor checks the name "/etc/shadow" so that you cannot access
> that name without explicit permission.
> o AppArmor cares about the integrity of what the OS returns
> when you access the name "/etc/shadow" and does not care a
> wit what happens to the inode that was *previously* named
> Now, without running off into the weeds again, tell me again why I
> should care about the *integrity* of an inode that was *previously*
> known as "/etc/shadow"?
But insufficient here.
If you rename /etc/shadow legitimately, after changing a password, there might be a program which still has a handle to the _old_ inode and is still reading it, still comparing a password against its contents.
If policy was entirely name based, so modifications may be possible to that file after it's renamed from /etc/shadow to /etc/shadow.bak, _while_ some programs are still reading it (because it was /etc/shadow when they opened it, and they got swapped for a moment), that's a failure.
So you *should* care about the integrity of an inode that was previously known as /etc/shadow - at least until you can prove that nobody is still dependent on it's earlier security properties. That's a garbage collection problem.
> So associating a security property with a name is ok if you do it
> statically at some arbitrary point in time, but not if you consider it
> at the time of access? WtF? Isn't that a gigantic race condition?
Both are race conditions.
> To the contrary, I argue that the *current* name of a file is vastly
> more meaningful for security properties than the name the file had some
> months ago when someone ran restorecon over the file system.
I agree that the current name is meaningful, but it's not watertight when your systems change. To avoid unexpected weaknesses, you'll need to apply the intersection of permissions over a time period, using name based policy but having it follow renames until you can prove it's safe to release the following.