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 > "/etc/shadow". > > 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. -- Jamie -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
