On Wed, 18 Apr 2007, Crispin Cowan wrote:
Please explain why labels are necessary for effective confinement. Many
systems besides AppArmor have used non-label schemes for effective
confinement: TRON, Janus, LIDS, Systrace, BSD Jail, EROS, PSOS, KeyOS,
AS400, to name just a few. This claim seems bogus. Labels may be your
method of choice for confinement, but they are far from the only way.
One problem with AppArmor and Janus and Systrace and everything else that
relies on pathname resolution is the point where they do the pathname
resolution.
If you read the janus, systrace, subdomain (apparmor's predecssor?)
papers, you'll see how they have to jump through hoops to handle things
like symlinks, when there's no fundamental reason why they have to.
If one simply worked at the FS level, all one cares about is lookup() and
permission. You have a set of rules that lookup() is able to use to
dynamically tag dentries and permission() then checks that tag. One
doesn't jump through hoops anymore.
So, while I sound like a broken record, something like a stackable file
system works wonders here (I know, I implemented one). Now, stackable
file systems aren't perfect here (mount point crossing, additional mounted
file systems on top of the stackable file system) can cause problems,
overall it seems like a cleaner solution.
Another option would be if the LSM could be extended to allow a simple
method of storing "private" data along with every dentry/inode (the main
reason one needs a stackable file system). In this way, if the lookup()
oepration was extended to be able to take a function that filled in that
data and permission() was able to be extended to take a function that
could use that data, one wouldn't even need a stackable file system, but
one would still be operating at the simplest layer (which is the file
system).
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