On Tue, 2008-12-30 at 15:00 -0800, Muntz, Daniel wrote:

Yes, and if you have a single user on the machine at a time (with cache
flushed inbetween, kernel refreshed), root can read /dev/kmem, swap,
intercept traffic and read cachefs data to its heart's content--hence,
those requirements.

Unless you _are_ root and can check every executable, after presumably
rebooting into your own trusted kernel, then those requirements won't
mean squat. If you're that paranoid, then you will presumably also be
using a cryptfs-encrypted partition for cachefs, which you unmount when
you're not logged in.

That said, most cluster environments will tend to put most of their
security resources into keeping untrusted users out altogether. The
client nodes tend to be a homogeneous lot with presumably only a trusted
few sysadmins...

Trond

-----Original Message-----
From: Trond Myklebust [mailto:trond.myklebust@fys.uio.no] 
Sent: Tuesday, December 30, 2008 2:36 PM
To: Muntz, Daniel
Cc: Andrew Morton; Stephen Rothwell; Bernd Schubert;
nfsv4@linux-nfs.org; linux-kernel@vger.kernel.org; steved@redhat.com;
dhowells@redhat.com; linux-next@vger.kernel.org;
linux-fsdevel@vger.kernel.org; rwheeler@redhat.com
Subject: RE: Pull request for FS-Cache, including NFS patches

On Tue, 2008-12-30 at 14:15 -0800, Muntz, Daniel wrote:

quoted

quoted

quoted

As for security, look at what MIT had to do to prevent local disk 
caching from breaking the security guarantees of AFS.

See what David has added to the LSM code to provide the same 
guarantees

for cachefs...

quoted

Trond

Unless it (at least) leverages TPM, the issues I had in mind can't 
really be addressed in code.  One requirement is to prevent a local 
root user from accessing fs information without appropriate

permissions.

quoted

This leads to unwieldly requirements such as allowing only one user on

quoted

a machine at a time, blowing away the cache on logout, validating 
(e.g.,
refreshing) the kernel on each boot, etc.  Sure, some applications 
won't care, but you're also potentially opening holes that users may 
not consider.

You can't prevent a local root user from accessing cached data: that's
true with or without cachefs. root can typically access the data using
/dev/kmem, swap, intercepting tty traffic, spoofing user creds,...
If root can't be trusted, then find another machine.

The worry is rather that privileged daemons may be tricked into
revealing said data to unprivileged users, or that unprivileged users
may attempt to read data from files to which they have no rights using
the cachefs itself. That is a problem that is addressable by means of
LSM, and is what David has attempted to solve.

  Trond

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/