On Thu, 2017-11-16 at 10:23 +0100, Roberto Sassu wrote: > On 11/16/2017 3:13 AM, Mimi Zohar wrote: > > On Wed, 2017-11-15 at 16:05 -0800, Matthew Garrett wrote: > > > On Wed, Nov 15, 2017 at 4:02 PM, James Morris <james.l.morris@ora > > > cle.com> wrote: > > > > On Wed, 15 Nov 2017, Patrick Ohly wrote: > > > > > > > > > I have some experience with SMACK, but not with Apparmor. At > > > > > least with > > > > > SMACK the problem is that the LSM depends on integrity > > > > > protection of > > > > > the xattrs, but the integrity protection itself depends on > > > > > the LSM, so > > > > > there's a cycle. An attacker can much too easily make offline > > > > > changes > > > > > which then defeat whatever IMA policy the system might be > > > > > using. > > > > > > > > Isn't this what EVM is supposed to mitigate? > > With the default appraisal policy, it can't. IMA determines if a file > must be appraised depending on metadata whose integrity has not been > verified yet. A root process is able to load appraised files with > i_uid = 0 and files with missing/invalid HMAC and i_uid != 0, at the > same time. Exactly. Therefore a policy that says "appraise only files owned by root" or "with a certain SMACK label" is vulnerable to offline attacks. > Me and Matthew are considering policies based on subject criteria > rather than object criteria. The integrity of a process can be > guaranteed because everything that process reads or executes will be > appraised. Even then you still have the problem that the integrity of the process may also depend on the presence (or absence) of files. My favorite example for that is systemd: suppose that the integrity of the system depends on starting a certain service via systemd. It's trivial for an attacker to remove the corresponding unit file when the system is offline. Adding a custom service written by an attacker gets prevented, but an attacker can still install unit files prepared by the vendor. For example, suppose a device is not supposed to have an ssh daemon, but there is a package for OpenSSH properly signed by the vendor. Then an attacker can take those files and add them to the device while offline. It could get even worse (telnet? A debugging service?), so a vendor has to be very careful about what is getting signed. Another attack vector that also remains open is replacing files with other files from the vendor. Suppose there's a binary that does some check and signals the result to the calling process with its exit code. An attacker can control the result of the check by replacing the binary with /bin/true or /bin/false, depending on what result is desired. -- Best Regards, Patrick Ohly The content of this message is my personal opinion only and although I am an employee of Intel, the statements I make here in no way represent Intel's position on the issue, nor am I authorized to speak on behalf of Intel on this matter.
