On Sat, Apr 22, 2017 at 1:28 AM, Andy Lutomirski [off-list ref] wrote:

On Fri, Apr 21, 2017 at 4:19 PM, Kees Cook [off-list ref] wrote:

quoted

On Wed, Apr 19, 2017 at 7:41 PM, Andy Lutomirski [off-list ref] wrote:

quoted

On Wed, Apr 19, 2017 at 4:43 PM, Kees Cook [off-list ref] wrote:

quoted

On Wed, Apr 19, 2017 at 4:15 PM, Andy Lutomirski [off-list ref] wrote:

quoted

On Wed, Apr 19, 2017 at 3:20 PM, Djalal Harouni [off-list ref] wrote:

quoted

+/* Sets task's modules_autoload */
+static inline int task_set_modules_autoload(struct task_struct *task,
+                                           unsigned long value)
+{
+       if (value > MODULES_AUTOLOAD_DISABLED)
+               return -EINVAL;
+       else if (task->modules_autoload > value)
+               return -EPERM;
+       else if (task->modules_autoload < value)
+               task->modules_autoload = value;
+
+       return 0;
+}

This needs to be more locked down.  Otherwise someone could set this
and then run a setuid program.  Admittedly, it would be quite odd if
this particular thing causes a problem, but the issue exists
nonetheless.

Eeeh, I don't agree this needs to be changed. APIs provided by modules
are different than the existing privilege-manipulation syscalls this
concern stems from. Applications are already forced to deal with
things being missing like this in the face of it simply not being
built into the kernel.

Having to hide this behind nnp seems like it'd reduce its utility...

I think that adding an inherited boolean to task_struct that can be
set by unprivileged tasks and passed to privileged tasks is a terrible
precedent.  Ideally someone would try to find all the existing things
like this and kill them off.

(Tristate, not boolean, but yeah.)

I see two others besides seccomp and nnp:

PR_MCE_KILL

Well, that's interesting.  That should presumably be reset on setuid
exec or something.

quoted

PR_SET_THP_DISABLE

Um.  At least that's just a performance issue.

quoted

I really don't think this needs nnp protection.

quoted

I agree that I don't see how one would exploit this particular
feature, but I still think I dislike the approach.  This is a slippery
slope to adding a boolean for perf_event_open(), unshare(), etc, and
we should solve these for real rather than half-arsing them IMO.

I disagree (obviously); this would be protecting the entire module
autoload attack surface. That's hardly a specific control, and it's a
demonstrably needed flag.

The list is just going to get longer.  We should probably have controls for:

 - Use of perf.  Unclear how fine grained they should be.

 - Creation of new user namespaces.  Possibly also use of things like
iptables without global privilege.

 - Ability to look up tasks owned by different uids (or maybe other
tasks *at all*) by pid/tid.  Conceptually, this is easy.  The API is
the only hard part, I think.

 - Ability to bind ports, maybe?

My point is that all of these need some way to handle configuration
and inheritance, and I don't think that a bunch of per-task prctls is
the right way.  As just an example, saying that interactive users can
autoload modules but other users can't, or that certain systemd
services can, etc, might be nice.  Linus already complained that he
(i.e. user "torvalds" or whatever) should be able to profile the
kernel but that other uids should not be able to.

Neat, maybe this could already be achieved with this interface and
systemd-logind,  "ModulesAutoloadUsers=andy" in logind.conf where
"andy" is the only logged-in user able to trigger and autoload kernel
modules. However maybe we should not restrict too much other bits or
functionality of the other users, please let me will follow up later
on it.

I personally like my implicit_rights idea, and it might be interesting
to prototype it.

Yes I would use that if it is available, in mean time there was
several bugs and 2 public exploits last months.. and we should use
available features.

-- 
tixxdz