On Fri, Apr 21, 2017 at 4:28 PM, 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.

This can already be "given up" by a process by using seccomp. The
system-wide setting is what's missing here, and that's a whole other
thread already even though basically every distro has implemented the
= 3 sysctl knob level.

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

This is another one that can be controlled by seccomp. The system-wide
setting already exists in /proc/sys/user/max_user_namespaces.

 - 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.

The attack surface here is relatively small compared to the other examples.

 - Ability to bind ports, maybe?

seccomp and maybe a sysctl? I'd have to look at that more carefully,
but again, this isn't a comparable attack-surface/confinement issue.

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.

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

I don't like blocking a needed feature behind a large super-feature
that doesn't exist yet. We'd be able to refactor this code into using
such a thing in the future, so I'd prefer to move ahead with this
since it would stop actual exploits.

-Kees

-- 
Kees Cook
Pixel Security