On Fri, Apr 21, 2017 at 4:52 PM, Casey Schaufler [off-list ref] wrote:

On 4/21/2017 4:28 PM, Andy Lutomirski wrote:

quoted

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?

One of my longer term (i.e. after stacking) projects
is to create sensible access control on ports. Why shouldn't
they have owners and mode bits (or ACLs, if you prefer)
or real names. I kind of think we should be able to eliminate
the need for dbus without resorting to kdbus.

My implicit_rights concept gives any type of access control you can
use on inodes because they *are* inodes.  So you get ACLs, etc.

Brief summary for those who didn't read my old email: We add a new
kind of filesystem object called a "right".  It's a special kind of
socket inode that can't be bound or connected but is instead created
by a new syscall.  It has a name, so "port:1234" might be a name of a
right.

To use an implicit right, you do whatever syscall you would do
normally.  The kernel looks for a right object at
/dev/implicit_rights/<name>.  If that object exists, is a right of the
correct type (i.e. the right's name matches <name>) and you have
execute access, you win.  Otherwise you lose.

To avoid breaking existing distros, for things like modules_autoload,
you would set a sysctl
/proc/sys/kernel/required_implicit_rights/modules_autoload=1.  With
that set, to autoload a module without CAP_SYS_MODULE, you need the
/dev/implicit_rights/modules_autoload.

So I don't like the idea of treating that as a special case.
I'd rather see ports controlled properly. (Of course, the
SELinux crowd will point out they have this handled, but I
remain unconvinced of the overall solution)

Agreed.  But I think we should address all of these things together.