Re: [PATCH v5 next 1/5] modules:capabilities: add request_module_cap()

On Tue, Nov 28, 2017 at 10:33:27PM +0100, Djalal Harouni wrote:

On Tue, Nov 28, 2017 at 10:16 PM, Luis R. Rodriguez [off-list ref] wrote:

quoted

On Tue, Nov 28, 2017 at 12:11:34PM -0800, Kees Cook wrote:

quoted

On Tue, Nov 28, 2017 at 11:14 AM, Luis R. Rodriguez [off-list ref] wrote:

quoted

kmod is just a helper to poke userpsace to load a module, that's it.

The old init_module() and newer finit_module() do the real handy work or
module loading, and both currently only use may_init_module():

static int may_init_module(void)
{
        if (!capable(CAP_SYS_MODULE) || modules_disabled)
                return -EPERM;

        return 0;
}

This begs the question:

  o If userspace just tries to just use raw finit_module() do we want similar
    checks?

Otherwise, correct me if I'm wrong this all seems pointless.

Hm? That's direct-loading, not auto-loading. This series is only about
auto-loading.

And *all* auto-loading uses aliases? What's the difference between auto-loading
and direct-loading?

Not all auto-loading uses aliases, auto-loading is when kernel code
calls request_module() to loads the feature that was not present, 

It seems the actual interest here is system call implicated request_module()
calls? Because there are uses of request_module() which may be module hacks,
and not implicated via system calls.

and direct-loading in this thread is the direct syscalls like
finit_module().

OK.

quoted

quoted

We already have a global sysctl for blocking direct-loading (modules_disabled).

My point was that even if you have a CAP_NET_ADMIN check on request_module(),
finit_module() will not check for it, so a crafty userspace could still try
to just finit_module() directly, and completely then bypass the CAP_NET_ADMIN
check.

The finit_module() uses CAP_SYS_MODULE which should allow all modules
and in this context it should be more privileged than CAP_NET_ADMIN
which is only for "netdev-%s" (to not load arbitrary modules with it).

finit_module() coming from request_module() always has the
CAP_NET_ADMIN, hence the check is done before.

But since CAP_SYS_MODULE is more restrictive, what's the point in checking
for CAP_NET_ADMIN?

quoted

So unless I'm missing something, I see no point in adding extra checks for
request_module() but nothing for the respective load_module().

I see, request_module() is called from kernel context which runs in
init namespace will full capabilities, the spawned userspace modprobe
will get CAP_SYS_MODULE and all other caps, then after comes modprobe
and load_module().

Right, so defining the gains of adding this extra check is not very clear
yet. It would seem a benefit exists, what is it?

Btw as suggested by Linus I will update with request_module_cap() and > I can
offer my help maintaining these bits too.

Can you start by extending lib/test_module.c and
tools/testing/selftests/kmod/kmod.sh with a proof of concept of the gains here,
as well as ensuring things work as expected ?

  Luis