[RFC 1/1] Use ioctl selinux callback io_uring commands that implement the ioctl op convention

[Paul Moore]

On Mon, Nov 21, 2022 at 2:53 PM Luis Chamberlain <mcgrof at kernel.org> wrote:
> On Thu, Nov 17, 2022 at 05:10:07PM -0500, Paul Moore wrote:
> > On Thu, Nov 17, 2022 at 4:40 AM Joel Granados <j.granados at samsung.com> wrote:
> > > On Wed, Nov 16, 2022 at 02:21:14PM -0500, Paul Moore wrote:
> >
> > ...
> >
> > > > * As we discussed previously, the real problem is the fact that we are
> > > > missing the necessary context in the LSM hook to separate the
> > > > different types of command targets.  With traditional ioctls we can
> > > > look at the ioctl number and determine both the type of
> > > > device/subsystem/etc. as well as the operation being requested; there
> > > > is no such information available with the io_uring command
> > > > passthrough.  In this sense, the io_uring command passthrough is
> > > > actually worse than traditional ioctls from an access control
> > > > perspective.  Until we have an easy(ish)[1] way to determine the
> > > > io_uring command target type, changes like the one suggested here are
> > > > going to be doomed as each target type is free to define their own
> > > > io_uring commands.
> > >
> > > The only thing that comes immediately to mind is that we can have
> > > io_uring users define a function that is then passed to the LSM
> > > infrastructure. This function will have all the logic to give relative
> > > context to LSM. It would be general enough to fit all the possible commands
> > > and the logic would be implemented in the "drivers" side so there is no
> > > need for LSM folks to know all io_uring users.
> >
> > Passing a function pointer to the LSM to fetch, what will likely be
> > just a constant value, seems kinda ugly, but I guess we only have ugly
> > options at this point.
>
> I am not sure if this helps yet, but queued on modules-next we now have
> an improvement in speed of about 1500x for kallsyms_lookup_name(), and
> so symbol lookups are now fast. Makes me wonder if a type of special
> export could be drawn up for specific calls which follow a structure
> and so the respective lsm could be inferred by a prefix instead of
> placing the calls in-place. Then it would not mattter where a call is
> used, so long as it would follow a specific pattern / structure with
> all the crap you need on it.

I suspect we may be talking about different things here, I don't think
the issue is which LSM(s) may be enabled, as the call is to
security_uring_cmd() regardless.  I believe the issue is more of how
do the LSMs determine the target of the io_uring command, e.g. nvme or
ublk.

My understanding is that Joel was suggesting a change to the LSM hook
to add a function specific pointer (presumably defined as part of the
file_operations struct) that could be called by the LSM to determine
the target.

Although now that I'm looking again at the file_operations struct, I
wonder if we would be better off having the LSMs inspect the
file_operations::owner field, potentially checking the module::name
field.  It's a little painful in the sense that it is potentially
multiple strcmp() calls for each security_uring_cmd() call, but I'm
not sure the passed function approach would be much better.  Do we
have a consistent per-module scalar value we can use instead of a
character string?

--
paul-moore.com