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

Joel Granados
2022-11-22T11:23:56Z

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On Mon, Nov 21, 2022 at 04:05:37PM -0500, Paul Moore wrote:
> On Mon, Nov 21, 2022 at 2:53 PM Luis Chamberlain <mcgrof@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@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.
I agree, but we might be able to use kallsysms_lookup_name to execute a
callback once we know where the call comes from.

>
> 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.
Indeed. I just sent out the RFC. Its at an idea stage and would be great
to hear what you think

>
> 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?
This is also a possibility. And with that we might just be able to call
some sort of callback with kallsysms_lookup_name or whatever makes
sense.
>
> --
> paul-moore.com

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