Re: [PATCH v4 4/4] vduse: Add LSM hooks to check Virtio device type

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Nov 03, 2023 at 08:55:19AM +0100, Maxime Coquelin wrote:
> 
> 
> On 11/2/23 19:59, Michael S. Tsirkin wrote:
> > On Thu, Nov 02, 2023 at 06:56:59PM +0100, Maxime Coquelin wrote:
> > > 
> > > 
> > > On 10/24/23 17:30, Casey Schaufler wrote:
> > > > On 10/24/2023 2:49 AM, Maxime Coquelin wrote:
> > > > > 
> > > > > 
> > > > > On 10/23/23 17:13, Casey Schaufler wrote:
> > > > > > On 10/23/2023 12:28 AM, Maxime Coquelin wrote:
> > > > > > > 
> > > > > > > 
> > > > > > > On 10/21/23 00:20, Casey Schaufler wrote:
> > > > > > > > On 10/20/2023 8:58 AM, Maxime Coquelin wrote:
> > > > > > > > > This patch introduces LSM hooks for devices creation,
> > > > > > > > > destruction and opening operations, checking the
> > > > > > > > > application is allowed to perform these operations for
> > > > > > > > > the Virtio device type.
> > > > > > > > 
> > > > > > > > Why do you think that there needs to be a special LSM check for virtio
> > > > > > > > devices? What can't existing device attributes be used?
> > > > > > > 
> > > > > > > Michael asked for a way for SELinux to allow/prevent the creation of
> > > > > > > some types of devices [0].
> > > > > > > 
> > > > > > > A device is created using ioctl() on VDUSE control chardev. Its type is
> > > > > > > specified via a field in the structure passed in argument.
> > > > > > > 
> > > > > > > I didn't see other way than adding dedicated LSM hooks to achieve this,
> > > > > > > but it is possible that their is a better way to do it?
> > > > > > 
> > > > > > At the very least the hook should be made more general, and I'd have to
> > > > > > see a proposal before commenting on that. security_dev_destroy(dev)
> > > > > > might
> > > > > > be a better approach. If there's reason to control destruction of vduse
> > > > > > devices it's reasonable to assume that there are other devices with the
> > > > > > same or similar properties.
> > > > > 
> > > > > VDUSE is different from other devices as the device is actually
> > > > > implemented by the user-space application, so this is very specific in
> > > > > my opinion.
> > > > 
> > > > This is hardly unique. If you're implementing the device
> > > > in user-space you may well be able to implement the desired
> > > > controls there.
> > > > 
> > > > > 
> > > > > > 
> > > > > > Since SELinux is your target use case, can you explain why you can't
> > > > > > create SELinux policy to enforce the restrictions you're after? I
> > > > > > believe
> > > > > > (but can be proven wrong, of course) that SELinux has mechanism for
> > > > > > dealing
> > > > > > with controls on ioctls.
> > > > > > 
> > > > > 
> > > > > I am not aware of such mechanism to deal with ioctl(), if you have a
> > > > > pointer that would be welcome.
> > > > 
> > > > security/selinux/hooks.c
> > > 
> > > We might be able to extend selinux_file_ioctl(), but that will only
> > > covers the ioctl for the control file, this patch also adds hook for the
> > > device file opening that would need dedicated hook as the device type
> > > information is stored in the device's private data.
> > > 
> > > Michael, before going further, I would be interested in your feedback.
> > > Was this patch what you had in mind when requesting for a way to
> > > allow/deny devices types for a given application?
> > > 
> > > Regards,
> > > Maxime
> > 
> > 
> > Yes, this is more or less what I had in mind.
> 
> Great.
> 
> Do you think we need to cover both ioctl() on the control file and
> open() on the device file, or only ioctl() is enough?
> 
> If the former, we will need VDUSE-specific hooks. I may be able to
> improve my patch to have a single hook instead of 3 by passing the type
> of operation as an extra argument (create/destroy/open).
> 
> If the latter, we may be able to extend the generic ioctl hook.
> 
> Personally, I think it would make sense to also ensure a given
> application can only open existing VDUSE devices it supports. For
> example, openvswitch should only be allowed to open networking VDUSE
> devices.
> 
> Thanks,
> Maxime

I agree here. I think an open hook is important.
Make sure to document the need in the cover letter
and commit log.

> > 
> > > > 
> > > > > 
> > > > > Thanks,
> > > > > Maxime
> > > > > 
> > > > > > 
> > > > > > > 
> > > > > > > Thanks,
> > > > > > > Maxime
> > > > > > > 
> > > > > > > [0]:
> > > > > > > https://lore.kernel.org/all/20230829130430-mutt-send-email-mst@kernel.org/
> > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> >