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Date:   Fri, 3 Nov 2023 04:04:50 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Maxime Coquelin <maxime.coquelin@redhat.com>
Cc:     Casey Schaufler <casey@schaufler-ca.com>, jasowang@redhat.com,
        xuanzhuo@linux.alibaba.com, paul@paul-moore.com, jmorris@namei.org,
        serge@hallyn.com, stephen.smalley.work@gmail.com,
        eparis@parisplace.org, xieyongji@bytedance.com,
        virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org,
        linux-security-module@vger.kernel.org, selinux@vger.kernel.org,
        david.marchand@redhat.com, lulu@redhat.com
Subject: Re: [PATCH v4 4/4] vduse: Add LSM hooks to check Virtio device type
Message-ID: <20231103040224-mutt-send-email-mst@kernel.org>
References: <20231020155819.24000-1-maxime.coquelin@redhat.com>
 <20231020155819.24000-5-maxime.coquelin@redhat.com>
 <c8f189e6-c79b-429a-ab36-2193bb68e3e9@schaufler-ca.com>
 <923f87a1-1871-479e-832e-db67b5ae87fd@redhat.com>
 <64626db9-e37a-4c65-a455-fc3985382216@schaufler-ca.com>
 <7524dee3-7c48-4864-8182-1b166b0f6faa@redhat.com>
 <b307ec62-7cfd-4a58-88ef-ea549c64e75e@schaufler-ca.com>
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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/
> > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> >