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

[Michael S. Tsirkin]

On Fri, Dec 08, 2023 at 12:01:15PM +0100, Maxime Coquelin wrote:
> Hello Paul,
> 
> On 11/8/23 03:31, Paul Moore wrote:
> > On Oct 20, 2023 "Michael S. Tsirkin" <mst@xxxxxxxxxx> 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.
> > > 
> > > Signed-off-by: Maxime Coquelin <maxime.coquelin@xxxxxxxxxx>
> > > ---
> > >   drivers/vdpa/vdpa_user/vduse_dev.c  | 12 +++++++
> > >   include/linux/lsm_hook_defs.h       |  4 +++
> > >   include/linux/security.h            | 15 ++++++++
> > >   security/security.c                 | 42 ++++++++++++++++++++++
> > >   security/selinux/hooks.c            | 55 +++++++++++++++++++++++++++++
> > >   security/selinux/include/classmap.h |  2 ++
> > >   6 files changed, 130 insertions(+)
> > 
> > My apologies for the late reply, I've been trying to work my way through
> > the review backlog but it has been taking longer than expected; comments
> > below ...
> 
> No worries, I have also been busy these days.
> 
> > > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> > > index 2aa0e219d721..65d9262a37f7 100644
> > > --- a/security/selinux/hooks.c
> > > +++ b/security/selinux/hooks.c
> > > @@ -21,6 +21,7 @@
> > >    *  Copyright (C) 2016 Mellanox Technologies
> > >    */
> > > +#include "av_permissions.h"
> > >   #include <linux/init.h>
> > >   #include <linux/kd.h>
> > >   #include <linux/kernel.h>
> > > @@ -92,6 +93,7 @@
> > >   #include <linux/fsnotify.h>
> > >   #include <linux/fanotify.h>
> > >   #include <linux/io_uring.h>
> > > +#include <uapi/linux/virtio_ids.h>
> > >   #include "avc.h"
> > >   #include "objsec.h"
> > > @@ -6950,6 +6952,56 @@ static int selinux_uring_cmd(struct io_uring_cmd *ioucmd)
> > >   }
> > >   #endif /* CONFIG_IO_URING */
> > > +static int vduse_check_device_type(u32 sid, u32 device_id)
> > > +{
> > > +	u32 requested;
> > > +
> > > +	if (device_id == VIRTIO_ID_NET)
> > > +		requested = VDUSE__NET;
> > > +	else if (device_id == VIRTIO_ID_BLOCK)
> > > +		requested = VDUSE__BLOCK;
> > > +	else
> > > +		return -EINVAL;
> > > +
> > > +	return avc_has_perm(sid, sid, SECCLASS_VDUSE, requested, NULL);
> > > +}
> > > +
> > > +static int selinux_vduse_dev_create(u32 device_id)
> > > +{
> > > +	u32 sid = current_sid();
> > > +	int ret;
> > > +
> > > +	ret = avc_has_perm(sid, sid, SECCLASS_VDUSE, VDUSE__DEVCREATE, NULL);
> > > +	if (ret)
> > > +		return ret;
> > > +
> > > +	return vduse_check_device_type(sid, device_id);
> > > +}
> > 
> > I see there has been some discussion about the need for a dedicated
> > create hook as opposed to using the existing ioctl controls.  I think
> > one important point that has been missing from the discussion is the
> > idea of labeling the newly created device.  Unfortunately prior to a
> > few minutes ago I hadn't ever looked at VDUSE so please correct me if
> > I get some things wrong :)
> > 
> >  From what I can see userspace creates a new VDUSE device with
> > ioctl(VDUSE_CREATE_DEV), which trigger the creation of a new
> > /dev/vduse/XXX device which will be labeled according to the udev
> > and SELinux configuration, likely with a generic udev label.  My
> > question is if we want to be able to uniquely label each VDUSE
> > device based on the process that initiates the device creation
> > with the call to ioctl()?  If that is the case, we would need a
> > create hook not only to control the creation of the device, but to
> > record the triggering process' label in the new device; this label
> > would then be used in subsequent VDUSE open and destroy operations.
> > The normal device file I/O operations would still be subject to the
> > standard SELinux file I/O permissions using the device file label
> > assigned by systemd/udev when the device was created.
> 
> I don't think we need a unique label for VDUSE devices, but maybe
> Michael thinks otherwise?

I don't know.
All this is consumed by libvirt, you need to ask these guys.


> > 
> > > +static int selinux_vduse_dev_destroy(u32 device_id)
> > > +{
> > > +	u32 sid = current_sid();
> > > +	int ret;
> > > +
> > > +	ret = avc_has_perm(sid, sid, SECCLASS_VDUSE, VDUSE__DEVDESTROY, NULL);
> > > +	if (ret)
> > > +		return ret;
> > > +
> > > +	return vduse_check_device_type(sid, device_id);
> > > +}
> > > +
> > > +static int selinux_vduse_dev_open(u32 device_id)
> > > +{
> > > +	u32 sid = current_sid();
> > > +	int ret;
> > > +
> > > +	ret = avc_has_perm(sid, sid, SECCLASS_VDUSE, VDUSE__DEVOPEN, NULL);
> > > +	if (ret)
> > > +		return ret;
> > > +
> > > +	return vduse_check_device_type(sid, device_id);
> > > +}
> > > +
> > >   /*
> > >    * IMPORTANT NOTE: When adding new hooks, please be careful to keep this order:
> > >    * 1. any hooks that don't belong to (2.) or (3.) below,
> > > @@ -7243,6 +7295,9 @@ static struct security_hook_list selinux_hooks[] __ro_after_init = {
> > >   #ifdef CONFIG_PERF_EVENTS
> > >   	LSM_HOOK_INIT(perf_event_alloc, selinux_perf_event_alloc),
> > >   #endif
> > > +	LSM_HOOK_INIT(vduse_dev_create, selinux_vduse_dev_create),
> > > +	LSM_HOOK_INIT(vduse_dev_destroy, selinux_vduse_dev_destroy),
> > > +	LSM_HOOK_INIT(vduse_dev_open, selinux_vduse_dev_open),
> > >   };
> > >   static __init int selinux_init(void)
> > > diff --git a/security/selinux/include/classmap.h b/security/selinux/include/classmap.h
> > > index a3c380775d41..d3dc37fb03d4 100644
> > > --- a/security/selinux/include/classmap.h
> > > +++ b/security/selinux/include/classmap.h
> > > @@ -256,6 +256,8 @@ const struct security_class_mapping secclass_map[] = {
> > >   	  { "override_creds", "sqpoll", "cmd", NULL } },
> > >   	{ "user_namespace",
> > >   	  { "create", NULL } },
> > > +	{ "vduse",
> > > +	  { "devcreate", "devdestroy", "devopen", "net", "block", NULL} },
> > 
> > I think we can just call the permissions "create", "open", and "destroy"
> > since the "dev" prefix is somewhat implied by this being a dedicated
> > VDUSE object class.
> 
> Ack, I can remove the "dev" prefix in next revision.
> 
> > 
> > I don't see where you are using the "net" and "block" permissions above,
> > is this a leftover from a prior draft of this patch or are you planning
> > to do something with these permissions?
> 
> It is actually used, but maybe not in a correct way.
> If you look at each hook, there are two checks performed:
> 1. Check for the operation type: create/destroy/open
> 2. Check for the device type: block/net
> 
> It means that the application will have to combine one (or more)
> operation type with one (or more) device type.
> 
> Does that make sense?
> 
> Thanks,
> Maxime
> 
> > 
> > >   	{ NULL }
> > >     };
> > > -- 
> > > 2.41.0
> > 
> > --
> > paul-moore.com
> >