Quoting Pavel Emelyanov (xemul@openvz.org):
> Serge E. Hallyn wrote:
> > Quoting Pavel Emelyanov (xemul@openvz.org):
> >> Each new group will have its own maps for char and block
> >> layers. The devices access list is tuned via the 
> >> devices.permissions file. One may read from the file to get 
> >> the configured state.
> >>
> >> The top container isn't initialized, so that the char 
> >> and block layers will use the global maps to lookup 
> >> their devices. I did that not to export the static maps
> >> to the outer world.
> >>
> >> Good news is that this patch now contains more comments 
> >> and Documentation file :)
> > 
> > Seems to work as advertised  :)  I can't reproduce Suka's null/zero
> > bug.
> > 
> > You're relying fully on uid-0 to stop writes into the
> > devices.permissions files.  Would you mind adding a check for
> > CAP_SYS_ADMIN (or CAP_NS_OVERRIDE+CAP_MKNOD)?  Or were you really
> > counting on using the filesystem visibility cgroup to stop a container
> > from making changes to its device access whitelist?
> 
> Yup. I strongly believe that a controller itself should not bring 
> any security policy of its own, but the infrastructure should 
> take care of this.

That would be ok if the controller gave the infrastructure some way of
knowing what sort of thing the controller does.  I.e. I wouldn't mind
having the cgroup infrastructe check for capabilities, but I suspect
some cgroups will really be best represented by different capabilities.

Paul (actually both Menage and Jackson :) do you have an opinion on
this?  Are there sites which eg do 'chown -R some_user_id /cgroup/cpusets/'
to have some non-root user be able to dole out cpusets?  Is there any
way it would be ok to have cgroup_file_write() check for CAP_SYS_ADMIN?

> However, I'm open to change my mind if I see good explanation of
> why it is wrong.

Well the thing is that it currently leaves no way to keep root in
another namespace from manipulating it.  I don't think we can even use
SELinux unless we're willing to prevent containers from having write
access to everything under the cgroup - which is only ok depending on
what is composed with the devices cgroup.

We have the same kind of problem with the
/proc/sys/filesystems/<fs>/fs_safe flag.  There are a few possibilities,
and your fs visibility cgroup (plus splitting /proc/sys/filesystems into
its own fs) is one.  More complicated things pop into my head, but I
think until we get more comfortable with all this the simplest way is
the best.

But really imo the simplest way is to have a capable(CAP_SYS_ADMIN)
check inside your _write function :)

Ideally if you didn't mind I would float a patch (cousin to the userns
4-patch set) defining CAP_NS_OVERRIDE and requiring both CAP_NS_OVERRIDE
and CAP_SYS_ADMIN to access _write.

-serge

> > thanks,
> > -serge
> > 
> >> Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
> >>
> >> ---
> >>
> >> diff --git a/Documentation/controllers/devices.txt b/Documentation/controllers/devices.txt
> >> new file mode 100644
> >> index 0000000..dbd0c7a
> >> --- /dev/null
> >> +++ b/Documentation/controllers/devices.txt
> >> @@ -0,0 +1,61 @@
> >> +
> >> +	Devices visibility controller
> >> +
> >> +This controller allows to tune the devices accessibility by tasks,
> >> +i.e. grant full access for /dev/null, /dev/zero etc, grant read-only
> >> +access to IDE devices and completely hide SCSI disks.
> >> +
> >> +Tasks still can call mknod to create device files, regardless of
> >> +whether the particular device is visible or accessible, but they
> >> +may not be able to open it later.
> >> +
> >> +This one hides under CONFIG_CGROUP_DEVS option.
> >> +
> >> +
> >> +Configuring
> >> +
> >> +The controller provides a single file to configure itself -- the
> >> +devices.permissions one. To change the accessibility level for some
> >> +device write the following string into it:
> >> +
> >> +[cb] <major>:(<minor>|*) [r-][w-]
> >> + ^          ^               ^
> >> + |          |               |
> >> + |          |               +--- access rights (1)
> >> + |          |
> >> + |          +-- device major and minor numbers (2)
> >> + |
> >> + +-- device type (character / block)
> >> +
> >> +1) The access rights set to '--' remove the device from the group's
> >> +access list, so that it will not even be shown in this file later.
> >> +
> >> +2) Setting the minor to '*' grants access to all the minors for
> >> +particular major.
> >> +
> >> +When reading from it, one may see something like
> >> +
> >> +	c 1:5 rw
> >> +	b 8:* r-
> >> +
> >> +Security issues, concerning who may grant access to what are governed
> >> +at the cgroup infrastructure level.
> >> +
> >> +
> >> +Examples:
> >> +
> >> +1. Grand full access to /dev/null
> >> +	# echo c 1:3 rw > /cgroups/<id>/devices.permissions
> >> +
> >> +2. Grant the read-only access to /dev/sda and partitions
> >> +	# echo b 8:* r- > ...
> >> +
> >> +3. Change the /dev/null access to write-only
> >> +	# echo c 1:3 -w > ...
> >> +
> >> +4. Revoke access to /dev/sda
> >> +	# echo b 8:* -- > ...
> >> +
> >> +
> >> +		Written by Pavel Emelyanov <xemul@openvz.org>
> >> +
> >> diff --git a/fs/Makefile b/fs/Makefile
> >> index 7996220..5ad03be 100644
> >> --- a/fs/Makefile
> >> +++ b/fs/Makefile
> >> @@ -64,6 +64,8 @@ obj-y				+= devpts/
> >>
> >>  obj-$(CONFIG_PROFILING)		+= dcookies.o
> >>  obj-$(CONFIG_DLM)		+= dlm/
> >> +
> >> +obj-$(CONFIG_CGROUP_DEVS)	+= devscontrol.o
> >>   
> >>  # Do not add any filesystems before this line
> >>  obj-$(CONFIG_REISERFS_FS)	+= reiserfs/
> >> diff --git a/fs/devscontrol.c b/fs/devscontrol.c
> >> new file mode 100644
> >> index 0000000..48c5f69
> >> --- /dev/null
> >> +++ b/fs/devscontrol.c
> >> @@ -0,0 +1,314 @@
> >> +/*
> >> + * devscontrol.c - Device Controller
> >> + *
> >> + * Copyright 2007 OpenVZ SWsoft Inc
> >> + * Author: Pavel Emelyanov <xemul at openvz dot org>
> >> + *
> >> + * This program is free software; you can redistribute it and/or modify
> >> + * it under the terms of the GNU General Public License as published by
> >> + * the Free Software Foundation; either version 2 of the License, or
> >> + * (at your option) any later version.
> >> + *
> >> + * This program is distributed in the hope that it will be useful,
> >> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> >> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> >> + * GNU General Public License for more details.
> >> + */
> >> +
> >> +#include <linux/cgroup.h>
> >> +#include <linux/cdev.h>
> >> +#include <linux/err.h>
> >> +#include <linux/devscontrol.h>
> >> +#include <linux/uaccess.h>
> >> +#include <linux/fs.h>
> >> +#include <linux/genhd.h>
> >> +
> >> +struct devs_cgroup {
> >> +	/*
> >> +	 * The subsys state to build into cgrous infrastructure
> >> +	 */
> >> +	struct cgroup_subsys_state css;
> >> +
> >> +	/*
> >> +	 * The maps of character and block devices. They provide a
> >> +	 * map from dev_t-s to struct cdev/gendisk. See fs/char_dev.c
> >> +	 * and block/genhd.c to find out how the ->open() callbacks
> >> +	 * work when opening a device.
> >> +	 *
> >> +	 * Each group will have its onw maps, and at the open()
> >> +	 * time code will lookup in this map to get the device
> >> +	 * and permissions by its dev_t.
> >> +	 */
> >> +	struct kobj_map *cdev_map;
> >> +	struct kobj_map *bdev_map;
> >> +};
> >> +
> >> +static inline
> >> +struct devs_cgroup *css_to_devs(struct cgroup_subsys_state *css)
> >> +{
> >> +	return container_of(css, struct devs_cgroup, css);
> >> +}
> >> +
> >> +static inline
> >> +struct devs_cgroup *cgroup_to_devs(struct cgroup *cont)
> >> +{
> >> +	return css_to_devs(cgroup_subsys_state(cont, devs_subsys_id));
> >> +}
> >> +
> >> +struct kobj_map *task_cdev_map(struct task_struct *tsk)
> >> +{
> >> +	struct cgroup_subsys_state *css;
> >> +
> >> +	css = task_subsys_state(tsk, devs_subsys_id);
> >> +	if (css->cgroup->parent == NULL)
> >> +		return NULL;
> >> +	else
> >> +		return css_to_devs(css)->cdev_map;
> >> +}
> >> +
> >> +struct kobj_map *task_bdev_map(struct task_struct *tsk)
> >> +{
> >> +	struct cgroup_subsys_state *css;
> >> +
> >> +	css = task_subsys_state(tsk, devs_subsys_id);
> &g