Here is an LSM-based alternative to Pavel's device control
cgroup, purely for discussion, not for any sort of code
review (please :).

thanks,
-serge

>From 4266131c40b629e3b04c0d9d01569a95fa967e3e Mon Sep 17 00:00:00 2001
From: Serge E. Hallyn <serue@us.ibm.com>
Date: Thu, 11 Oct 2007 15:27:48 -0400
Subject: [PATCH RFC] cgroups: implement device whitelist cgroup+lsm

Implement a cgroup using the LSM interface to enforce open and mknod
on device files.  Not a line of this code is expected to be used in a
final version, this is just a proof of concept.

No stacking is implemented, so to test this you must have

	CGROUPS=y
	SECURITY=y

but all other LSMs =n (no capabilities, no selinux, no rootplug).

This implements a simple device access whitelist.  A whitelist entry
has 4 fields.  'type' is a (all), c (char), or b (block).  'all' means it
applies to all types, all major numbers, and all minor numbers.  Major and
minor are obvious.  Access is a composition of r (read), w (write), and
m (mknod).

The root devcgroup starts with rwm to 'all'.  A child devcg gets a copy
of the parent.  Admins can then add and remove devices to the whitelist.
Once CAP_HOST_ADMIN is introduced it will be needed to add entries as
well or remove entries from another cgroup, though just CAP_SYS_ADMIN
will suffice to remove entries for your own group.

An entry is added by doing "echo <type> <maj> <min> <access>" > devcg.allow,
for instance:

	echo b 7 0 mrw > /cgroups/1/devcg.allow

An entry is removed by doing likewise into devcg.deny.  Since this is a
pure whitelist, not acls, you can only remove entries which exist in the
whitelist.  You must explicitly

	echo a 0 0 mrw > /cgroups/1/devcg.deny

to remove the "allow all" entry which is automatically inherited from
the root cgroup.

While composing this with the ns_cgroup may seem logical, it may not
be the right thing to do.  Note that each newly created devcg gets
a copy of the parent whitelist.  So if you had done

	mount -t cgroup -o ns,devcg none /cgroups

then once a process in /cgroup/1 had done an unshare(CLONE_NEWNS)
it would be under /cgroup/1/node_<pid>
if an admin did

	echo b 7 0 m > /cgroups/1/devcg.deny

then the entry would still be in the whitelist for /cgroups/1/node_<pid>.
Something to discuss if we get that far before nixing this whole idea.

Signed-off-by: Serge E. Hallyn <serue@us.ibm.com>
---
 include/linux/cgroup_subsys.h |    6 +
 init/Kconfig                  |    7 +
 kernel/Makefile               |    1 +
 kernel/dev_cgroup.c           |  554 +++++++++++++++++++++++++++++++++++++++++
 4 files changed, 568 insertions(+), 0 deletions(-)
 create mode 100644 kernel/dev_cgroup.c

diff --git a/include/linux/cgroup_subsys.h b/include/linux/cgroup_subsys.h
index d822977..cf55cb2 100644
--- a/include/linux/cgroup_subsys.h
+++ b/include/linux/cgroup_subsys.h
@@ -36,3 +36,9 @@ SUBSYS(mem_cgroup)
 #endif
 
 /* */
+
+#ifdef CONFIG_CGROUP_DEV
+SUBSYS(devcg)
+#endif
+
+/* */
diff --git a/init/Kconfig b/init/Kconfig
index 6bb603a..0b3b684 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -319,6 +319,13 @@ config CPUSETS
 
 	  Say N if unsure.
 
+config CGROUP_DEV
+	bool "Device controller for cgroups"
+	depends on CGROUPS && SECURITY && EXPERIMENTAL
+	help
+	  Provides a cgroup implementing whitelists for devices which
+	  a process in the cgroup can mknod or open.
+
 config FAIR_GROUP_SCHED
 	bool "Fair group CPU scheduler"
 	default y
diff --git a/kernel/Makefile b/kernel/Makefile
index 76f782f..6ded46d 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -43,6 +43,7 @@ obj-$(CONFIG_CGROUP_DEBUG) += cgroup_debug.o
 obj-$(CONFIG_CPUSETS) += cpuset.o
 obj-$(CONFIG_CGROUP_CPUACCT) += cpu_acct.o
 obj-$(CONFIG_CGROUP_NS) += ns_cgroup.o
+obj-$(CONFIG_CGROUP_DEV) += dev_cgroup.o
 obj-$(CONFIG_IKCONFIG) += configs.o
 obj-$(CONFIG_STOP_MACHINE) += stop_machine.o
 obj-$(CONFIG_AUDIT) += audit.o auditfilter.o
diff --git a/kernel/dev_cgroup.c b/kernel/dev_cgroup.c
new file mode 100644
index 0000000..87c8fb4
--- /dev/null
+++ b/kernel/dev_cgroup.c
@@ -0,0 +1,554 @@
+/*
+ * dev_cgroup.c - device cgroup subsystem
+ *
+ * Copyright 2007 IBM Corp
+ */
+
+#include <linux/module.h>
+#include <linux/cgroup.h>
+#include <linux/fs.h>
+#include <linux/list.h>
+#include <linux/security.h>
+
+#include <asm/uaccess.h>
+
+#define ACC_MKNOD 1
+#define ACC_READ  2
+#define ACC_WRITE 4
+
+#define DEV_BLOCK 1
+#define DEV_CHAR  2
+#define DEV_ALL   4  /* this represents all devices */
+
+/*
+ * whitelist locking rules:
+ * cgroup_lock() cannot be taken under cgroup->lock.
+ * cgroup->lock can be taken with or without cgroup_lock().
+ *
+ * modifications always require cgroup_lock
+ * modifications to a list which is visible require the
+ *   cgroup->lock *and* cgroup_lock()
+ * walking the list requires cgroup->lock or cgroup_lock().
+ *
+ * reasoning: dev_whitelist_copy() needs to kmalloc, so needs
+ *   a mutex, which the cgroup_lock() is.  Since modifying
+ *   a visible list requires both locks, either lock can be
+ *   taken for walking the list.  Since the wh->spinlock is taken
+ *   for modifying a public-accessible list, the spinlock is
+ *   sufficient for just walking the list.
+ */
+
+struct dev_whitelist_item {
+	u32 major, minor;
+	short type;
+	short access;
+	struct list_head list;
+};
+
+struct dev_cgroup {
+	struct cgroup_subsys_state css;
+	struct list_head whitelist;
+	spinlock_t lock;
+};
+
+struct cgroup_subsys devcg_subsys;
+
+static inline struct dev_cgroup *cgroup_to_devcg(
+		struct cgroup *cgroup)
+{
+	return container_of(cgroup_subsys_state(cgroup, devcg_subsys_id),
+			    struct dev_cgroup, css);
+}
+
+/*
+ * Once 64-bit caps and CAP_HOST_ADMIN exist, we will be
+ * requiring (CAP_HOST_ADMIN|CAP_MKNOD) to create a device
+ * not in the whitelist, * (CAP_HOST_ADMIN|CAP_SYS_ADMIN)
+ * to edit the whitelist,
+ */
+static int devcg_can_attach(struct cgroup_subsys *ss,
+		struct cgroup *new_cgroup, struct task_struct *task)
+{
+	struct cgroup *orig;
+
+	if (current != task) {
+		if (!cgroup_is_descendant(new_cgroup))
+			return -EPERM;
+	}
+
+	if (atomic_read(&new_cgroup->count) != 0)
+		return -EPERM;
+
+	orig = task_cgroup(task, devcg_subsys_id);
+	if (orig && orig != new_cgroup->parent)
+		return -EPERM;
+
+	return 0;
+}
+
+/*
+ * called under cgroup_lock()
+ */
+int dev_whitelist_copy(struct list_head *dest, struct list_head *orig)
+{
+	struct dev_whitelist_item *wh, *tmp, *new;
+
+	list_for_each_entry(wh, orig, list) {
+		new = kmalloc(sizeof(*wh), GFP_KERNEL);
+		if (!new)
+			goto free_and_exit;
+		new->major = wh->major;
+		new->minor = wh->minor;
+		new->type = wh->type;
+		new->access = wh->access;
+		list_add_tail(&new->list, dest);
+	}
+
+	return 0;
+
+free_and_exit:
+	list_for_each_entry_safe(wh, tmp, dest, list) {
+		list_del(&wh->list);
+		kfree(wh);
+	}
+	return -ENOMEM;
+}
+
+/* Stupid prototype - don't bother combining existing entries */
+/*
+ * called under cgroup_lock()
+ * since the list is visible to other tasks, we need the spinlock also
+ */
+void dev_whitelist_add(struct dev_cgroup *dev_cgroup, struct dev_whitelist_item *wh)
+{
+	spin_lock(&dev_cgroup->lock);
+	list_add_tail(&wh->list, &dev_cgroup->whitelist);
+	spin_unlock(&dev_cgroup->lock);
+}
+
+/*
+ * called under cgroup_lock()
+ * since the list is visible to other tasks, we need the spinlock also
+ */
+void dev_whitelist_rm(struct dev_cgroup *dev_cgroup, struct dev_whitelist_item *wh)
+{
+	struct dev_whitelist_item *walk, *tmp;
+
+	spin_lock(&dev_cgroup->lock);
+	list_for_each_entry_safe(walk, tmp, &dev_cgroup->whitelist, list) {
+		if (walk->type & DEV_ALL) {
+			list_del(&walk->list);
+			kfree(walk);
+			continue;
+		}
+		if (walk->type != wh->type)
+			continue;
+		if (walk->major != wh->major || walk->minor != wh->minor)
+			continue;
+		walk->access &= ~wh->access;
+		if (!walk->access) {
+			list_del(&walk->list);
+			kfree(walk);
+		}
+	}
+	spin_unlock(&dev_cgroup->lock);
+}
+
+/*
+ * Rules: you can only create a cgroup if
+ *     1. you are capable(CAP_SYS_ADMIN)
+ *     2. the target cgroup is a descendant of your own cgroup
+ *
+ * Note: called from kernel/cgroup.c with cgroup_lock() held.
+ */
+static struct cgroup_subsys_state *devcg_create(struct cgroup_subsys *ss,
+						struct cgroup *cgroup)
+{
+	struct dev_cgroup *dev_cgroup, *parent_dev_cgroup;
+	struct cgroup *parent_cgroup;
+	int ret;
+
+	if (!capable(CAP_SYS_ADMIN))
+		return ERR_PTR(-EPERM);
+	if (!cgroup_is_descendant(cgroup))
+		return ERR_PTR(-EPERM);
+
+	dev_cgroup = kzalloc(sizeof(*dev_cgroup), GFP_KERNEL);
+	if (!dev_cgroup)
+		return ERR_PTR(-ENOMEM);
+	INIT_LIST_HEAD(&dev_cgroup->whitelist);
+	parent_cgroup = cgroup->parent;
+
+	if (parent_cgroup == NULL) {
+		struct dev_whitelist_item *wh;
+		wh = kmalloc(sizeof(*wh), GFP_KERNEL);
+		wh->minor = wh->major = 0;
+		wh->type = DEV_ALL;
+		wh->access = ACC_MKNOD | ACC_READ | ACC_WRITE;
+		list_add(&wh->list, &dev_cgroup->whitelist);
+	} else {
+		parent_dev_cgroup = cgroup_to_devcg(parent_cgroup);
+		ret = dev_whitelist_copy(&dev_cgroup->whitelist,
+				&parent_dev_cgroup->whitelist);
+		if (ret) {
+			kfree(dev_cgroup);
+			return ERR_PTR(ret);
+		}
+	}
+
+	spin_lock_init(&dev_cgroup->lock);
+	return &dev_cgroup->css;
+}
+
+static void devcg_destroy(struct cgroup_subsys *ss,
+			struct cgroup *cgroup)
+{
+	struct dev_cgroup *dev_cgroup;
+	struct dev_whitelist_item *wh, *tmp;
+
+	dev_cgroup = cgroup_to_devcg(cgroup);
+	list_for_each_entry_safe(wh, tmp, &dev_cgroup->whitelist, list) {
+		list_del(&wh->list);
+		kfree(wh);
+	}
+	kfree(dev_cgroup);
+}
+
+#define DEVCG_ALLOW