[PATCH 1/1] cgroups: implement device whitelist cgroup+lsm

[Serge E . Hallyn]

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@xxxxxxxxxx>
---
 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 1
+#define DEVCG_DENY 2
+
+void set_access(char *acc, short access)
+{
+	int idx = 0;
+	memset(acc, 0, 4);
+	if (access & ACC_READ)
+		acc[idx++] = 'r';
+	if (access & ACC_WRITE)
+		acc[idx++] = 'w';
+	if (access & ACC_MKNOD)
+		acc[idx++] = 'm';
+}
+
+char type_to_char(short type)
+{
+	if (type == DEV_ALL)
+		return 'a';
+	if (type == DEV_CHAR)
+		return 'c';
+	if (type == DEV_BLOCK)
+		return 'b';
+	return 'X';
+}
+
+char *print_whitelist(struct dev_cgroup *devcgroup, int *len)
+{
+	char *buf, *s, acc[4];
+	struct dev_whitelist_item *wh;
+	int ret;
+	int count = 0;
+
+	buf = kmalloc(4096, GFP_KERNEL);
+	if (!buf)
+		return ERR_PTR(-ENOMEM);
+	s = buf;
+	*s = '\0';
+	*len = 0;
+
+	spin_lock(&devcgroup->lock);
+	list_for_each_entry(wh, &devcgroup->whitelist, list) {
+		set_access(acc, wh->access);
+		printk(KERN_NOTICE "%s (count%d): whtype %hd maj %u min %u acc %hd\n", __FUNCTION__,
+			count, wh->type, wh->major, wh->minor, wh->access);
+		ret = snprintf(s, 4095-(s-buf), "%c %u %u %s\n",
+			type_to_char(wh->type), wh->major, wh->minor, acc);
+		if (s+ret >= buf+4095) {
+			kfree(buf);
+			buf = ERR_PTR(-ENOMEM);
+			break;
+		}
+		s += ret;
+		*len += ret;
+		count++;
+	}
+	spin_unlock(&devcgroup->lock);
+
+	return buf;
+}
+
+static ssize_t devcg_access_read(struct cgroup *cgroup,
+			struct cftype *cft, struct file *file,
+			char __user *userbuf, size_t nbytes, loff_t *ppos)
+{
+	struct dev_cgroup *devcgrp = cgroup_to_devcg(cgroup);
+	int filetype = cft->private;
+	char *buffer;
+	int len, retval;
+
+	if (filetype != DEVCG_ALLOW)
+		return -EINVAL;
+	buffer = print_whitelist(devcgrp, &len);
+	if (IS_ERR(buffer))
+		return PTR_ERR(buffer);
+
+	retval = simple_read_from_buffer(userbuf, nbytes, ppos, buffer, len);
+	kfree(buffer);
+	return retval;
+}
+
+static inline short convert_access(char *acc)
+{
+	short access = 0;
+
+	while (*acc) {
+		switch (*acc) {
+			case 'r':
+			case 'R': access |= ACC_READ; break;
+			case 'w':
+			case 'W': access |= ACC_WRITE; break;
+			case 'm':
+			case 'M': access |= ACC_MKNOD; break;
+			case '\n': break;
+			default:
+				return -EINVAL;
+		}
+		acc++;
+	}
+
+	return access;
+}
+
+static inline short convert_type(char intype)
+{
+	short type = 0;
+	switch(intype) {
+		case 'a': type = DEV_ALL; break;
+		case 'c': type = DEV_CHAR; break;
+		case 'b': type = DEV_BLOCK; break;
+		default: type = -EACCES; break;
+	}
+	return type;
+}
+
+/*
+ * Current rules:
+ * CAP_SYS_ADMIN needed for all writes.
+ * when we have CAP_HOST_ADMIN, the rules will become:
+ *   if (!writetoself)
+ *      require capable(CAP_HOST_ADMIN | CAP_SYS_ADMIN);
+ *   if (is_allow)
+ *      require capable(CAP_HOST_ADMIN | CAP_SYS_ADMIN);
+ *   require capable(CAP_SYS_ADMIN);
+ */
+static int have_write_permission(int is_allow, int writetoself)
+{
+	if (!capable(CAP_SYS_ADMIN))
+		return 0;
+	return 1;
+}
+
+static ssize_t devcg_access_write(struct cgroup *cgroup, struct cftype *cft,
+				struct file *file, const char __user *userbuf,
+				size_t nbytes, loff_t *ppos)
+{
+	struct cgroup *cur_cgroup;
+	struct dev_cgroup *devcgrp, *cur_devcgroup;
+	int filetype = cft->private;
+	char *buffer, acc[4];
+	int retval = 0;
+	int nitems;
+	char type;
+	struct dev_whitelist_item *wh;
+
+	devcgrp = cgroup_to_devcg(cgroup);
+	cur_cgroup = task_cgroup(current, devcg_subsys.subsys_id);
+	cur_devcgroup = cgroup_to_devcg(cur_cgroup);
+
+	if (!have_write_permission(filetype==DEVCG_ALLOW, cur_devcgroup==devcgrp))
+		return -EPERM;
+
+	buffer = kmalloc(nbytes+1, GFP_KERNEL);
+	if (!buffer)
+		return -ENOMEM;
+
+	wh = kmalloc(sizeof(*wh), GFP_KERNEL);
+	if (!wh) {
+		kfree(buffer);
+		return -ENOMEM;
+	}
+
+	if (copy_from_user(buffer, userbuf, nbytes)) {
+		retval = -EFAULT;
+		goto out1;
+	}
+	buffer[nbytes] = 0;	/* nul-terminate */
+
+	cgroup_lock();
+	if (cgroup_is_removed(cgroup)) {
+		retval = -ENODEV;
+		goto out2;
+	}
+
+	memset(wh, 0, sizeof(*wh));
+	memset(acc, 0, 4);
+	nitems = sscanf(buffer, "%c %u %u %3s", &type, &wh->major, &wh->minor, acc);
+	retval = -EINVAL;
+	if (nitems != 4)
+		goto out2;
+	wh->type = convert_type(type);
+	if (wh->type < 0)
+		goto out2;
+	wh->access = convert_access(acc);
+	if (wh->access < 0)
+		goto out2;
+	retval = 0;
+	switch (filetype) {
+		case DEVCG_ALLOW:
+			printk(KERN_NOTICE "%s: add whtype %hd maj %u min %u acc %hd\n", __FUNCTION__,
+				wh->type, wh->major, wh->minor, wh->access);
+			dev_whitelist_add(devcgrp, wh);
+			break;
+		case DEVCG_DENY:
+			dev_whitelist_rm(devcgrp, wh);
+			break;
+	default:
+		retval = -EINVAL;
+		goto out2;
+	}
+
+	if (retval == 0)
+		retval = nbytes;
+
+out2:
+	cgroup_unlock();
+out1:
+	kfree(buffer);
+	return retval;
+}
+
+static struct cftype dev_cgroup_files[] = {
+	{
+		.name = "allow",
+		.read = devcg_access_read,
+		.write  = devcg_access_write,
+		.private = DEVCG_ALLOW,
+	},
+	{
+		.name = "deny",
+		.write = devcg_access_write,
+		.private = DEVCG_DENY,
+	},
+};
+
+static int devcg_populate(struct cgroup_subsys *ss,
+				struct cgroup *cont)
+{
+	return cgroup_add_files(cont, ss, dev_cgroup_files,
+					ARRAY_SIZE(dev_cgroup_files));
+}
+
+struct cgroup_subsys devcg_subsys = {
+	.name = "devcg",
+	.can_attach = devcg_can_attach,
+	.create = devcg_create,
+	.destroy  = devcg_destroy,
+	.populate = devcg_populate,
+	.subsys_id = devcg_subsys_id,
+};
+
+static int devcgroup_inode_permission(struct inode *inode, int mask,
+				    struct nameidata *nd)
+{
+	struct cgroup *cgroup;
+	struct dev_cgroup *dev_cgroup;
+	struct dev_whitelist_item *wh;
+
+	dev_t device = inode->i_rdev;
+	if (!device)
+		return 0;
+	if (!S_ISBLK(inode->i_mode) && !S_ISCHR(inode->i_mode))
+		return 0;
+	cgroup = task_cgroup(current, devcg_subsys.subsys_id);
+	dev_cgroup = cgroup_to_devcg(cgroup);
+	if (!dev_cgroup)
+		return 0;
+
+	spin_lock(&dev_cgroup->lock);
+	/* if capable(CAP_HOST_ADMIN) return 0; */
+	list_for_each_entry(wh, &dev_cgroup->whitelist, list) {
+		if (wh->type & DEV_ALL)
+			goto acc_check;
+		if (S_ISBLK(inode->i_mode) && !(wh->type & DEV_BLOCK))
+			continue;
+		if (S_ISCHR(inode->i_mode) && !(wh->type & DEV_CHAR))
+			continue;
+		if (wh->major != imajor(inode) || wh->minor != iminor(inode))
+			continue;
+acc_check:
+		if ((mask & MAY_WRITE) && !(wh->access & ACC_WRITE))
+			continue;
+		if ((mask & MAY_READ) && !(wh->access & ACC_READ))
+			continue;
+		spin_unlock(&dev_cgroup->lock);
+		return 0;
+	}
+	spin_unlock(&dev_cgroup->lock);
+
+	printk(KERN_NOTICE "%s: %d denied %d access to %s (%lu)\n", __FUNCTION__,
+		current->pid, mask, nd ? nd->dentry->d_name.name : "null",
+		inode->i_ino);
+	return -EPERM;
+}
+
+static int devcgroup_inode_mknod(struct inode *dir, struct dentry *dentry, int mode, dev_t dev)
+{
+	struct cgroup *cgroup;
+	struct dev_cgroup *dev_cgroup;
+	struct dev_whitelist_item *wh;
+	
+	/* if capable(CAP_HOST_ADMIN) return 0; */
+	cgroup = task_cgroup(current, devcg_subsys.subsys_id);
+	dev_cgroup = cgroup_to_devcg(cgroup);
+	if (!dev_cgroup)
+		return 0;
+
+	spin_lock(&dev_cgroup->lock);
+	list_for_each_entry(wh, &dev_cgroup->whitelist, list) {
+		if (wh->type & DEV_ALL)
+			goto ok;
+		if (S_ISBLK(mode) && !(wh->type & DEV_BLOCK))
+			continue;
+		if (S_ISCHR(mode) && !(wh->type & DEV_CHAR))
+			continue;
+		if (wh->major != MAJOR(dev) || wh->minor != MINOR(dev))
+			continue;
+		if (wh->access & ACC_MKNOD)
+			goto ok;
+	}
+	spin_unlock(&dev_cgroup->lock);
+
+	printk(KERN_NOTICE "%s: %d denied %d access to (%d %d)\n", __FUNCTION__,
+		current->pid, mode, MAJOR(dev), MINOR(dev));
+	return -EPERM;
+
+ok:
+	spin_unlock(&dev_cgroup->lock);
+	return 0;
+}
+
+static struct security_operations devcgroup_security_ops = {
+	.inode_mknod = devcgroup_inode_mknod,
+	.inode_permission = devcgroup_inode_permission,
+};
+
+static int __init dev_cgroup_security_init (void)
+{
+	/* register ourselves with the security framework */
+	if (register_security (&devcgroup_security_ops)) {
+		printk (KERN_INFO "Failure registering device cgroup lsm\n");
+		return -EINVAL;
+	}
+	printk (KERN_INFO "Device cgroup LSM initialized\n");
+	return 0;
+}
+
+security_initcall (dev_cgroup_security_init);
-- 
1.5.1.1.GIT

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