Message-ID: <47DF4FD9.7090308@cn.fujitsu.com>
Date: Tue, 18 Mar 2008 14:15:05 +0900
From: Li Zefan <lizf@cn.fujitsu.com>
User-Agent: Thunderbird 2.0.0.12 (X11/20080226)
MIME-Version: 1.0
To: "Serge E. Hallyn" <serue@us.ibm.com>
CC: lkml <linux-kernel@vger.kernel.org>, linux-security-module@vger.kernel.org,
       Linux Containers <containers@lists.osdl.org>,
       Stephen Smalley <sds@epoch.ncsc.mil>,
       Pavel Emelianov <xemul@openvz.org>, James Morris <jmorris@namei.org>,
       Greg KH <greg@kroah.com>, Casey Schaufler <casey@schaufler-ca.com>,
       Paul Menage <menage@google.com>
Subject: Re: [PATCH] cgroups: implement device whitelist (v4)
References: <20080317180722.GA17111@sergelap.austin.ibm.com>
In-Reply-To: <20080317180722.GA17111@sergelap.austin.ibm.com>
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Sender: linux-kernel-owner@vger.kernel.org
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Serge E. Hallyn wrote:
> Implement a cgroup to track and enforce open and mknod restrictions on device
> files.  A device cgroup associates a device access whitelist with each
> cgroup.  A whitelist entry has 4 fields.  'type' is a (all), c (char), or
> b (block).  'all' means it applies to all types and all major and minor
> numbers.  Major and minor are either an integer or * for all.
> Access is a composition of r (read), w (write), and m (mknod).
> 
> The root device cgroup starts with rwm to 'all'.  A child devcg gets
> a copy of the parent.  Admins can then remove devices from the
> whitelist or add new entries.  A child cgroup can never receive a
> device access which is denied its parent.  However when a device
> access is removed from a parent it will not also be removed from the
> child(ren).
> 
> An entry is added using devices.allow, and removed using
> devices.deny.  For instance
> 
> 	echo 'c 1:3 mr' > /cgroups/1/devices.allow
> 
> allows cgroup 1 to read and mknod the device usually known as
> /dev/null.  Doing
> 
> 	echo a > /cgroups/1/devices.deny
> 
> will remove the default 'a *:* mrw' entry.
> 
> CAP_SYS_ADMIN is needed to change permissions or move another task
> to a new cgroup.  A cgroup may not be granted more permissions than
> the cgroup's parent has.  Any task can move itself between cgroups.
> This won't be sufficient, but we can decide the best way to
> adequately restrict movement later.
> 
> The parsing of devices.allow/deny needs to be cleaned up a bit and
> Documented.  I'd like to get an idea whether this approach is otherwise
> acceptable.
> 
> Changelog:
> 	Mar 17 2008: Place specific device cgroup hooks next to
> 		security_inode_{mknod,permission} rather than using
> 		the security hooks.
> 		Also remove most of the controls over tasks moving
> 		between cgroups and playing with the allow and deny
> 		permissions.
> 		Switch to major:minor format.
> 		Rename devcg to 'devices' to conform to cgroup naming.
> 	Mar 13 2008: move the dev_cgroup support into
> 		capability hooks instead of having it
> 		as a separate security module.
> 		Support root_plug with devcg.
> 		Note that due to this change, devcg will
> 		not be enforcing if the dummy module is
> 		loaded, or if selinux is loaded without
> 		capabilities.
> 	Mar 12 2008: allow dev_cgroup lsm to be used when
> 		SECURITY=n, and allow stacking with SELinux
> 		and Smack.  Don't work too hard in Kconfig
> 		to prevent a warning when smack+devcg are
> 		both compiled in, worry about that later.
> 

I would like to give some comments in the code. :)

> Signed-off-by: Serge E. Hallyn <serue@us.ibm.com>
> ---
>  fs/namei.c                    |    9 +
>  include/linux/cgroup_subsys.h |    6 +
>  include/linux/device_cgroup.h |   12 +
>  init/Kconfig                  |    7 +
>  security/Makefile             |    1 +
>  security/device_cgroup.c      |  597 +++++++++++++++++++++++++++++++++++++++++
>  6 files changed, 632 insertions(+), 0 deletions(-)
>  create mode 100644 include/linux/device_cgroup.h
>  create mode 100644 security/device_cgroup.c
> 
> diff --git a/fs/namei.c b/fs/namei.c
> index dfb3cb8..6caed32 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -30,6 +30,7 @@
>  #include <linux/capability.h>
>  #include <linux/file.h>
>  #include <linux/fcntl.h>
> +#include <linux/device_cgroup.h>
>  #include <asm/namei.h>
>  #include <asm/uaccess.h>
>  
> @@ -281,6 +282,10 @@ int permission(struct inode *inode, int mask, struct nameidata *nd)
>  	if (retval)
>  		return retval;
>  
> +	retval = devcgroup_inode_permission(inode, mask);
> +	if (retval)
> +		return retval;
> +
>  	return security_inode_permission(inode, mask, nd);
>  }
>  
> @@ -2028,6 +2033,10 @@ int vfs_mknod(struct inode *dir, struct dentry *dentry, int mode, dev_t dev)
>  	if (!dir->i_op || !dir->i_op->mknod)
>  		return -EPERM;
>  
> +	error = devcgroup_inode_mknod(mode, dev);
> +	if (error)
> +		return error;
> +
>  	error = security_inode_mknod(dir, dentry, mode, dev);
>  	if (error)
>  		return error;
> diff --git a/include/linux/cgroup_subsys.h b/include/linux/cgroup_subsys.h
> index 1ddebfc..e287745 100644
> --- a/include/linux/cgroup_subsys.h
> +++ b/include/linux/cgroup_subsys.h
> @@ -42,3 +42,9 @@ SUBSYS(mem_cgroup)
>  #endif
>  
>  /* */
> +
> +#ifdef CONFIG_CGROUP_DEVICE
> +SUBSYS(devices)
> +#endif
> +
> +/* */
> diff --git a/include/linux/device_cgroup.h b/include/linux/device_cgroup.h
> new file mode 100644
> index 0000000..0b0d9c3
> --- /dev/null
> +++ b/include/linux/device_cgroup.h
> @@ -0,0 +1,12 @@
> +#include <linux/module.h>
> +#include <linux/fs.h>
> +
> +#ifdef CONFIG_CGROUP_DEVICE
> +extern int devcgroup_inode_permission(struct inode *inode, int mask);
> +extern int devcgroup_inode_mknod(int mode, dev_t dev);
> +#else
> +static inline int devcgroup_inode_permission(struct inode *inode, int mask)
> +{ return 0; }
> +static inline int devcgroup_inode_mknod(int mode, dev_t dev)
> +{ return 0; }
> +#endif
> diff --git a/init/Kconfig b/init/Kconfig
> index 009f2d8..30868cd 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -298,6 +298,13 @@ config CGROUP_NS
>            for instance virtual servers and checkpoint/restart
>            jobs.
>  
> +config CGROUP_DEVICE
> +	bool "Device controller for cgroups"
> +	depends on CGROUPS && EXPERIMENTAL
> +	help
> +	  Provides a cgroup implementing whitelists for devices which
> +	  a process in the cgroup can mknod or open.
> +
>  config CPUSETS
>  	bool "Cpuset support"
>  	depends on SMP && CGROUPS
> diff --git a/security/Makefile b/security/Makefile
> index 9e8b025..7ef1107 100644
> --- a/security/Makefile
> +++ b/security/Makefile
> @@ -18,3 +18,4 @@ obj-$(CONFIG_SECURITY_SELINUX)		+= selinux/built-in.o
>  obj-$(CONFIG_SECURITY_SMACK)		+= commoncap.o smack/built-in.o
>  obj-$(CONFIG_SECURITY_CAPABILITIES)	+= commoncap.o capability.o
>  obj-$(CONFIG_SECURITY_ROOTPLUG)		+= commoncap.o root_plug.o
> +obj-$(CONFIG_CGROUP_DEVICE)		+= device_cgroup.o
> diff --git a/security/device_cgroup.c b/security/device_cgroup.c
> new file mode 100644
> index 0000000..33d8fd8
> --- /dev/null
> +++ b/security/device_cgroup.c
> @@ -0,0 +1,597 @@
> +/*
> + * dev_cgroup.c - device cgroup subsystem
> + *
> + * Copyright 2007 IBM Corp
> + */
> +
> +#include <linux/device_cgroup.h>
> +#include <linux/cgroup.h>
> +#include <linux/ctype.h>
> +#include <linux/list.h>
> +#include <asm/uaccess.h>
> +
> +#define ACC_MKNOD 1
> +#define ACC_READ  2
> +#define ACC_WRITE 4
> +#define ACC_MASK (ACC_MKNOD | ACC_READ | ACC_WRITE)
> +
> +#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.

When you say cgroup->lock, you mean dev_cgroup->lock, right?
So would it be better to make it clear in the comment?

> + * 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;
> +};
> +
> +static inline struct dev_cgroup *cgroup_to_devcgroup(
> +		struct cgroup *cgroup)
> +{
> +	return container_of(cgroup_subsys_state(cgroup, devices_subsys_id),
> +			    struct dev_cgroup, css);
> +}
> +
> +
> +struct cgroup_subsys devices_subsys;
> +
> +static int devcgroup_can_attach(struct cgroup_subsys *ss,
> +		struct cgroup *new_cgroup, struct task_struct *task)
> +{
> +

redundant empty line

> +	if (current != task && !capable(CAP_SYS_ADMIN))
> +			return -EPERM;
> +
> +	return 0;
> +}
> +
> +/*
> + * called under cgroup_lock()
> + */
> +int dev_whitelist_copy(struct list_head *dest, struct list_head *orig)

static int

> +{
> +	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 dev_whitelist_addother tasks, we need the spinlock also
> + */
> +int dev_whitelist_add(struct dev_cgroup *dev_cgroup,
> +			struct dev_whitelist_item *wh)

ditoo

> +{
> +	struct dev_whitelist_item *whcopy;
> +
> +	whcopy = kmalloc(sizeof(*whcopy), GFP_KERNEL);
> +	if (!whcopy)
> +		return -ENOMEM;
> +
> +	memcpy(whcopy, wh, sizeof(*whcopy));
> +	spin_lock(&dev_cgroup->lock);
> +	list_add_tail(&whcopy->list, &dev_cgroup->whitelist);
> +	spin_unlock(&dev_cgroup->lock);
> +	return 0;
> +}
> +
> +/*
> + * 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)

ditto

> +{
> +	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)
> +			goto remove;
> +		if (walk->type != wh->type)
> +			continue;
> +		if (walk->major != ~0 && walk->major != wh->major)
> +			continue;
> +		if (walk->minor != ~0 && walk->minor != wh->minor)
> +			continue;
> +
> +remove:
> +		walk->access &= ~wh->access;
> +		if (!walk->access) {
> +			list_del(&walk->list);
> +			kfree(walk);
> +		}
> +	}
> +	spin_unlock(&dev_cgroup->lock);
> +}
> +
> +/*
> + * called from kernel/cgroup.c with cgroup_lock() held.
> + */
> +static struct cgroup_subsys_state *devcgroup_create(struct cgroup_subsys *ss,
> +						struct cgroup *cgroup)
> +{
> +	struct dev_cgroup *dev_cgroup, *parent_dev_cgroup;
> +	struct cgroup *parent_cgroup;
> +	int ret;
> +
> +	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);

if (wh == NULL) ..

> +		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_devcgroup(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 devcgroup_destroy(struct cgroup_subsys *ss,
> +			struct cgroup *cgroup)
> +{
> +	struct dev_cgroup *dev_cgroup;
> +	struct dev_whitelist_item *wh, *tmp;
> +
> +	dev_cgroup = cgroup_to_devcgroup(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)

static

> +{
> +	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)

static

> +{
> +	if (type == DEV_ALL)
> +		return 'a';
> +	if (type == DEV_CHAR)
> +		return 'c';
> +	if (type == DEV_BLOCK)
> +		return 'b';
> +	return 'X';
> +}
> +
> +static void set_majmin(char *str, int len, unsigned m)
> +{
> +	memset(str, 0, len);
> +	if (m == ~0)
> +		sprintf(str, "*");
> +	else
> +		snprintf(str, len, "%d", m);
> +}
> +
> +char *print_whitelist(struct dev_cgroup *devcgroup, int *len)

static

> +{
> +	char *buf, *s, acc[4];
> +	struct dev_whitelist_item *wh;
> +	int ret;
> +	int count = 0;
> +	char maj[10], min[10];
> +
> +	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);
> +		set_majmin(maj, 10, wh->major);
> +		set_majmin(min, 10, wh->minor);
> +		ret = snprintf(s, 4095-(s-buf), "%c %s:%s %s\n",
> +			type_to_char(wh->type), maj, min, 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 devcgroup_access_read(struct cgroup *cgroup,
> +			struct cftype *cft, struct file *file,
> +			char __user *userbuf, size_t nbytes, loff_t *ppos)
> +{
> +	struct dev_cgroup *devcgroup = cgroup_to_devcgroup(cgroup);
> +	int filetype = cft->private;
> +	char *buffer;
> +	int len, retval;
> +
> +	if (filetype != DEVCG_ALLOW)
> +		return -EINVAL;
> +	buffer = print_whitelist(devcgroup, &len);
> +	if (IS_ERR(buffer))
> +		return PTR_ERR(buffer);
> +
> +	retval = simple_read_from_buffer(userbuf, nbytes, ppos, buffer, len);
> +	kfree(buffer);
> +	return retval;
> +}
> +
> +/*
> + * may_access_whitelist:
> + * does the access granted to dev_cgroup c contain the access
> + * requested in whitelist item refwh.
> + * return 1 if yes, 0 if no.
> + * call with c->lock held
> + */
> +static int may_access_whitelist(struct dev_cgroup *c,
> +				       struct dev_whitelist_item *refwh)
> +{
> +	struct dev_whitelist_item *whitem;
> +
> +	list_for_each_entry(whitem, &c->whitelist, list) {
> +		if (whitem->type & DEV_ALL)
> +			return 1;
> +		if ((refwh->type & DEV_BLOCK) && !(whitem->type & DEV_BLOCK))
> +			continue;
> +		if ((refwh->type & DEV_CHAR) && !(whitem->type & DEV_CHAR))
> +			continue;
> +		if (whitem->major != ~0 && whitem->major != refwh->major)
> +			continue;
> +		if (whitem->minor != ~0 && whitem->minor != refwh->minor)
> +			continue;
> +		if (refwh->access & (~(whitem->access | ACC_MASK)))
> +			continue;
> +		return 1;
> +	}
> +	return 0;
> +}
> +
> +/*
> + * parent_has_perm:
> + * when adding a new allow rule to a device whitelist, the rule
> + * must be allowed in the parent device
> + */
> +static int parent_has_perm(struct cgroup *childcg,
> +				  struct dev_whitelist_item *wh)
> +{
> +	struct cgroup *pcg = childcg->parent;
> +	struct dev_cgroup *parent;
> +	int ret;
> +
> +	if (!pcg)
> +		return 1;
> +	parent = cgroup_to_devcgroup(pcg);
> +	spin_lock(&parent->lock);
> +	ret = may_access_whitelist(parent, wh);
> +	spin_unlock(&parent->lock);
> +	return ret;
> +}
> +
> +/*
> + * Modify the whitelist using allow/deny rules.
> + * CAP_SYS_ADMIN is needed for this.  It's at least separate from CAP_MKNOD
> + * so we can give a container CAP_MKNOD to let it create devices but not
> + * modify the whitelist.
> + * It seems likely we'll want to add a CAP_CONTAINER capability to allow
> + * us to also grant CAP_SYS_ADMIN to containers without giving away the
> + * device whitelist controls, but for now we'll stick with CAP_SYS_ADMIN
> + *
> + * Taking rules away is always allowed (given CAP_SYS_ADMIN).  Granting
> + * new access is only allowed if you're in the top-level cgroup, or your
> + * parent cgroup has the access you're asking for.
> + */
> +static ssize_t devcgroup_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 *devcgroup, *cur_devcgroup;
> +	int filetype = cft->private;
> +	char *buffer, *b;
> +	int retval = 0, count;
> +	struct dev_whitelist_item wh;
> +
> +	if (!capable(CAP_SYS_ADMIN))
> +		return -EPERM;
> +
> +	devcgroup = cgroup_to_devcgroup(cgroup);
> +	cur_cgroup = task_cgroup(current, devices_subsys.subsys_id);
> +	cur_devcgroup = cgroup_to_devcgroup(cur_cgroup);
> +
> +	buffer = kmalloc(nbytes+1, GFP_KERNEL);
> +	if (!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));
> +	b = buffer;
> +
> +	switch (*b) {
> +	case 'a':
> +		wh.type = DEV_ALL;
> +		wh.access = ACC_MASK;
> +		goto handle;
> +	case 'b':
> +		wh.type = DEV_BLOCK;
> +		break;
> +	case 'c':
> +		wh.type = DEV_CHAR;
> +		break;
> +	default:
> +		retval = -EINVAL;
> +		goto out2;
> +	}
> +	b += 2;
> +	if (*b == '*') {
> +		wh.major = ~0;
> +		b++;
> +	} else if (isdigit(*b)) {
> +		wh.major = 0;
> +		while (isdigit(*b)) {
> +			wh.major = wh.major*10+(*b-'0');
> +			b++;
> +		}
> +	} else {
> +		retval = -EINVAL;
> +		goto out2;
> +	}
> +	if (*b != ':') {
> +		retval = -EINVAL;
> +		goto out2;
> +	}
> +	b++;
> +
> +	/* read minor */
> +	if (*b == '*') {
> +		wh.minor = ~0;
> +		b++;
> +	} else if (isdigit(*b)) {
> +		wh.minor = 0;
> +		while (isdigit(*b)) {
> +			wh.minor = wh.minor*10+(*b-'0');
> +			b++;
> +		}
> +	} else {
> +		retval = -EINVAL;
> +		goto out2;
> +	}
> +	if (!isspace(*b)) {
> +		retval = -EINVAL;
> +		goto out2;
> +	}
> +	for (b++, count = 0; count < 3; count++, b++) {
> +		switch (*b) {
> +		case 'r':
> +			wh.access |= ACC_READ;
> +			break;
> +		case 'w':
> +			wh.access |= ACC_WRITE;
> +			break;
> +		case 'm':
> +			wh.access |= ACC_MKNOD;
> +			break;
> +		case '\n':
> +		case '\0':
> +			break;
> +		default:
> +			retval = -EINVAL;
> +			goto out2;
> +		}
> +	}
> +
> +handle:
> +	retval = 0;
> +	switch (filetype) {
> +	case DEVCG_ALLOW:
> +		if (!parent_has_perm(cgroup, &wh))
> +			retval = -EPERM;
> +		else
> +			retval = dev_whitelist_add(devcgroup, &wh);
> +		break;
> +	case DEVCG_DENY:
> +		dev_whitelist_rm(devcgroup, &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 = devcgroup_access_read,
> +		.write  = devcgroup_access_write,
> +		.private = DEVCG_ALLOW,
> +	},
> +	{
> +		.name = "deny",
> +		.write = devcgroup_access_write,
> +		.private = DEVCG_DENY,
> +	},
> +};
> +
> +static int devcgroup_populate(struct cgroup_subsys *ss,
> +				struct cgroup *cont)

could you use the name 'cgroup' or 'cgrp' instead of old name 'cont'?

> +	.name = "devices",
> +	.can_attach = devcgroup_can_attach,
> +	.create = devcgroup_create,
> +	.destroy  = devcgroup_destroy,
> +	.populate = devcgroup_populate,
> +	.subsys_id = devices_subsys_id,
> +};

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