binderfs.c

/* SPDX-License-Identifier: GPL-2.0 */

#include <linux/compiler_types.h>
#include <linux/errno.h>
#include <linux/fs.h>
#include <linux/fsnotify.h>
#include <linux/gfp.h>
#include <linux/idr.h>
#include <linux/init.h>
#include <linux/ipc_namespace.h>
#include <linux/kdev_t.h>
#include <linux/kernel.h>
#include <linux/list.h>
#include <linux/magic.h>
#include <linux/major.h>
#include <linux/miscdevice.h>
#include <linux/module.h>
#include <linux/mutex.h>
#include <linux/mount.h>
#include <linux/parser.h>
#include <linux/radix-tree.h>
#include <linux/sched.h>
#include <linux/slab.h>
#include <linux/spinlock_types.h>
#include <linux/stddef.h>
#include <linux/string.h>
#include <linux/types.h>
#include <linux/uaccess.h>
#include <linux/user_namespace.h>
#include <linux/xarray.h>
#include <uapi/asm-generic/errno-base.h>
#include <uapi/linux/android/binder.h>
#include <uapi/linux/android/binder_ctl.h>

#include "binder_internal.h"

#define FIRST_INODE 1
#define SECOND_INODE 2
#define INODE_OFFSET 3
#define INTSTRLEN 21
#define BINDERFS_MAX_MINOR (1U << MINORBITS)

static struct vfsmount *binderfs_mnt;

static dev_t binderfs_dev;
static DEFINE_MUTEX(binderfs_minors_mutex);
static DEFINE_IDA(binderfs_minors);

/**
 * binderfs_info - information about a binderfs mount
 * @ipc_ns:         The ipc namespace the binderfs mount belongs to.
 * @control_dentry: This records the dentry of this binderfs mount
 *                  binder-control device.
 * @root_uid:       uid that needs to be used when a new binder device is
 *                  created.
 * @root_gid:       gid that needs to be used when a new binder device is
 *                  created.
 */
struct binderfs_info {
	struct ipc_namespace *ipc_ns;
	struct dentry *control_dentry;
	kuid_t root_uid;
	kgid_t root_gid;

};

static inline struct binderfs_info *BINDERFS_I(const struct inode *inode)
{
	return inode->i_sb->s_fs_info;
}

bool is_binderfs_device(const struct inode *inode)
{
	if (inode->i_sb->s_magic == BINDERFS_SUPER_MAGIC)
		return true;

	return false;
}

/**
 * binderfs_binder_device_create - allocate inode from super block of a
 *                                 binderfs mount
 * @ref_inode: inode from wich the super block will be taken
 * @userp:     buffer to copy information about new device for userspace to
 * @req:       struct binderfs_device as copied from userspace
 *
 * This function allocated a new binder_device and reserves a new minor
 * number for it.
 * Minor numbers are limited and tracked globally in binderfs_minors. The
 * function will stash a struct binder_device for the specific binder
 * device in i_private of the inode.
 * It will go on to allocate a new inode from the super block of the
 * filesystem mount, stash a struct binder_device in its i_private field
 * and attach a dentry to that inode.
 *
 * Return: 0 on success, negative errno on failure
 */
static int binderfs_binder_device_create(struct inode *ref_inode,
					 struct binderfs_device __user *userp,
					 struct binderfs_device *req)
{
	int minor, ret;
	struct dentry *dentry, *dup, *root;
	struct binder_device *device;
	size_t name_len = BINDERFS_MAX_NAME + 1;
	char *name = NULL;
	struct inode *inode = NULL;
	struct super_block *sb = ref_inode->i_sb;
	struct binderfs_info *info = sb->s_fs_info;

	/* Reserve new minor number for the new device. */
	mutex_lock(&binderfs_minors_mutex);
	minor = ida_alloc_max(&binderfs_minors, BINDERFS_MAX_MINOR, GFP_KERNEL);
	mutex_unlock(&binderfs_minors_mutex);
	if (minor < 0)
		return minor;

	ret = -ENOMEM;
	device = kzalloc(sizeof(*device), GFP_KERNEL);
	if (!device)
		goto err;

	inode = new_inode(sb);
	if (!inode)
		goto err;

	inode->i_ino = minor + INODE_OFFSET;
	inode->i_mtime = inode->i_atime = inode->i_ctime = current_time(inode);
	init_special_inode(inode, S_IFCHR | 0600,
			   MKDEV(MAJOR(binderfs_dev), minor));
	inode->i_fop = &binder_fops;
	inode->i_uid = info->root_uid;
	inode->i_gid = info->root_gid;

	name = kmalloc(name_len, GFP_KERNEL);
	if (!name)
		goto err;

	strscpy(name, req->name, name_len);

	device->binderfs_inode = inode;
	device->context.binder_context_mgr_uid = INVALID_UID;
	device->context.name = name;
	device->miscdev.name = name;
	device->miscdev.minor = minor;
	mutex_init(&device->context.context_mgr_node_lock);

	req->major = MAJOR(binderfs_dev);
	req->minor = minor;

	ret = copy_to_user(userp, req, sizeof(*req));
	if (ret) {
		ret = -EFAULT;
		goto err;
	}

	root = sb->s_root;
	inode_lock(d_inode(root));
	dentry = d_alloc_name(root, name);
	if (!dentry) {
		inode_unlock(d_inode(root));
		ret = -ENOMEM;
		goto err;
	}

	/* Verify that the name userspace gave us is not already in use. */
	dup = d_lookup(root, &dentry->d_name);
	if (dup) {
		if (d_really_is_positive(dup)) {
			dput(dup);
			dput(dentry);
			inode_unlock(d_inode(root));
			ret = -EEXIST;
			goto err;
		}
		dput(dup);
	}

	inode->i_private = device;
	d_add(dentry, inode);
	fsnotify_create(root->d_inode, dentry);
	inode_unlock(d_inode(root));

	return 0;

err:
	kfree(name);
	kfree(device);
	mutex_lock(&binderfs_minors_mutex);
	ida_free(&binderfs_minors, minor);
	mutex_unlock(&binderfs_minors_mutex);
	iput(inode);

	return ret;
}

/**
 * binderfs_ctl_ioctl - handle binder device node allocation requests
 *
 * The request handler for the binder-control device. All requests operate on
 * the binderfs mount the binder-control device resides in:
 * - BINDER_CTL_ADD
 *   Allocate a new binder device.
 *
 * Return: 0 on success, negative errno on failure
 */
static long binder_ctl_ioctl(struct file *file, unsigned int cmd,
			     unsigned long arg)
{
	int ret = -EINVAL;
	struct inode *inode = file_inode(file);
	struct binderfs_device __user *device = (struct binderfs_device __user *)arg;
	struct binderfs_device device_req;

	switch (cmd) {
	case BINDER_CTL_ADD:
		ret = copy_from_user(&device_req, device, sizeof(device_req));
		if (ret) {
			ret = -EFAULT;
			break;
		}

		ret = binderfs_binder_device_create(inode, device, &device_req);
		break;
	default:
		break;
	}

	return ret;
}

static void binderfs_evict_inode(struct inode *inode)
{
	struct binder_device *device = inode->i_private;

	clear_inode(inode);

	if (!device)
		return;

	mutex_lock(&binderfs_minors_mutex);
	ida_free(&binderfs_minors, device->miscdev.minor);
	mutex_unlock(&binderfs_minors_mutex);

	kfree(device->context.name);
	kfree(device);
}

static const struct super_operations binderfs_super_ops = {
	.statfs = simple_statfs,
	.evict_inode = binderfs_evict_inode,
};

static int binderfs_rename(struct inode *old_dir, struct dentry *old_dentry,
			   struct inode *new_dir, struct dentry *new_dentry,
			   unsigned int flags)
{
	struct inode *inode = d_inode(old_dentry);

	/* binderfs doesn't support directories. */
	if (d_is_dir(old_dentry))
		return -EPERM;

	if (flags & ~RENAME_NOREPLACE)
		return -EINVAL;

	if (!simple_empty(new_dentry))
		return -ENOTEMPTY;

	if (d_really_is_positive(new_dentry))
		simple_unlink(new_dir, new_dentry);

	old_dir->i_ctime = old_dir->i_mtime = new_dir->i_ctime =
		new_dir->i_mtime = inode->i_ctime = current_time(old_dir);

	return 0;
}

static int binderfs_unlink(struct inode *dir, struct dentry *dentry)
{
	/*
	 * The control dentry is only ever touched during mount so checking it
	 * here should not require us to take lock.
	 */
	if (BINDERFS_I(dir)->control_dentry == dentry)
		return -EPERM;

	return simple_unlink(dir, dentry);
}

static const struct file_operations binder_ctl_fops = {
	.owner		= THIS_MODULE,
	.open		= nonseekable_open,
	.unlocked_ioctl	= binder_ctl_ioctl,
	.compat_ioctl	= binder_ctl_ioctl,
	.llseek		= noop_llseek,
};

/**
 * binderfs_binder_ctl_create - create a new binder-control device
 * @sb: super block of the binderfs mount
 *
 * This function creates a new binder-control device node in the binderfs mount
 * referred to by @sb.
 *
 * Return: 0 on success, negative errno on failure
 */
static int binderfs_binder_ctl_create(struct super_block *sb)
{
	int minor, ret;
	struct dentry *dentry;
	struct binder_device *device;
	struct inode *inode = NULL;
	struct dentry *root = sb->s_root;
	struct binderfs_info *info = sb->s_fs_info;

	device = kzalloc(sizeof(*device), GFP_KERNEL);
	if (!device)
		return -ENOMEM;

	inode_lock(d_inode(root));

	/* If we have already created a binder-control node, return. */
	if (info->control_dentry) {
		ret = 0;
		goto out;
	}

	ret = -ENOMEM;
	inode = new_inode(sb);
	if (!inode)
		goto out;

	/* Reserve a new minor number for the new device. */
	mutex_lock(&binderfs_minors_mutex);
	minor = ida_alloc_max(&binderfs_minors, BINDERFS_MAX_MINOR, GFP_KERNEL);
	mutex_unlock(&binderfs_minors_mutex);
	if (minor < 0) {
		ret = minor;
		goto out;
	}

	inode->i_ino = SECOND_INODE;
	inode->i_mtime = inode->i_atime = inode->i_ctime = current_time(inode);
	init_special_inode(inode, S_IFCHR | 0600,
			   MKDEV(MAJOR(binderfs_dev), minor));
	inode->i_fop = &binder_ctl_fops;
	inode->i_uid = info->root_uid;
	inode->i_gid = info->root_gid;

	device->binderfs_inode = inode;
	device->miscdev.minor = minor;

	dentry = d_alloc_name(root, "binder-control");
	if (!dentry)
		goto out;

	inode->i_private = device;
	info->control_dentry = dentry;
	d_add(dentry, inode);
	inode_unlock(d_inode(root));

	return 0;

out:
	inode_unlock(d_inode(root));
	kfree(device);
	iput(inode);

	return ret;
}

static const struct inode_operations binderfs_dir_inode_operations = {
	.lookup = simple_lookup,
	.rename = binderfs_rename,
	.unlink = binderfs_unlink,
};

static int binderfs_fill_super(struct super_block *sb, void *data, int silent)
{
	struct binderfs_info *info;
	int ret = -ENOMEM;
	struct inode *inode = NULL;
	struct ipc_namespace *ipc_ns = sb->s_fs_info;

	get_ipc_ns(ipc_ns);

	sb->s_blocksize = PAGE_SIZE;
	sb->s_blocksize_bits = PAGE_SHIFT;

	/*
	 * The binderfs filesystem can be mounted by userns root in a
	 * non-initial userns. By default such mounts have the SB_I_NODEV flag
	 * set in s_iflags to prevent security issues where userns root can
	 * just create random device nodes via mknod() since it owns the
	 * filesystem mount. But binderfs does not allow to create any files
	 * including devices nodes. The only way to create binder devices nodes
	 * is through the binder-control device which userns root is explicitly
	 * allowed to do. So removing the SB_I_NODEV flag from s_iflags is both
	 * necessary and safe.
	 */
	sb->s_iflags &= ~SB_I_NODEV;
	sb->s_iflags |= SB_I_NOEXEC;
	sb->s_magic = BINDERFS_SUPER_MAGIC;
	sb->s_op = &binderfs_super_ops;
	sb->s_time_gran = 1;

	info = kzalloc(sizeof(struct binderfs_info), GFP_KERNEL);
	if (!info)
		goto err_without_dentry;

	info->ipc_ns = ipc_ns;
	info->root_gid = make_kgid(sb->s_user_ns, 0);
	if (!gid_valid(info->root_gid))
		info->root_gid = GLOBAL_ROOT_GID;
	info->root_uid = make_kuid(sb->s_user_ns, 0);
	if (!uid_valid(info->root_uid))
		info->root_uid = GLOBAL_ROOT_UID;

	sb->s_fs_info = info;

	inode = new_inode(sb);
	if (!inode)
		goto err_without_dentry;

	inode->i_ino = FIRST_INODE;
	inode->i_fop = &simple_dir_operations;
	inode->i_mode = S_IFDIR | 0755;
	inode->i_mtime = inode->i_atime = inode->i_ctime = current_time(inode);
	inode->i_op = &binderfs_dir_inode_operations;
	set_nlink(inode, 2);

	sb->s_root = d_make_root(inode);
	if (!sb->s_root)
		goto err_without_dentry;

	ret = binderfs_binder_ctl_create(sb);
	if (ret)
		goto err_with_dentry;

	return 0;

err_with_dentry:
	dput(sb->s_root);
	sb->s_root = NULL;

err_without_dentry:
	put_ipc_ns(ipc_ns);
	iput(inode);
	kfree(info);

	return ret;
}

static int binderfs_test_super(struct super_block *sb, void *data)
{
	struct binderfs_info *info = sb->s_fs_info;

	if (info)
		return info->ipc_ns == data;

	return 0;
}

static int binderfs_set_super(struct super_block *sb, void *data)
{
	sb->s_fs_info = data;
	return set_anon_super(sb, NULL);
}

static struct dentry *binderfs_mount(struct file_system_type *fs_type,
				     int flags, const char *dev_name,
				     void *data)
{
	struct super_block *sb;
	struct ipc_namespace *ipc_ns = current->nsproxy->ipc_ns;

	if (!ns_capable(ipc_ns->user_ns, CAP_SYS_ADMIN))
		return ERR_PTR(-EPERM);

	sb = sget_userns(fs_type, binderfs_test_super, binderfs_set_super,
			 flags, ipc_ns->user_ns, ipc_ns);
	if (IS_ERR(sb))
		return ERR_CAST(sb);

	if (!sb->s_root) {
		int ret = binderfs_fill_super(sb, data, flags & SB_SILENT ? 1 : 0);
		if (ret) {
			deactivate_locked_super(sb);
			return ERR_PTR(ret);
		}

		sb->s_flags |= SB_ACTIVE;
	}

	return dget(sb->s_root);
}

static void binderfs_kill_super(struct super_block *sb)
{
	struct binderfs_info *info = sb->s_fs_info;

	if (info && info->ipc_ns)
		put_ipc_ns(info->ipc_ns);

	kfree(info);
	kill_litter_super(sb);
}

static struct file_system_type binder_fs_type = {
	.name		= "binder",
	.mount		= binderfs_mount,
	.kill_sb	= binderfs_kill_super,
	.fs_flags	= FS_USERNS_MOUNT,
};

static int __init init_binderfs(void)
{
	int ret;

	/* Allocate new major number for binderfs. */
	ret = alloc_chrdev_region(&binderfs_dev, 0, BINDERFS_MAX_MINOR,
				  "binder");
	if (ret)
		return ret;

	ret = register_filesystem(&binder_fs_type);
	if (ret) {
		unregister_chrdev_region(binderfs_dev, BINDERFS_MAX_MINOR);
		return ret;
	}

	binderfs_mnt = kern_mount(&binder_fs_type);
	if (IS_ERR(binderfs_mnt)) {
		ret = PTR_ERR(binderfs_mnt);
		binderfs_mnt = NULL;
		unregister_filesystem(&binder_fs_type);
		unregister_chrdev_region(binderfs_dev, BINDERFS_MAX_MINOR);
	}

	return ret;
}

device_initcall(init_binderfs);