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![Eric W. Biederman](/assets/img/avatar_default.png)
Modify the request_module to prefix the file system type with "fs-" and add aliases to all of the filesystems that can be built as modules to match. A common practice is to build all of the kernel code and leave code that is not commonly needed as modules, with the result that many users are exposed to any bug anywhere in the kernel. Looking for filesystems with a fs- prefix limits the pool of possible modules that can be loaded by mount to just filesystems trivially making things safer with no real cost. Using aliases means user space can control the policy of which filesystem modules are auto-loaded by editing /etc/modprobe.d/*.conf with blacklist and alias directives. Allowing simple, safe, well understood work-arounds to known problematic software. This also addresses a rare but unfortunate problem where the filesystem name is not the same as it's module name and module auto-loading would not work. While writing this patch I saw a handful of such cases. The most significant being autofs that lives in the module autofs4. This is relevant to user namespaces because we can reach the request module in get_fs_type() without having any special permissions, and people get uncomfortable when a user specified string (in this case the filesystem type) goes all of the way to request_module. After having looked at this issue I don't think there is any particular reason to perform any filtering or permission checks beyond making it clear in the module request that we want a filesystem module. The common pattern in the kernel is to call request_module() without regards to the users permissions. In general all a filesystem module does once loaded is call register_filesystem() and go to sleep. Which means there is not much attack surface exposed by loading a filesytem module unless the filesystem is mounted. In a user namespace filesystems are not mounted unless .fs_flags = FS_USERNS_MOUNT, which most filesystems do not set today. Acked-by: Serge Hallyn <serge.hallyn@canonical.com> Acked-by: Kees Cook <keescook@chromium.org> Reported-by: Kees Cook <keescook@google.com> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
184 lines
4.7 KiB
C
184 lines
4.7 KiB
C
/* -*- mode: c; c-basic-offset: 8; -*-
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* vim: noexpandtab sw=8 ts=8 sts=0:
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*
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* mount.c - operations for initializing and mounting configfs.
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*
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* This program is free software; you can redistribute it and/or
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* modify it under the terms of the GNU General Public
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* License as published by the Free Software Foundation; either
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* version 2 of the License, or (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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* General Public License for more details.
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*
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* You should have received a copy of the GNU General Public
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* License along with this program; if not, write to the
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* Free Software Foundation, Inc., 59 Temple Place - Suite 330,
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* Boston, MA 021110-1307, USA.
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*
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* Based on sysfs:
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* sysfs is Copyright (C) 2001, 2002, 2003 Patrick Mochel
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*
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* configfs Copyright (C) 2005 Oracle. All rights reserved.
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*/
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#include <linux/fs.h>
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#include <linux/module.h>
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#include <linux/mount.h>
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#include <linux/pagemap.h>
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#include <linux/init.h>
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#include <linux/slab.h>
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#include <linux/configfs.h>
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#include "configfs_internal.h"
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/* Random magic number */
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#define CONFIGFS_MAGIC 0x62656570
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static struct vfsmount *configfs_mount = NULL;
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struct kmem_cache *configfs_dir_cachep;
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static int configfs_mnt_count = 0;
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static const struct super_operations configfs_ops = {
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.statfs = simple_statfs,
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.drop_inode = generic_delete_inode,
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};
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static struct config_group configfs_root_group = {
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.cg_item = {
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.ci_namebuf = "root",
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.ci_name = configfs_root_group.cg_item.ci_namebuf,
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},
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};
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int configfs_is_root(struct config_item *item)
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{
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return item == &configfs_root_group.cg_item;
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}
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static struct configfs_dirent configfs_root = {
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.s_sibling = LIST_HEAD_INIT(configfs_root.s_sibling),
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.s_children = LIST_HEAD_INIT(configfs_root.s_children),
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.s_element = &configfs_root_group.cg_item,
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.s_type = CONFIGFS_ROOT,
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.s_iattr = NULL,
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};
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static int configfs_fill_super(struct super_block *sb, void *data, int silent)
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{
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struct inode *inode;
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struct dentry *root;
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sb->s_blocksize = PAGE_CACHE_SIZE;
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sb->s_blocksize_bits = PAGE_CACHE_SHIFT;
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sb->s_magic = CONFIGFS_MAGIC;
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sb->s_op = &configfs_ops;
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sb->s_time_gran = 1;
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inode = configfs_new_inode(S_IFDIR | S_IRWXU | S_IRUGO | S_IXUGO,
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&configfs_root, sb);
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if (inode) {
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inode->i_op = &configfs_root_inode_operations;
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inode->i_fop = &configfs_dir_operations;
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/* directory inodes start off with i_nlink == 2 (for "." entry) */
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inc_nlink(inode);
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} else {
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pr_debug("configfs: could not get root inode\n");
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return -ENOMEM;
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}
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root = d_make_root(inode);
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if (!root) {
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pr_debug("%s: could not get root dentry!\n",__func__);
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return -ENOMEM;
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}
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config_group_init(&configfs_root_group);
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configfs_root_group.cg_item.ci_dentry = root;
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root->d_fsdata = &configfs_root;
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sb->s_root = root;
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sb->s_d_op = &configfs_dentry_ops; /* the rest get that */
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return 0;
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}
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static struct dentry *configfs_do_mount(struct file_system_type *fs_type,
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int flags, const char *dev_name, void *data)
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{
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return mount_single(fs_type, flags, data, configfs_fill_super);
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}
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static struct file_system_type configfs_fs_type = {
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.owner = THIS_MODULE,
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.name = "configfs",
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.mount = configfs_do_mount,
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.kill_sb = kill_litter_super,
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};
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MODULE_ALIAS_FS("configfs");
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struct dentry *configfs_pin_fs(void)
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{
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int err = simple_pin_fs(&configfs_fs_type, &configfs_mount,
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&configfs_mnt_count);
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return err ? ERR_PTR(err) : configfs_mount->mnt_root;
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}
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void configfs_release_fs(void)
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{
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simple_release_fs(&configfs_mount, &configfs_mnt_count);
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}
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static struct kobject *config_kobj;
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static int __init configfs_init(void)
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{
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int err = -ENOMEM;
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configfs_dir_cachep = kmem_cache_create("configfs_dir_cache",
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sizeof(struct configfs_dirent),
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0, 0, NULL);
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if (!configfs_dir_cachep)
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goto out;
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config_kobj = kobject_create_and_add("config", kernel_kobj);
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if (!config_kobj)
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goto out2;
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err = configfs_inode_init();
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if (err)
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goto out3;
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err = register_filesystem(&configfs_fs_type);
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if (err)
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goto out4;
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return 0;
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out4:
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printk(KERN_ERR "configfs: Unable to register filesystem!\n");
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configfs_inode_exit();
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out3:
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kobject_put(config_kobj);
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out2:
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kmem_cache_destroy(configfs_dir_cachep);
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configfs_dir_cachep = NULL;
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out:
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return err;
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}
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static void __exit configfs_exit(void)
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{
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unregister_filesystem(&configfs_fs_type);
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kobject_put(config_kobj);
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kmem_cache_destroy(configfs_dir_cachep);
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configfs_dir_cachep = NULL;
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configfs_inode_exit();
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}
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MODULE_AUTHOR("Oracle");
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MODULE_LICENSE("GPL");
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MODULE_VERSION("0.0.2");
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MODULE_DESCRIPTION("Simple RAM filesystem for user driven kernel subsystem configuration.");
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module_init(configfs_init);
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module_exit(configfs_exit);
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