[RCF] [PATCH] unprivileged mount/umount

Miklos Szeredi
2005-05-03T14:39:34Z

This (lightly tested) patch against 2.6.12-rc* adds some
infrastructure and basic functionality for unprivileged mount/umount
system calls.

Details:

- new mnt_owner field in struct vfsmount
- if mnt_owner is NULL, it's a privileged mount
- global limit on unprivileged mounts in /proc/sys/fs/mount-max
- per user limit of mounts in rlimit
- allow umount for the owner (except force flag)
- allow unprivileged bind mount to files/directories writable by owner
- add nosuid,nodev flags to unprivileged mounts

Next step would be to add some policy for new mounts. I'm thinking of
either something static: e.g. FS_SAFE flag for "safe" filesystems, or
a more configurable approach through sysfs or something.

Comments?

Thanks,
Miklos

Index: fs/namespace.c
===================================================================
--- a6d962c4f559f3644678574a66310084fd13d130/fs/namespace.c (mode:100644 sha1:3b93e5d750ebf8452ea1264251c5b55cc89f48f8)
+++ uncommitted/fs/namespace.c (mode:100644)
@@ -42,7 +42,7 @@
static struct list_head *mount_hashtable;
static int hash_mask, hash_bits;
static kmem_cache_t *mnt_cache;
-
+struct mounts_stat_struct mounts_stat;
static inline unsigned long hash(struct vfsmount *mnt, struct dentry *dentry)
{
unsigned long tmp = ((unsigned long) mnt / L1_CACHE_BYTES);
@@ -51,10 +51,44 @@
return tmp & hash_mask;
}

-struct vfsmount *alloc_vfsmnt(const char *name)
+static inline int inc_nr_mounts(struct user_struct *owner)
{
- struct vfsmount *mnt = kmem_cache_alloc(mnt_cache, GFP_KERNEL);
- if (mnt) {
+ int err = 0;
+ spin_lock(&vfsmount_lock);
+ if (owner &&
+ (mounts_stat.nr_mounts >= mounts_stat.max_mounts ||
+ owner->mounts >= current->signal->rlim[RLIMIT_MOUNTS].rlim_cur))
+ err = -EPERM;
+ else {
+ mounts_stat.nr_mounts++;
+ if (owner)
+ owner->mounts++;
+ }
+ spin_unlock(&vfsmount_lock);
+ return err;
+}
+
+static inline void dec_nr_mounts(struct user_struct *owner)
+{
+ spin_lock(&vfsmount_lock);
+ mounts_stat.nr_mounts--;
+ if (owner)
+ owner->mounts--;
+ spin_unlock(&vfsmount_lock);
+}
+
+struct vfsmount *alloc_vfsmnt(const char *name, struct user_struct *owner)
+{
+ struct vfsmount *mnt;
+ int err = inc_nr_mounts(owner);
+ if (err)
+ return ERR_PTR(err);
+
+ mnt = kmem_cache_alloc(mnt_cache, GFP_KERNEL);
+ if (!mnt) {
+ dec_nr_mounts(owner);
+ return ERR_PTR(-ENOMEM);
+ } else {
memset(mnt, 0, sizeof(struct vfsmount));
atomic_set(&mnt->mnt_count,1);
INIT_LIST_HEAD(&mnt->mnt_hash);
@@ -70,14 +104,21 @@
mnt->mnt_devname = newname;
}
}
+ if (owner) {
+ atomic_inc(&owner->__count);
+ mnt->mnt_owner = owner;
+ }
}
return mnt;
}

void free_vfsmnt(struct vfsmount *mnt)
{
+ struct user_struct *owner = mnt->mnt_owner;
kfree(mnt->mnt_devname);
kmem_cache_free(mnt_cache, mnt);
+ dec_nr_mounts(owner);
+ free_uid(owner);
}

/*
@@ -148,13 +189,16 @@
}

static struct vfsmount *
-clone_mnt(struct vfsmount *old, struct dentry *root)
+clone_mnt(struct vfsmount *old, struct dentry *root, struct user_struct *owner)
{
struct super_block *sb = old->mnt_sb;
- struct vfsmount *mnt = alloc_vfsmnt(old->mnt_devname);
+ struct vfsmount *mnt = alloc_vfsmnt(old->mnt_devname, owner);

- if (mnt) {
+ if (!IS_ERR(mnt)) {
mnt->mnt_flags = old->mnt_flags;
+ /* Unprivileged mounts require NOSUID and NODEV */
+ if (owner)
+ mnt->mnt_flags |= MNT_NOSUID | MNT_NODEV;
atomic_inc(&sb->s_active);
mnt->mnt_sb = sb;
mnt->mnt_root = dget(root);
@@ -252,6 +296,8 @@
if (mnt->mnt_flags & fs_infop->flag)
seq_puts(m, fs_infop->str);
}
+ if (mnt->mnt_owner)
+ seq_printf(m, ",owner=%i", mnt->mnt_owner->uid);
if (mnt->mnt_sb->s_op->show_options)
err = mnt->mnt_sb->s_op->show_options(m, mnt);
seq_puts(m, " 0 0\n");
@@ -480,7 +526,8 @@
goto dput_and_out;

retval = -EPERM;
- if (!capable(CAP_SYS_ADMIN))
+ if (!capable(CAP_SYS_ADMIN) && (nd.mnt->mnt_owner != current->user ||
+ (flags & MNT_FORCE)))
goto dput_and_out;

retval = do_umount(nd.mnt, flags);
@@ -503,22 +550,21 @@

#endif

-static int mount_is_safe(struct nameidata *nd)
+static struct user_struct *mount_is_safe(struct nameidata *nd)
{
if (capable(CAP_SYS_ADMIN))
- return 0;
- return -EPERM;
-#ifdef notyet
- if (S_ISLNK(nd->dentry->d_inode->i_mode))
- return -EPERM;
+ return NULL;
+
+ if (!S_ISDIR(nd->dentry->d_inode->i_mode) &&
+ !S_ISREG(nd->dentry->d_inode->i_mode))
+ return ERR_PTR(-EPERM);
if (nd->dentry->d_inode->i_mode & S_ISVTX) {
- if (current->uid != nd->dentry->d_inode->i_uid)
- return -EPERM;
+ if (current->fsuid != nd->dentry->d_inode->i_uid)
+ return ERR_PTR(-EPERM);
}
if (permission(nd->dentry->d_inode, MAY_WRITE, nd))
- return -EPERM;
- return 0;
-#endif
+ return ERR_PTR(-EPERM);
+ return current->user;
}

static int
@@ -533,15 +579,16 @@
}
}

-static struct vfsmount *copy_tree(struct vfsmount *mnt, struct dentry *dentry)
+static struct vfsmount *copy_tree(struct vfsmount *mnt, struct dentry *dentry,
+ struct user_struct *owner)
{
struct vfsmount *res, *p, *q, *r, *s;
struct list_head *h;
struct nameidata nd;

- res = q = clone_mnt(mnt, dentry);
- if (!q)
- goto Enomem;
+ res = q = clone_mnt(mnt, dentry, owner);
+ if (IS_ERR(q))
+ goto out_error;
q->mnt_mountpoint = mnt->mnt_mountpoint;

p = mnt;
@@ -558,9 +605,9 @@
p = s;
nd.mnt = q;
nd.dentry = p->mnt_mountpoint;
- q = clone_mnt(p, p->mnt_root);
- if (!q)
- goto Enomem;
+ q = clone_mnt(p, p->mnt_root, owner);
+ if (IS_ERR(q))
+ goto out_error;
spin_lock(&vfsmount_lock);
list_add_tail(&q->mnt_list, &res->mnt_list);
attach_mnt(q, &nd);
@@ -568,13 +615,13 @@
}
}
return res;
- Enomem:
+ out_error:
if (res) {
spin_lock(&vfsmount_lock);
umount_tree(res);
spin_unlock(&vfsmount_lock);
}
- return NULL;
+ return q;
}

static int graft_tree(struct vfsmount *mnt, struct nameidata *nd)
@@ -620,11 +667,12 @@
*/
static int do_loopback(struct nameidata *nd, char *old_name, int recurse)
{
+ int err;
struct nameidata old_nd;
struct vfsmount *mnt = NULL;
- int err = mount_is_safe(nd);
- if (err)
- return err;
+ struct user_struct *owner = mount_is_safe(nd);
+ if (IS_ERR(owner))
+ return PTR_ERR(owner);
if (!old_name || !*old_name)
return -EINVAL;
err = path_lookup(old_name, LOOKUP_FOLLOW, &old_nd);
@@ -634,11 +682,14 @@
down_write(&current->namespace->sem);
err = -EINVAL;
if (check_mnt(nd->mnt) && (!recurse || check_mnt(old_nd.mnt))) {
- err = -ENOMEM;
if (recurse)
- mnt = copy_tree(old_nd.mnt, old_nd.dentry);
+ mnt = copy_tree(old_nd.mnt, old_nd.dentry, owner);
else
- mnt = clone_mnt(old_nd.mnt, old_nd.dentry);
+ mnt = clone_mnt(old_nd.mnt, old_nd.dentry, owner);
+ if (IS_ERR(mnt)) {
+ err = PTR_ERR(mnt);
+ goto out;
+ }
}

if (mnt) {
@@ -656,6 +707,7 @@
mntput(mnt);
}

+ out:
up_write(&current->namespace->sem);
path_release(&old_nd);
return err;
@@ -1066,6 +1118,7 @@
struct vfsmount *rootmnt = NULL, *pwdmnt = NULL, *altrootmnt = NULL;
struct fs_struct *fs = tsk->fs;
struct vfsmount *p, *q;
+ int err;

if (!namespace)
return 0;
@@ -1075,11 +1128,11 @@
if (!(flags & CLONE_NEWNS))
return 0;

- if (!capable(CAP_SYS_ADMIN)) {
- put_namespace(namespace);
- return -EPERM;
- }
+ err = -EPERM;
+ if (!capable(CAP_SYS_ADMIN))
+ goto out;

+ err = -ENOMEM;
new_ns = kmalloc(sizeof(struct namespace), GFP_KERNEL);
if (!new_ns)
goto out;
@@ -1090,8 +1143,9 @@

down_write(&tsk->namespace->sem);
/* First pass: copy the tree topology */
- new_ns->root = copy_tree(namespace->root, namespace->root->mnt_root);
- if (!new_ns->root) {
+ new_ns->root = copy_tree(namespace->root, namespace->root->mnt_root, 0);
+ if (IS_ERR(new_ns->root)) {
+ err = PTR_ERR(new_ns->root);
up_write(&tsk->namespace->sem);
kfree(new_ns);
goto out;
@@ -1142,7 +1196,7 @@

out:
put_namespace(namespace);
- return -ENOMEM;
+ return err;
}

asmlinkage long sys_mount(char __user * dev_name, char __user * dir_name,
Index: fs/super.c
===================================================================
--- a6d962c4f559f3644678574a66310084fd13d130/fs/super.c (mode:100644 sha1:3a1b8ca04ba601b37ffa5dadb8f9695272952e39)
+++ uncommitted/fs/super.c (mode:100644)
@@ -797,7 +797,7 @@
do_kern_mount(const char *fstype, int flags, const char *name, void *data)
{
struct file_system_type *type = get_fs_type(fstype);
- struct super_block *sb = ERR_PTR(-ENOMEM);
+ struct super_block *sb;
struct vfsmount *mnt;
int error;
char *secdata = NULL;
@@ -805,9 +805,11 @@
if (!type)
return ERR_PTR(-ENODEV);

- mnt = alloc_vfsmnt(name);
- if (!mnt)
- goto out;
+ mnt = alloc_vfsmnt(name, NULL);
+ if (IS_ERR(mnt)) {
+ put_filesystem(type);
+ return mnt;
+ }

if (data) {
secdata = alloc_secdata();
@@ -845,7 +847,6 @@
free_secdata(secdata);
out_mnt:
free_vfsmnt(mnt);
-out:
put_filesystem(type);
return (struct vfsmount *)sb;
}
Index: include/asm-generic/resource.h
===================================================================
--- a6d962c4f559f3644678574a66310084fd13d130/include/asm-generic/resource.h (mode:100644 sha1:cfe3692b23e580a01ccf0ad7a9102e854c8be792)
+++ uncommitted/include/asm-generic/resource.h (mode:100644)
@@ -44,8 +44,9 @@
#define RLIMIT_NICE 13 /* max nice prio allowed to raise to
0-39 for nice level 19 .. -20 */
#define RLIMIT_RTPRIO 14 /* maximum realtime priority */
+#define RLIMIT_MOUNTS 15 /* maximum number of mounts for user */

-#define RLIM_NLIMITS 15
+#define RLIM_NLIMITS 16

/*
* SuS says limits have to be unsigned.
@@ -86,6 +87,7 @@
[RLIMIT_MSGQUEUE] = { MQ_BYTES_MAX, MQ_BYTES_MAX }, \
[RLIMIT_NICE] = { 0, 0 }, \
[RLIMIT_RTPRIO] = { 0, 0 }, \
+ [RLIMIT_MOUNTS] = { RLIM_INFINITY, RLIM_INFINITY }, \
}

#endif /* __KERNEL__ */
Index: include/linux/fs.h
===================================================================
--- a6d962c4f559f3644678574a66310084fd13d130/include/linux/fs.h (mode:100644 sha1:4edba067a7178103f78544717a91ef98dae7994d)
+++ uncommitted/include/linux/fs.h (mode:100644)
@@ -36,6 +36,12 @@
};
extern struct files_stat_struct files_stat;

+struct mounts_stat_struct {
+ int nr_mounts;
+ int max_mounts;
+};
+extern struct mounts_stat_struct mounts_stat;
+
struct inodes_stat_t {
int nr_inodes;
int nr_unused;
Index: include/linux/mount.h
===================================================================
--- a6d962c4f559f3644678574a66310084fd13d130/include/linux/mount.h (mode:100644 sha1:8b8d3b9beefdc4f91cfa12d9275976408590cbcc)
+++ uncommitted/include/linux/mount.h (mode:100644)
@@ -33,6 +33,7 @@
int mnt_flags;
int mnt_expiry_mark; /* true if marked for expiry */
char *mnt_devname; /* Name of device e.g. /dev/dsk/hda1 */
+ struct user_struct *mnt_owner; /* Mount owner */
struct list_head mnt_list;
struct list_head mnt_fslink; /* link in fs-specific expiry list */
struct namespace *mnt_namespace; /* containing namespace */
@@ -64,7 +65,8 @@
}

extern void free_vfsmnt(struct vfsmount *mnt);
-extern struct vfsmount *alloc_vfsmnt(const char *name);
+extern struct vfsmount *alloc_vfsmnt(const char *name,
+ struct user_struct *owner);
extern struct vfsmount *do_kern_mount(const char *fstype, int flags,
const char *name, void *data);

Index: include/linux/sched.h
===================================================================
--- a6d962c4f559f3644678574a66310084fd13d130/include/linux/sched.h (mode:100644 sha1:5f868a5885811571fdc701037bf7b09b40a746b8)
+++ uncommitted/include/linux/sched.h (mode:100644)
@@ -408,6 +408,9 @@
unsigned long mq_bytes; /* How many bytes can be allocated to mqueue? */
unsigned long locked_shm; /* How many pages of mlocked shm ? */

+ /* protected by vfsmount_lock */
+ unsigned long mounts; /* Number of mounts this user owns */
+
#ifdef CONFIG_KEYS
struct key *uid_keyring; /* UID specific keyring */
struct key *session_keyring; /* UID's default session keyring */
Index: include/linux/sysctl.h
===================================================================
--- a6d962c4f559f3644678574a66310084fd13d130/include/linux/sysctl.h (mode:100644 sha1:772998147e3ef989988c184520f6dacba9fb601d)
+++ uncommitted/include/linux/sysctl.h (mode:100644)
@@ -667,8 +667,8 @@
FS_NRFILE=6, /* int:current number of allocated filedescriptors */
FS_MAXFILE=7, /* int:maximum number of filedescriptors that can be allocated */
FS_DENTRY=8,
- FS_NRSUPER=9, /* int:current number of allocated super_blocks */
- FS_MAXSUPER=10, /* int:maximum number of super_blocks that can be allocated */
+ FS_NRMOUNT=9, /* int:current number of mounts */
+ FS_MAXMOUNT=10, /* int:maximum number of mounts allowed */
FS_OVERFLOWUID=11, /* int: overflow UID */
FS_OVERFLOWGID=12, /* int: overflow GID */
FS_LEASES=13, /* int: leases enabled */
Index: kernel/sysctl.c
===================================================================
--- a6d962c4f559f3644678574a66310084fd13d130/kernel/sysctl.c (mode:100644 sha1:701d12c6306844136a8e525902d06082733127fb)
+++ uncommitted/kernel/sysctl.c (mode:100644)
@@ -885,6 +885,22 @@
.proc_handler = &proc_dointvec,
},
{
+ .ctl_name = FS_NRMOUNT,
+ .procname = "mount-nr",
+ .data = &mounts_stat.nr_mounts,
+ .maxlen = sizeof(int),
+ .mode = 0444,
+ .proc_handler = &proc_dointvec,
+ },
+ {
+ .ctl_name = FS_MAXMOUNT,
+ .procname = "mount-max",
+ .data = &mounts_stat.max_mounts,
+ .maxlen = sizeof(int),
+ .mode = 0644,
+ .proc_handler = &proc_dointvec,
+ },
+ {
.ctl_name = FS_OVERFLOWUID,
.procname = "overflowuid",
.data = &fs_overflowuid,