[RFC] [PATCH] allowing user mounts (updated for 2.6.5)

From: Miklos Szeredi
Date: Mon Apr 05 2004 - 05:40:24 EST

This patch allows the use of the mount syscall by non-root users in a
controlled, and secure way. For a detailed explanation on what this
patch does see the earlier relase notes:

http://marc.theaimsgroup.com/?l=linux-kernel&m=107701231914499&w=2

Comments, suggestions are welcome.

Miklos

diff -rup linux-2.6.5.orig/fs/filesystems.c linux-2.6.5/fs/filesystems.c
--- linux-2.6.5.orig/fs/filesystems.c 2003-12-18 03:59:18.000000000 +0100
+++ linux-2.6.5/fs/filesystems.c 2004-04-05 10:54:37.000000000 +0200
@@ -222,7 +222,8 @@ struct file_system_type *get_fs_type(con
if (fs && !try_module_get(fs->owner))
fs = NULL;
read_unlock(&file_systems_lock);
- if (!fs && (request_module("%s", name) == 0)) {
+ if (!fs && capable(CAP_SYS_ADMIN) &&
+ (request_module("%s", name) == 0)) {
read_lock(&file_systems_lock);
fs = *(find_filesystem(name));
if (fs && !try_module_get(fs->owner))
diff -rup linux-2.6.5.orig/fs/namespace.c linux-2.6.5/fs/namespace.c
--- linux-2.6.5.orig/fs/namespace.c 2004-04-05 10:50:46.000000000 +0200
+++ linux-2.6.5/fs/namespace.c 2004-04-05 10:54:37.000000000 +0200
@@ -25,13 +25,16 @@

extern int __init init_rootfs(void);
extern int __init sysfs_init(void);
+extern void put_filesystem(struct file_system_type *fs);
+
+#define MAX_MOUNTS 256

/* spinlock for vfsmount related operations, inplace of dcache_lock */
spinlock_t vfsmount_lock __cacheline_aligned_in_smp = SPIN_LOCK_UNLOCKED;
static struct list_head *mount_hashtable;
static int hash_mask, hash_bits;
static kmem_cache_t *mnt_cache;
-
+struct mounts_stat_struct mounts_stat = { .max_mounts = MAX_MOUNTS };
static inline unsigned long hash(struct vfsmount *mnt, struct dentry *dentry)
{
unsigned long tmp = ((unsigned long) mnt / L1_CACHE_BYTES);
@@ -40,10 +43,38 @@ static inline unsigned long hash(struct
return tmp & hash_mask;
}

+static inline int inc_nr_mounts(void)
+{
+ int err = 0;
+ spin_lock(&vfsmount_lock);
+ if (capable(CAP_SYS_ADMIN) ||
+ mounts_stat.nr_mounts < mounts_stat.max_mounts)
+ mounts_stat.nr_mounts++;
+ else
+ err = mounts_stat.max_mounts ? -EMFILE : -EPERM;
+ spin_unlock(&vfsmount_lock);
+ return err;
+}
+
+static inline void dec_nr_mounts(void)
+{
+ spin_lock(&vfsmount_lock);
+ mounts_stat.nr_mounts--;
+ spin_unlock(&vfsmount_lock);
+}
+
struct vfsmount *alloc_vfsmnt(const char *name)
{
- struct vfsmount *mnt = kmem_cache_alloc(mnt_cache, GFP_KERNEL);
- if (mnt) {
+ struct vfsmount *mnt;
+ int err = inc_nr_mounts();
+ if (err)
+ return ERR_PTR(err);
+
+ mnt = kmem_cache_alloc(mnt_cache, GFP_KERNEL);
+ if (!mnt) {
+ dec_nr_mounts();
+ return ERR_PTR(-ENOMEM);
+ } else {
memset(mnt, 0, sizeof(struct vfsmount));
atomic_set(&mnt->mnt_count,1);
INIT_LIST_HEAD(&mnt->mnt_hash);
@@ -66,6 +97,7 @@ void free_vfsmnt(struct vfsmount *mnt)
{
kfree(mnt->mnt_devname);
kmem_cache_free(mnt_cache, mnt);
+ dec_nr_mounts();
}

/*
@@ -147,13 +179,14 @@ clone_mnt(struct vfsmount *old, struct d
struct super_block *sb = old->mnt_sb;
struct vfsmount *mnt = alloc_vfsmnt(old->mnt_devname);

- if (mnt) {
+ if (!IS_ERR(mnt)) {
mnt->mnt_flags = old->mnt_flags;
atomic_inc(&sb->s_active);
mnt->mnt_sb = sb;
mnt->mnt_root = dget(root);
mnt->mnt_mountpoint = mnt->mnt_root;
mnt->mnt_parent = mnt;
+ mnt->user = capable(CAP_SYS_ADMIN) ? 0 : current->fsuid;
}
return mnt;
}
@@ -238,6 +271,8 @@ static int show_vfsmnt(struct seq_file *
if (mnt->mnt_flags & fs_infop->flag)
seq_puts(m, fs_infop->str);
}
+ if (mnt->user)
+ seq_printf(m, ",user=%i", mnt->user);
if (mnt->mnt_sb->s_op->show_options)
err = mnt->mnt_sb->s_op->show_options(m, mnt);
seq_puts(m, " 0 0\n");
@@ -388,8 +423,10 @@ asmlinkage long sys_umount(char __user *
goto dput_and_out;

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

retval = do_umount(nd.mnt, flags);
dput_and_out:
@@ -409,20 +446,15 @@ asmlinkage long sys_oldumount(char __use

static int 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;
- if (nd->dentry->d_inode->i_mode & S_ISVTX) {
- if (current->uid != nd->dentry->d_inode->i_uid)
+ if (!capable(CAP_SYS_ADMIN)) {
+ if (!S_ISDIR(nd->dentry->d_inode->i_mode) &&
+ !S_ISREG(nd->dentry->d_inode->i_mode))
+ return -EPERM;
+ if (current->fsuid != nd->dentry->d_inode->i_uid ||
+ permission(nd->dentry->d_inode, MAY_WRITE, nd))
return -EPERM;
}
- if (permission(nd->dentry->d_inode, MAY_WRITE, nd))
- return -EPERM;
return 0;
-#endif
}

static int
@@ -444,8 +476,8 @@ static struct vfsmount *copy_tree(struct
struct nameidata nd;

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

p = mnt;
@@ -463,8 +495,8 @@ static struct vfsmount *copy_tree(struct
nd.mnt = q;
nd.dentry = p->mnt_mountpoint;
q = clone_mnt(p, p->mnt_root);
- if (!q)
- goto Enomem;
+ if (IS_ERR(q))
+ goto out_error;
spin_lock(&vfsmount_lock);
list_add_tail(&q->mnt_list, &res->mnt_list);
attach_mnt(q, &nd);
@@ -472,13 +504,13 @@ static struct vfsmount *copy_tree(struct
}
}
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)
@@ -538,11 +570,14 @@ static int do_loopback(struct nameidata
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);
else
mnt = clone_mnt(old_nd.mnt, old_nd.dentry);
+ if (IS_ERR(mnt)) {
+ err = PTR_ERR(mnt);
+ goto out;
+ }
}

if (mnt) {
@@ -555,6 +590,7 @@ static int do_loopback(struct nameidata
mntput(mnt);
}

+ out:
up_write(&current->namespace->sem);
path_release(&old_nd);
return err;
@@ -654,14 +690,28 @@ static int do_add_mount(struct nameidata
int mnt_flags, char *name, void *data)
{
struct vfsmount *mnt;
- int err;
+ int err = mount_is_safe(nd);
+ if(err)
+ return err;

if (!type || !memchr(type, 0, PAGE_SIZE))
return -EINVAL;

/* we need capabilities... */
- if (!capable(CAP_SYS_ADMIN))
- return -EPERM;
+ if (!capable(CAP_SYS_ADMIN)) {
+ /* but allow "safe" filesystems anyway */
+ int issafe = 0;
+ struct file_system_type *t = get_fs_type(type);
+ if(t) {
+ issafe = t->fs_flags & FS_SAFE;
+ put_filesystem(t);
+ }
+ if(!issafe)
+ return -EPERM;
+
+ /* users should not have suid or dev files */
+ mnt_flags |= (MNT_NOSUID | MNT_NODEV);
+ }

mnt = do_kern_mount(type, flags, name, data);
err = PTR_ERR(mnt);
@@ -801,6 +851,7 @@ int copy_namespace(int flags, struct tas
struct namespace *new_ns;
struct vfsmount *rootmnt = NULL, *pwdmnt = NULL, *altrootmnt = NULL;
struct fs_struct *fs = tsk->fs;
+ int err;

if (!namespace)
return 0;
@@ -810,11 +861,7 @@ int copy_namespace(int flags, struct tas
if (!(flags & CLONE_NEWNS))
return 0;

- if (!capable(CAP_SYS_ADMIN)) {
- put_namespace(namespace);
- return -EPERM;
- }
-
+ err = -ENOMEM;
new_ns = kmalloc(sizeof(struct namespace), GFP_KERNEL);
if (!new_ns)
goto out;
@@ -826,7 +873,8 @@ int copy_namespace(int flags, struct tas
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) {
+ if (IS_ERR(new_ns->root)) {
+ err = PTR_ERR(new_ns->root);
up_write(&tsk->namespace->sem);
kfree(new_ns);
goto out;
@@ -876,7 +924,7 @@ int copy_namespace(int flags, struct tas

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

asmlinkage long sys_mount(char __user * dev_name, char __user * dir_name,
diff -rup linux-2.6.5.orig/fs/super.c linux-2.6.5/fs/super.c
--- linux-2.6.5.orig/fs/super.c 2004-04-05 10:50:46.000000000 +0200
+++ linux-2.6.5/fs/super.c 2004-04-05 11:02:10.000000000 +0200
@@ -726,7 +726,7 @@ struct vfsmount *
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;
@@ -735,24 +735,23 @@ do_kern_mount(const char *fstype, int fl
return ERR_PTR(-ENODEV);

mnt = alloc_vfsmnt(name);
- if (!mnt)
+ error = PTR_ERR(mnt);
+ if (IS_ERR(mnt))
goto out;

if (data) {
secdata = alloc_secdata();
- if (!secdata) {
- sb = ERR_PTR(-ENOMEM);
+ error = -ENOMEM;
+ if (!secdata)
goto out_mnt;
- }

error = security_sb_copy_data(type, data, secdata);
- if (error) {
- sb = ERR_PTR(error);
+ if (error)
goto out_free_secdata;
- }
}

sb = type->get_sb(type, flags, name, data);
+ error = PTR_ERR(sb);
if (IS_ERR(sb))
goto out_free_secdata;
error = security_sb_kern_mount(sb, secdata);
@@ -762,20 +761,20 @@ do_kern_mount(const char *fstype, int fl
mnt->mnt_root = dget(sb->s_root);
mnt->mnt_mountpoint = sb->s_root;
mnt->mnt_parent = mnt;
+ mnt->user = capable(CAP_SYS_ADMIN) ? 0 : current->fsuid;
up_write(&sb->s_umount);
put_filesystem(type);
return mnt;
out_sb:
up_write(&sb->s_umount);
deactivate_super(sb);
- sb = ERR_PTR(error);
out_free_secdata:
free_secdata(secdata);
out_mnt:
free_vfsmnt(mnt);
out:
put_filesystem(type);
- return (struct vfsmount *)sb;
+ return ERR_PTR(error);
}

struct vfsmount *kern_mount(struct file_system_type *type)
diff -rup linux-2.6.5.orig/include/linux/fs.h linux-2.6.5/include/linux/fs.h
--- linux-2.6.5.orig/include/linux/fs.h 2004-04-05 10:50:50.000000000 +0200
+++ linux-2.6.5/include/linux/fs.h 2004-04-05 11:03:55.000000000 +0200
@@ -55,6 +55,12 @@ struct files_stat_struct {
};
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;
@@ -90,6 +96,7 @@ extern int leases_enable, dir_notify_ena
/* public flags for file_system_type */
#define FS_REQUIRES_DEV 1
#define FS_BINARY_MOUNTDATA 2
+#define FS_SAFE 4 /* Safe to mount by user */
#define FS_REVAL_DOT 16384 /* Check the paths ".", ".." for staleness */
#define FS_ODD_RENAME 32768 /* Temporary stuff; will go away as soon
* as nfs_rename() will be cleaned up
diff -rup linux-2.6.5.orig/include/linux/mount.h linux-2.6.5/include/linux/mount.h
--- linux-2.6.5.orig/include/linux/mount.h 2003-12-18 03:58:08.000000000 +0100
+++ linux-2.6.5/include/linux/mount.h 2004-04-05 10:54:37.000000000 +0200
@@ -30,6 +30,7 @@ struct vfsmount
atomic_t mnt_count;
int mnt_flags;
char *mnt_devname; /* Name of device e.g. /dev/dsk/hda1 */
+ uid_t user;
struct list_head mnt_list;
};

diff -rup linux-2.6.5.orig/include/linux/sysctl.h linux-2.6.5/include/linux/sysctl.h
--- linux-2.6.5.orig/include/linux/sysctl.h 2004-04-05 10:50:50.000000000 +0200
+++ linux-2.6.5/include/linux/sysctl.h 2004-04-05 10:54:37.000000000 +0200
@@ -624,8 +624,8 @@ enum
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 */
diff -rup linux-2.6.5.orig/kernel/sysctl.c linux-2.6.5/kernel/sysctl.c
--- linux-2.6.5.orig/kernel/sysctl.c 2004-04-05 10:50:51.000000000 +0200
+++ linux-2.6.5/kernel/sysctl.c 2004-04-05 10:54:37.000000000 +0200
@@ -793,6 +793,22 @@ static ctl_table fs_table[] = {
.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,

-
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