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Subject[patch 06/10] unprivileged mounts: allow unprivileged mounts
From: Miklos Szeredi <mszeredi@suse.cz>

For "safe" filesystems allow unprivileged mounting and forced
unmounting.

A filesystem type is considered "safe", if mounting it by an
unprivileged user may not cause a security problem. This is somewhat
subjective, so setting this property is left to userspace (implemented
in the next patch).

Since most filesystems haven't been designed with unprivileged
mounting in mind, a thorough audit is recommended before setting this
property.

Make this a separate integer member in 'struct file_system_type'
instead of a flag, since that is easier to handle by sysctl code.

Move subtype handling from do_kern_mount() into do_new_mount(). All
other callers are kernel-internal and do not need subtype support.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Acked-by: Serge Hallyn <serue@us.ibm.com>
---
Index: linux/fs/namespace.c
===================================================================
--- linux.orig/fs/namespace.c 2008-02-04 23:48:00.000000000 +0100
+++ linux/fs/namespace.c 2008-02-04 23:48:02.000000000 +0100
@@ -1105,14 +1105,16 @@ static bool is_mount_owner(struct vfsmou
/*
* umount is permitted for
* - sysadmin
- * - mount owner, if not forced umount
+ * - mount owner
+ * o if not forced umount,
+ * o if forced umount, and filesystem is "safe"
*/
static bool permit_umount(struct vfsmount *mnt, int flags)
{
if (capable(CAP_SYS_ADMIN))
return true;

- if (flags & MNT_FORCE)
+ if ((flags & MNT_FORCE) && !(mnt->mnt_sb->s_type->fs_safe))
return false;

return is_mount_owner(mnt, current->fsuid);
@@ -1170,13 +1172,17 @@ asmlinkage long sys_oldumount(char __use
* - mountpoint is not a symlink
* - mountpoint is in a mount owned by the user
*/
-static bool permit_mount(struct nameidata *nd, int *flags)
+static bool permit_mount(struct nameidata *nd, struct file_system_type *type,
+ int *flags)
{
struct inode *inode = nd->path.dentry->d_inode;

if (capable(CAP_SYS_ADMIN))
return true;

+ if (type && !type->fs_safe)
+ return false;
+
if (S_ISLNK(inode->i_mode))
return false;

@@ -1430,7 +1436,7 @@ static int do_loopback(struct nameidata
struct vfsmount *mnt = NULL;
int err;

- if (!permit_mount(nd, &flags))
+ if (!permit_mount(nd, NULL, &flags))
return -EPERM;
if (!old_name || !*old_name)
return -EINVAL;
@@ -1611,30 +1617,76 @@ out:
return err;
}

+static struct vfsmount *fs_set_subtype(struct vfsmount *mnt, const char *fstype)
+{
+ int err;
+ const char *subtype = strchr(fstype, '.');
+ if (subtype) {
+ subtype++;
+ err = -EINVAL;
+ if (!subtype[0])
+ goto err;
+ } else
+ subtype = "";
+
+ mnt->mnt_sb->s_subtype = kstrdup(subtype, GFP_KERNEL);
+ err = -ENOMEM;
+ if (!mnt->mnt_sb->s_subtype)
+ goto err;
+ return mnt;
+
+ err:
+ mntput(mnt);
+ return ERR_PTR(err);
+}
+
/*
* create a new mount for userspace and request it to be added into the
* namespace's tree
*/
-static int do_new_mount(struct nameidata *nd, char *type, int flags,
+static int do_new_mount(struct nameidata *nd, char *fstype, int flags,
int mnt_flags, char *name, void *data)
{
+ int err;
struct vfsmount *mnt;
+ struct file_system_type *type;

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

- /* we need capabilities... */
- if (!capable(CAP_SYS_ADMIN))
- return -EPERM;
-
- mnt = do_kern_mount(type, flags & ~MS_SETUSER, name, data);
- if (IS_ERR(mnt))
+ type = get_fs_type(fstype);
+ if (!type)
+ return -ENODEV;
+
+ err = -EPERM;
+ if (!permit_mount(nd, type, &flags))
+ goto out_put_filesystem;
+
+ if (flags & MS_SETUSER) {
+ err = reserve_user_mount();
+ if (err)
+ goto out_put_filesystem;
+ }
+
+ mnt = vfs_kern_mount(type, flags & ~MS_SETUSER, name, data);
+ if (!IS_ERR(mnt) && (type->fs_flags & FS_HAS_SUBTYPE) &&
+ !mnt->mnt_sb->s_subtype)
+ mnt = fs_set_subtype(mnt, fstype);
+ put_filesystem(type);
+ if (IS_ERR(mnt)) {
+ if (flags & MS_SETUSER)
+ dec_nr_user_mounts();
return PTR_ERR(mnt);
+ }

if (flags & MS_SETUSER)
- set_mnt_user(mnt);
+ __set_mnt_user(mnt);

return do_add_mount(mnt, nd, mnt_flags, NULL);
+
+ out_put_filesystem:
+ put_filesystem(type);
+ return err;
}

/*
@@ -1665,7 +1717,7 @@ int do_add_mount(struct vfsmount *newmnt
if (S_ISLNK(newmnt->mnt_root->d_inode->i_mode))
goto unlock;

- /* MNT_USER was set earlier */
+ /* some flags may have been set earlier */
newmnt->mnt_flags |= mnt_flags;
if ((err = graft_tree(newmnt, nd)))
goto unlock;
Index: linux/fs/super.c
===================================================================
--- linux.orig/fs/super.c 2008-02-04 23:47:46.000000000 +0100
+++ linux/fs/super.c 2008-02-04 23:48:02.000000000 +0100
@@ -925,29 +925,6 @@ out:

EXPORT_SYMBOL_GPL(vfs_kern_mount);

-static struct vfsmount *fs_set_subtype(struct vfsmount *mnt, const char *fstype)
-{
- int err;
- const char *subtype = strchr(fstype, '.');
- if (subtype) {
- subtype++;
- err = -EINVAL;
- if (!subtype[0])
- goto err;
- } else
- subtype = "";
-
- mnt->mnt_sb->s_subtype = kstrdup(subtype, GFP_KERNEL);
- err = -ENOMEM;
- if (!mnt->mnt_sb->s_subtype)
- goto err;
- return mnt;
-
- err:
- mntput(mnt);
- return ERR_PTR(err);
-}
-
struct vfsmount *
do_kern_mount(const char *fstype, int flags, const char *name, void *data)
{
@@ -956,9 +933,6 @@ do_kern_mount(const char *fstype, int fl
if (!type)
return ERR_PTR(-ENODEV);
mnt = vfs_kern_mount(type, flags, name, data);
- if (!IS_ERR(mnt) && (type->fs_flags & FS_HAS_SUBTYPE) &&
- !mnt->mnt_sb->s_subtype)
- mnt = fs_set_subtype(mnt, fstype);
put_filesystem(type);
return mnt;
}
Index: linux/include/linux/fs.h
===================================================================
--- linux.orig/include/linux/fs.h 2008-02-04 23:47:58.000000000 +0100
+++ linux/include/linux/fs.h 2008-02-04 23:48:02.000000000 +0100
@@ -1437,6 +1437,7 @@ int sync_inode(struct inode *inode, stru
struct file_system_type {
const char *name;
int fs_flags;
+ int fs_safe;
int (*get_sb) (struct file_system_type *, int,
const char *, void *, struct vfsmount *);
void (*kill_sb) (struct super_block *);
--


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