[patch 1/9] unprivileged mounts: add user mounts to the kernel

[Miklos Szeredi]

From: Miklos Szeredi <mszeredi@xxxxxxx>

This patchset adds support for keeping mount ownership information in the
kernel, and allow unprivileged mount(2) and umount(2) in certain cases.

The mount owner has the following privileges:

  - unmount the owned mount
  - create a submount under the owned mount

The sysadmin can set the owner explicitly on mount and remount.  When an
unprivileged user creates a mount, then the owner is automatically set to the
user.

The following use cases are envisioned:

1) Private namespace, with selected mounts owned by user.  E.g.
   /home/$USER is a good candidate for allowing unpriv mounts and unmounts
   within.

2) Private namespace, with all mounts owned by user and having the "nosuid"
   flag.  User can mount and umount anywhere within the namespace, but suid
   programs will not work.

3) Global namespace, with a designated directory, which is a mount owned by
   the user.  E.g.  /mnt/users/$USER is set up so that it is bind mounted onto
   itself, and set to be owned by $USER.  The user can add/remove mounts only
   under this directory.

The following extra security measures are taken for unprivileged mounts:

 - usermounts are limited by a sysctl tunable
 - force "nosuid,nodev" mount options on the created mount

For testing unprivileged mounts (and for other purposes) simple
mount/umount utilities are available from:

  http://www.kernel.org/pub/linux/kernel/people/mszeredi/mmount/

After this series I'll be posting a preliminary patch for util-linux-ng,
to add the same functionality to mount(8) and umount(8).

This patch:

A new mount flag, MS_SETUSER is used to make a mount owned by a user.  If this
flag is specified, then the owner will be set to the current fsuid and the
mount will be marked with the MNT_USER flag.  On remount don't preserve
previous owner, and treat MS_SETUSER as for a new mount.  The MS_SETUSER flag
is ignored on mount move.

The MNT_USER flag is not copied on any kind of mount cloning: namespace
creation, binding or propagation.  For bind mounts the cloned mount(s) are set
to MNT_USER depending on the MS_SETUSER mount flag.  In all the other cases
MNT_USER is always cleared.

For MNT_USER mounts a "user=UID" option is added to /proc/PID/mounts.  This is
compatible with how mount ownership is stored in /etc/mtab.

The rationale for using MS_SETUSER and MNT_USER, to distinguish "user"
mounts from "non-user" or "legacy" mounts are follows:

  a) Mount(2) and umount(2) on legacy mounts always need CAP_SYS_ADMIN
     capability.  As opposed to user mounts, which will only require,
     that the mount owner matches the current fsuid.  So a process
     with fsuid=0 should not be able to mount/umount legacy mounts
     without the CAP_SYS_ADMIN capability.

  b) Legacy userspace programs may set fsuid to nonzero before calling
     mount(2).  In such an unlikely case, this patchset would cause
     an unintended side effect of making the mount owned by the fsuid.

  c) For legacy mounts, no "user=UID" option should be shown in
     /proc/mounts for backwards compatibility.

Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxx>
---

Index: linux/fs/namespace.c
===================================================================
--- linux.orig/fs/namespace.c	2008-01-03 22:10:10.000000000 +0100
+++ linux/fs/namespace.c	2008-01-04 13:46:33.000000000 +0100
@@ -477,6 +477,13 @@ static struct vfsmount *skip_mnt_tree(st
 	return p;
 }
 
+static void set_mnt_user(struct vfsmount *mnt)
+{
+	BUG_ON(mnt->mnt_flags & MNT_USER);
+	mnt->mnt_uid = current->fsuid;
+	mnt->mnt_flags |= MNT_USER;
+}
+
 static struct vfsmount *clone_mnt(struct vfsmount *old, struct dentry *root,
 					int flag)
 {
@@ -491,6 +498,11 @@ static struct vfsmount *clone_mnt(struct
 		mnt->mnt_mountpoint = mnt->mnt_root;
 		mnt->mnt_parent = mnt;
 
+		/* don't copy the MNT_USER flag */
+		mnt->mnt_flags &= ~MNT_USER;
+		if (flag & CL_SETUSER)
+			set_mnt_user(mnt);
+
 		if (flag & CL_SLAVE) {
 			list_add(&mnt->mnt_slave, &old->mnt_slave_list);
 			mnt->mnt_master = old;
@@ -644,6 +656,8 @@ static int show_vfsmnt(struct seq_file *
 		if (mnt->mnt_flags & fs_infop->flag)
 			seq_puts(m, fs_infop->str);
 	}
+	if (mnt->mnt_flags & MNT_USER)
+		seq_printf(m, ",user=%i", mnt->mnt_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");
@@ -1181,8 +1195,9 @@ static int do_change_type(struct nameida
 /*
  * do loopback mount.
  */
-static int do_loopback(struct nameidata *nd, char *old_name, int recurse)
+static int do_loopback(struct nameidata *nd, char *old_name, int flags)
 {
+	int clone_fl;
 	struct nameidata old_nd;
 	struct vfsmount *mnt = NULL;
 	int err = mount_is_safe(nd);
@@ -1202,11 +1217,12 @@ static int do_loopback(struct nameidata 
 	if (!check_mnt(nd->path.mnt) || !check_mnt(old_nd.path.mnt))
 		goto out;
 
+	clone_fl = (flags & MS_SETUSER) ? CL_SETUSER : 0;
 	err = -ENOMEM;
-	if (recurse)
-		mnt = copy_tree(old_nd.path.mnt, old_nd.path.dentry, 0);
+	if (flags & MS_REC)
+		mnt = copy_tree(old_nd.path.mnt, old_nd.path.dentry, clone_fl);
 	else
-		mnt = clone_mnt(old_nd.path.mnt, old_nd.path.dentry, 0);
+		mnt = clone_mnt(old_nd.path.mnt, old_nd.path.dentry, clone_fl);
 
 	if (!mnt)
 		goto out;
@@ -1268,8 +1284,11 @@ static int do_remount(struct nameidata *
 		err = change_mount_flags(nd->path.mnt, flags);
 	else
 		err = do_remount_sb(sb, flags, data, 0);
-	if (!err)
+	if (!err) {
 		nd->path.mnt->mnt_flags = mnt_flags;
+		if (flags & MS_SETUSER)
+			set_mnt_user(nd->path.mnt);
+	}
 	up_write(&sb->s_umount);
 	if (!err)
 		security_sb_post_remount(nd->path.mnt, flags, data);
@@ -1378,10 +1397,13 @@ static int do_new_mount(struct nameidata
 	if (!capable(CAP_SYS_ADMIN))
 		return -EPERM;
 
-	mnt = do_kern_mount(type, flags, name, data);
+	mnt = do_kern_mount(type, flags & ~MS_SETUSER, name, data);
 	if (IS_ERR(mnt))
 		return PTR_ERR(mnt);
 
+	if (flags & MS_SETUSER)
+		set_mnt_user(mnt);
+
 	return do_add_mount(mnt, nd, mnt_flags, NULL);
 }
 
@@ -1413,7 +1435,8 @@ int do_add_mount(struct vfsmount *newmnt
 	if (S_ISLNK(newmnt->mnt_root->d_inode->i_mode))
 		goto unlock;
 
-	newmnt->mnt_flags = mnt_flags;
+	/* MNT_USER was set earlier */
+	newmnt->mnt_flags |= mnt_flags;
 	if ((err = graft_tree(newmnt, nd)))
 		goto unlock;
 
@@ -1735,7 +1758,7 @@ long do_mount(char *dev_name, char *dir_
 		retval = do_remount(&nd, flags & ~MS_REMOUNT, mnt_flags,
 				    data_page);
 	else if (flags & MS_BIND)
-		retval = do_loopback(&nd, dev_name, flags & MS_REC);
+		retval = do_loopback(&nd, dev_name, flags);
 	else if (flags & (MS_SHARED | MS_PRIVATE | MS_SLAVE | MS_UNBINDABLE))
 		retval = do_change_type(&nd, flags);
 	else if (flags & MS_MOVE)
Index: linux/fs/pnode.h
===================================================================
--- linux.orig/fs/pnode.h	2008-01-03 22:10:10.000000000 +0100
+++ linux/fs/pnode.h	2008-01-04 13:45:45.000000000 +0100
@@ -23,6 +23,7 @@
 #define CL_MAKE_SHARED 		0x08
 #define CL_PROPAGATION 		0x10
 #define CL_PRIVATE 		0x20
+#define CL_SETUSER		0x40
 
 static inline void set_mnt_shared(struct vfsmount *mnt)
 {
Index: linux/include/linux/fs.h
===================================================================
--- linux.orig/include/linux/fs.h	2008-01-03 22:10:10.000000000 +0100
+++ linux/include/linux/fs.h	2008-01-04 13:45:46.000000000 +0100
@@ -125,6 +125,7 @@ extern int dir_notify_enable;
 #define MS_RELATIME	(1<<21)	/* Update atime relative to mtime/ctime. */
 #define MS_KERNMOUNT	(1<<22) /* this is a kern_mount call */
 #define MS_I_VERSION	(1<<23) /* Update inode I_version field */
+#define MS_SETUSER	(1<<24) /* set mnt_uid to current user */
 #define MS_ACTIVE	(1<<30)
 #define MS_NOUSER	(1<<31)
 
Index: linux/include/linux/mount.h
===================================================================
--- linux.orig/include/linux/mount.h	2008-01-03 22:10:10.000000000 +0100
+++ linux/include/linux/mount.h	2008-01-04 13:45:45.000000000 +0100
@@ -33,6 +33,7 @@ struct mnt_namespace;
 
 #define MNT_SHRINKABLE	0x100
 #define MNT_IMBALANCED_WRITE_COUNT	0x200 /* just for debugging */
+#define MNT_USER	0x400
 
 #define MNT_SHARED	0x1000	/* if the vfsmount is a shared mount */
 #define MNT_UNBINDABLE	0x2000	/* if the vfsmount is a unbindable mount */
@@ -69,6 +70,8 @@ struct vfsmount {
 	 * are held, and all mnt_writer[]s on this mount have 0 as their ->count
 	 */
 	atomic_t __mnt_writers;
+
+	uid_t mnt_uid;			/* owner of the mount */
 };
 
 static inline struct vfsmount *mntget(struct vfsmount *mnt)

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