[patch 10/10] unprivileged mounts: add "no submounts" flag

[Miklos Szeredi]

From: Miklos Szeredi <mszeredi at suse.cz>

Add a new mount flag "nomnt", which denies submounts for the owner.
This would be useful, if we want to support traditional /etc/fstab
based user mounts.

In this case mount(8) would still have to be suid-root, to check the
mountpoint against the user/users flag in /etc/fstab, but /etc/mtab
would no longer be mandatory for storing the actual owner of the
mount.

Signed-off-by: Miklos Szeredi <mszeredi at suse.cz>
---

Index: linux/fs/namespace.c
===================================================================
--- linux.orig/fs/namespace.c	2007-04-27 12:57:11.000000000 +0200
+++ linux/fs/namespace.c	2007-04-27 12:57:14.000000000 +0200
@@ -449,6 +449,7 @@ static int show_vfsmnt(struct seq_file *
 		{ MNT_NOATIME, ",noatime" },
 		{ MNT_NODIRATIME, ",nodiratime" },
 		{ MNT_RELATIME, ",relatime" },
+		{ MNT_NOMNT, ",nomnt" },
 		{ 0, NULL }
 	};
 	struct proc_fs_info *fs_infop;
@@ -806,6 +807,9 @@ static bool permit_mount(struct nameidat
 	if (S_ISLNK(inode->i_mode))
 		return false;
 
+	if (nd->mnt->mnt_flags & MNT_NOMNT)
+		return false;
+
 	if (!is_mount_owner(nd->mnt, current->fsuid))
 		return false;
 
@@ -1575,9 +1579,11 @@ long do_mount(char *dev_name, char *dir_
 		mnt_flags |= MNT_NODIRATIME;
 	if (flags & MS_RELATIME)
 		mnt_flags |= MNT_RELATIME;
+	if (flags & MS_NOMNT)
+		mnt_flags |= MNT_NOMNT;
 
 	flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_ACTIVE |
-		   MS_NOATIME | MS_NODIRATIME | MS_RELATIME);
+		   MS_NOATIME | MS_NODIRATIME | MS_RELATIME | MS_NOMNT);
 
 	/* ... and get the mountpoint */
 	retval = path_lookup(dir_name, LOOKUP_FOLLOW, &nd);
Index: linux/include/linux/fs.h
===================================================================
--- linux.orig/include/linux/fs.h	2007-04-27 12:57:11.000000000 +0200
+++ linux/include/linux/fs.h	2007-04-27 12:57:14.000000000 +0200
@@ -128,6 +128,7 @@ extern int dir_notify_enable;
 #define MS_SHARED	(1<<20)	/* change to shared */
 #define MS_RELATIME	(1<<21)	/* Update atime relative to mtime/ctime. */
 #define MS_SETUSER	(1<<22) /* set mnt_uid to current user */
+#define MS_NOMNT	(1<<23) /* don't allow unprivileged submounts */
 #define MS_ACTIVE	(1<<30)
 #define MS_NOUSER	(1<<31)
 
Index: linux/include/linux/mount.h
===================================================================
--- linux.orig/include/linux/mount.h	2007-04-27 12:57:01.000000000 +0200
+++ linux/include/linux/mount.h	2007-04-27 12:57:14.000000000 +0200
@@ -28,6 +28,7 @@ struct mnt_namespace;
 #define MNT_NOATIME	0x08
 #define MNT_NODIRATIME	0x10
 #define MNT_RELATIME	0x20
+#define MNT_NOMNT	0x40
 
 #define MNT_SHRINKABLE	0x100
 #define MNT_USER	0x200

--