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

[Miklos Szeredi <miklos@szeredi.hu>]

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From: Miklos Szeredi <mszeredi@suse.cz>

Add a new mount flag "nosubmnt", 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@suse.cz>
Acked-by: Serge Hallyn <serue@us.ibm.com>
---

Index: linux/fs/namespace.c
===================================================================
--- linux.orig/fs/namespace.c	2008-02-04 23:48:08.000000000 +0100
+++ linux/fs/namespace.c	2008-02-04 23:48:10.000000000 +0100
@@ -783,6 +783,7 @@ static void show_mnt_opts(struct seq_fil
 		{ MNT_NOATIME, ",noatime" },
 		{ MNT_NODIRATIME, ",nodiratime" },
 		{ MNT_RELATIME, ",relatime" },
+		{ MNT_NOSUBMNT, ",nosubmnt" },
 		{ 0, NULL }
 	};
 	const struct proc_fs_info *fs_infop;
@@ -1189,6 +1190,9 @@ static bool permit_mount(struct nameidat
 	if (S_ISLNK(inode->i_mode))
 		return false;
 
+	if (nd->path.mnt->mnt_flags & MNT_NOSUBMNT)
+		return false;
+
 	if (!is_mount_owner(nd->path.mnt, current->fsuid))
 		return false;
 
@@ -2033,9 +2037,11 @@ long do_mount(char *dev_name, char *dir_
 		mnt_flags |= MNT_RELATIME;
 	if (flags & MS_RDONLY)
 		mnt_flags |= MNT_READONLY;
+	if (flags & MS_NOSUBMNT)
+		mnt_flags |= MNT_NOSUBMNT;
 
-	flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_ACTIVE |
-		   MS_NOATIME | MS_NODIRATIME | MS_RELATIME| MS_KERNMOUNT);
+	flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_ACTIVE | MS_NOATIME |
+		   MS_NODIRATIME | MS_RELATIME | MS_KERNMOUNT | MS_NOSUBMNT);
 
 	/* ... and get the mountpoint */
 	retval = path_lookup(dir_name, LOOKUP_FOLLOW, &nd);
Index: linux/include/linux/fs.h
===================================================================
--- linux.orig/include/linux/fs.h	2008-02-04 23:48:08.000000000 +0100
+++ linux/include/linux/fs.h	2008-02-04 23:48:10.000000000 +0100
@@ -129,6 +129,7 @@ extern int dir_notify_enable;
 #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_NOSUBMNT	(1<<25) /* 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	2008-02-04 23:47:50.000000000 +0100
+++ linux/include/linux/mount.h	2008-02-04 23:48:10.000000000 +0100
@@ -30,6 +30,7 @@ struct mnt_namespace;
 #define MNT_NODIRATIME	0x10
 #define MNT_RELATIME	0x20
 #define MNT_READONLY	0x40	/* does the user want this to be r/o? */
+#define MNT_NOSUBMNT	0x80
 
 #define MNT_SHRINKABLE	0x100
 #define MNT_IMBALANCED_WRITE_COUNT	0x200 /* just for debugging */

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