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Subject[patch 9/9] unprivileged mounts: add "no submounts" flag
From: Miklos Szeredi <mszeredi@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@suse.cz>
---

Index: linux/fs/namespace.c
===================================================================
--- linux.orig/fs/namespace.c 2008-01-04 13:49:52.000000000 +0100
+++ linux/fs/namespace.c 2008-01-04 13:50:28.000000000 +0100
@@ -694,6 +694,7 @@ 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;
@@ -1044,6 +1045,9 @@ static bool permit_mount(struct nameidat
if (S_ISLNK(inode->i_mode))
return false;

+ if (nd->path.mnt->mnt_flags & MNT_NOMNT)
+ return false;
+
if (!is_mount_owner(nd->path.mnt, current->fsuid))
return false;

@@ -1888,9 +1892,11 @@ long do_mount(char *dev_name, char *dir_
mnt_flags |= MNT_RELATIME;
if (flags & MS_RDONLY)
mnt_flags |= MNT_READONLY;
+ if (flags & MS_NOMNT)
+ mnt_flags |= MNT_NOMNT;

- 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_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 2008-01-04 13:49:12.000000000 +0100
+++ linux/include/linux/fs.h 2008-01-04 13:49:58.000000000 +0100
@@ -130,6 +130,7 @@ 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_NOMNT (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 2008-01-04 13:45:45.000000000 +0100
+++ linux/include/linux/mount.h 2008-01-04 13:49:58.000000000 +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_NOMNT 0x80

#define MNT_SHRINKABLE 0x100
#define MNT_IMBALANCED_WRITE_COUNT 0x200 /* just for debugging */
--


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