unprivileged mount problems: device permissions ignored, mount sharing

[utz lehmann <lkml123@s2y4n2c.de>]

Hello

I tried the unprivileged mount v5 patches with 2.6.21.1. I made some
experiments with normal filesystems (ext3, xfs, iso9660). I removed the
FS_SAFE checks for that.

Mounting and umounting as unprivileged user (user1) works, e.g.
(/mnt/user1 is a mount owned by user1)

[user1@segv ~]$ mmount -t xfs /dev/mapper/vg00-test /mnt/user1

But the device permissions are ignored. The unprivileged user can mount
the block device even there are no permissions to access it:

brw------- 1 root root 253, 5 Apr 29 18:32 /dev/mapper/vg00-test


And there is another problem with sharing the device (superblock?).
If user1 mount a device readonly:

[user1@segv ~]$ mmount -r -t xfs /dev/mapper/vg00-test /mnt/user1

Than root cannot mount the device rw:

[root@segv ~]# mount /dev/mapper/vg00-test /mnt/test/
mount: /dev/mapper/vg00-test already mounted or /mnt/test/ busy

Mounting ro works and than remounting rw work. But than /mnt/user1 is rw
too.

This can DoS e.g. the automounter mounting /dev/mapper/vg00-test rw.


utz