linux-kernel - unprivileged mount problems: device permissions ignored, mount sharing

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Message-Id: <1177873151.4343.29.camel@segv.aura.of.mankind>
Date:	Sun, 29 Apr 2007 20:59:11 +0200
From:	utz lehmann <lkml123@...4n2c.de>
To:	Miklos Szeredi <miklos@...redi.hu>,
	LKML <linux-kernel@...r.kernel.org>
Subject: unprivileged mount problems: device permissions ignored, mount
	sharing

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

Than root cannot mount the device rw:

[root@...v ~]# 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

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