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

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Date:	Wed, 02 May 2007 09:56:52 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	lkml123@...4n2c.de
CC:	linux-kernel@...r.kernel.org
Subject: Re: unprivileged mount problems: device permissions ignored, mount
	sharing

> 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.

Thanks for looking at this.

> 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

Yes, I'm aware of this.  Before we enable FS_SAFE for block
filesystems, this must be addressed.

But I'm not sure _if_ we'll ever want this.  It is very likely that
there are some other security holes in most filesystems that are
difficult to address.  One example is checking for hard-linked
directories, which is normally only done during an fsck.

> 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.

Right.

There are patches for enabling per-mount read-only, but this shows
well, that unprivileged block device mounts are far from trivial.

Miklos
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