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

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Date:	Sun, 6 May 2007 12:21:51 +0200
From:	Frank van Maarseveen <frankvm@...nkvm.com>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	lkml123@...4n2c.de, linux-kernel@...r.kernel.org
Subject: Re: unprivileged mount problems: device permissions ignored, mount sharing

On Wed, May 02, 2007 at 09:56:52AM +0200, Miklos Szeredi wrote:
> > 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.

Some filesystems are not hardened against mounting a corrupted image and
can Oops. This is also a problem for automounting USB sticks except that
in that case you're (probably) sitting next to the hardware so there
are other ways to do bad things.

>From a security point of view I'd like to restrict unprivileged mounts
to a configurable list of filesystem types.

-- 
Frank
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