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