Analysis of mounts/unmounts issues.

Julian Elischer julian at
Mon Jan 26 15:19:16 PST 2004

On Tue, 27 Jan 2004, Pawel Jakub Dawidek wrote:

> Hello.
> This is a short analysis of mount(2)/unmount(2) problems related to
> usermounts, unprivileged root and jails.
> I've found many issues related to this topic, here is a list of those issues:
> 1. Root from inside of jail is able to unmount _any_ file system
>    (except /) from even outside of jail.
> 2. Even if security.bsd.suser is set to 0, root is able to unmount
>    file systems mounted by provileged root (except /).
> 3. If usermount is set to 1, user from inside of jail is able to
>    mount file system (if support for required file system is
>    compiled in kernel of loaded as a kld module), but with
>    MNT_NOSUID and MNT_NODEV flags set.
>    Insufficient check is in two place: for normal mounts and
>    for mounts with MNT_UPDATE flag set.
> 4. Let's assume that usermount is set to 1 and user mounts file system,
>    now we're setting usermount to 0 and user is still able to
>    unmount file system mounted by him previously.
> My fix deny any mounts/unmounts inside of jail and deny mounts/unmounts
> for unprivileged root, because there is no chance to check if
> security.bsd.suser was 0 or 1 while file system was mounted.
> Patch is here:
> Things to discuss.
> Should we permit mounts/unmounts inside of jail if usermount is set to 1?
> Maybe there should be 'jailmount' variable to control this?

we need a generic way to specify what is limited in a jail..

I have applications for jails that need to be able to run 'ping' 
and some that need chflags().. 
My answer is to just "allow it" in source
but that's not a general solution..

> Should we store in mount structure value of security.bsd.suser while
> file system is mounted to permit unmount and mount with MNT_UPDATE flag set
> operations for unprivileged root? This will give as a complete solution.
> -- 
> Pawel Jakub Dawidek                       pawel at
> UNIX Systems Programmer/Administrator
> Am I Evil? Yes, I Am!           

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