tree ae7189cec50de78be582c7923658b17b5eb5ada5
parent 356fadf41f9c985649f06dec5bbeb308fe720fd6
author Eric W. Biederman <ebiederm@xmission.com> 1519233487 -0600
committer Eric W. Biederman <ebiederm@xmission.com> 1519688103 -0600

fuse: Support fuse filesystems outside of init_user_ns

In order to support mounts from namespaces other than init_user_ns,
fuse must translate uids and gids to/from the userns of the process
servicing requests on /dev/fuse. This patch does that, with a couple
of restrictions on the namespace:

 - The userns for the fuse connection is fixed to the namespace
   from which /dev/fuse is opened.

 - The namespace must be the same as s_user_ns.

These restrictions simplify the implementation by avoiding the need to
pass around userns references and by allowing fuse to rely on the
checks in setattr_prepare for ownership changes.  Either restriction
could be relaxed in the future if needed.

For cuse the userns used is the opener of /dev/cuse.  Semantically the
cuse support does not appear safe for unprivileged users.  Practically
the permissions on /dev/cuse only make it accessible to the global root
user.  If something slips through the cracks in a user namespace the only
users who will be able to use the cuse device are those users mapped into
the user namespace.

Translation in the posix acl is updated to use the uuser namespace of
the filesystem.  Avoiding cases which might bypass this translation is
handled in a following change.

This change is stronlgy based on a similar change from Seth Forshee
and Dongsu Park.

Cc: linux-fsdevel@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: Miklos Szeredi <mszeredi@redhat.com>
Cc: <seth.forshee@canonical.com>
Cc: Dongsu Park <dongsu@kinvolk.io>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>