Re: [RFC v2 v2 1/1] ns: add binfmt_misc to the mount namespace

On Wed, Oct 3, 2018 at 8:07 AM Eric W. Biederman [off-list ref] wrote:

Laurent Vivier [off-list ref] writes:

quoted

This patch allows to have a different binftm_misc configuration
in each container we mount binfmt_misc filesystem with mount namespace
enabled.

A container started without the CLONE_NEWNS will use the host binfmt_misc
configuration, otherwise the container starts with an empty binfmt_misc
interpreters list.

For instance, using "unshare" we can start a chroot of an another
architecture and configure the binfmt_misc interpreted without being root
to run the binaries in this chroot.

A couple of things.
As has already been mentioned on your previous version anything that
comes through the filesystem interface needs to lookup up the local
binfmt context not through current but through file->f_dentry->d_sb.
AKA the superblock of the mounted filesystem.

Something else: bm_register_write() currently calls into open_exec(),
which uses the credentials of current. That's not really allowed in
this context - but so far, it's not a big deal because only
init-namespace root can reach this code. Before you expose this stuff
to unprivileged userspace, this needs to get fixed; perhaps by
wrapping the open_exec() call in override_creds(file->f_cred) and
revert_creds().