Re: [PATCH v4 2/5] fuse: Support fuse filesystems outside of init_user_ns

[Seth Forshee <seth.forshee@canonical.com>]

On Wed, Oct 15, 2014 at 10:05:46AM -0700, Andy Lutomirski wrote:
> On Wed, Oct 15, 2014 at 8:05 AM, Seth Forshee
> <seth.forshee@canonical.com> wrote:
> > On Wed, Oct 15, 2014 at 07:49:39AM -0700, Andy Lutomirski wrote:
> >> On 10/14/2014 07:25 AM, Seth Forshee wrote:
> >> > Update fuse to translate uids and gids to/from the user namspace
> >> > of the process servicing requests on /dev/fuse. Any ids which do
> >> > not map into the namespace will result in errors. inodes will
> >> > also be marked bad when unmappable ids are received from
> >> > userspace.
> >> >
> >> > Due to security concerns the namespace used should be fixed,
> >> > otherwise a user might be able to gain elevated privileges or
> >> > influence processes that the user would otherwise be unable to
> >> > manipulate. Thus the namespace of the mounting process is used
> >> > for all translations, and this namespace is required to be the
> >> > same as the one in use when /dev/fuse was opened.
> >> >
> >>
> >> I'm not sure that this is necessary if my nosuid patch goes in, but I
> >> also don't think it makes any sense to hold this up while we find a
> >> perfect solution.
> >>
> >> Is there a decent way to extend this to different translation schemes in
> >> the future (e.g. a flag at fs setup that could be used)?
> >
> > I think it would be possible to relax the translation scheme
> > restrictions in the future, certainly that's easier than tightening down
> > a looser restriction. I still favor picking one namespace to use for
> > translation (surely that's how it would work with other filesystems
> > anyway) rather than using the current namespace during /dev/fuse I/O. I
> > did an implementation using the latter technique, and it's far more
> > complex with no benefits that I can see.
> 
> Long term, I think we'll want more flexible translations for
> filesystems on removable media, even when both the mounter and the
> accessing process are in the init user namespace.  But this can wait.

You've piqued my interest. What are you thinking of which would require
this flexibility?

Thanks,
Seth