[PATCH 18/19] fuse: Restrict allow_other to the superblock's namespace or a descendant

[Seth Forshee]

On Fri, Dec 04, 2015 at 02:05:41PM -0600, Serge E. Hallyn wrote:
> Quoting Seth Forshee (seth.forshee at canonical.com):
> > Unprivileged users are normally restricted from mounting with the
> > allow_other option by system policy, but this could be bypassed
> > for a mount done with user namespace root permissions. In such
> > cases allow_other should not allow users outside the userns
> > to access the mount as doing so would give the unprivileged user
> > the ability to manipulate processes it would otherwise be unable
> > to manipulate. Restrict allow_other to apply to users in the same
> > userns used at mount or a descendant of that namespace.
> > 
> > Signed-off-by: Seth Forshee <seth.forshee at canonical.com>
> > ---
> >  fs/fuse/dir.c | 10 ++++++++--
> >  1 file changed, 8 insertions(+), 2 deletions(-)
> > 
> > diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
> > index f67f4dd86b36..5b8edb1203b8 100644
> > --- a/fs/fuse/dir.c
> > +++ b/fs/fuse/dir.c
> > @@ -1018,8 +1018,14 @@ int fuse_allow_current_process(struct fuse_conn *fc)
> >  {
> >  	const struct cred *cred;
> >  
> > -	if (fc->flags & FUSE_ALLOW_OTHER)
> > -		return 1;
> > +	if (fc->flags & FUSE_ALLOW_OTHER) {
> > +		struct user_namespace *ns;
> > +		for (ns = current_user_ns(); ns; ns = ns->parent) {
> > +			if (ns == fc->user_ns)
> > +				return 1;
> > +		}
> 
> 	use current_in_userns() ?

Yes, it should. I wrote this before I wrote the patch which adds that
function and never thought to go back to change it here.