[Devel] Re: [patch 05/10] add "permit user mounts in new namespace" clone flag

[Serge E. Hallyn]

Quoting Miklos Szeredi (miklos at szeredi.hu):
> > > > > > Also for bind-mount and remount operations the flag has to be propagated
> > > > > > down its propagation tree.  Otherwise a unpriviledged mount in a shared
> > > > > > mount wont get reflected in its peers and slaves, leading to unidentical
> > > > > > shared-subtrees.
> > > > > 
> > > > > That's an interesting question.  Do we want shared mounts to be
> > > > > totally identical, including mnt_flags?  It doesn't look as if
> > > > > do_remount() guarantees that currently.
> > > > 
> > > > Depends on the semantics of each of the flags. Some flags like of the
> > > > read/write flag, would not interfere with the propagation semantics
> > > > AFAICT.  But this one certainly seems to interfere.
> > > 
> > > That depends.  Current patches check the "unprivileged submounts
> > > allowed under this mount" flag only on the requested mount and not on
> > > the propagated mounts.  Do you see a problem with this?
> > 
> > Don't see a problem if the flag is propagated to all peers and slave
> > mounts. 
> > 
> > If not, I see a problem. What if the propagated mount has its flag set
> > to not do un-priviledged mounts, whereas the requested mount has it
> > allowed?
> 
> Then the mount is allowed.
> 
> It is up to the sysadmin/distro to design set up the propagations in a
> way that this is not a problem.
> 
> I think it would be much less clear conceptually, if unprivileged
> mounting would have to check propagations as well.
> 
> Miklos

I'm a bit lost about what is currently done and who advocates for what.

It seems to me the MNT_ALLOWUSERMNT (or whatever :) flag should be
propagated.  In the /share rbind+chroot example, I assume the admin
would start by doing

	mount --bind /share /share
	mount --make-slave /share
	mount --bind -o allow_user_mounts /share (or whatever)
	mount --make-shared /share

then on login, pam does

	chroot /share/$USER

or some sort of

	mount --bind /share /home/$USER/root
	chroot /home/$USER/root

or whatever.  In any case, the user cannot make user mounts except under
/share, and any cloned namespaces will still allow user mounts.

Or are you guys talking about something else?

-serge