[patch 8/9] unprivileged mounts: propagation: inherit owner from parent

[Serge E. Hallyn]

Quoting Miklos Szeredi (miklos at szeredi.hu):
> > > > > On mount propagation, let the owner of the clone be inherited from the
> > > > > parent into which it has been propagated.  Also if the parent has the
> > > > > "nosuid" flag, set this flag for the child as well.
> > > > 
> > > > What about nodev?
> > > 
> > > Hmm, I think the nosuid thing is meant to prevent suid mounts being
> > > introduced into a "suidless" namespace.  This doesn't apply to dev
> > > mounts, which are quite safe in a suidless environment, as long as the
> > > user is not able to create devices.  But that should be taken care of
> > > by capability tests.
> > > 
> > > I'll update the description.
> > 
> > Hmm,
> > 
> > Part of me wants to say the safest thing for now would be to refuse
> > mounts propagation from non-user mounts to user mounts.
> > 
> > I assume you're thinking about a fully user-mounted chroot, where
> > the user woudl still want to be able to stick in a cdrom and have
> > it automounted under /mnt/cdrom, propagated from the root mounts ns?
> 
> Right.
> 
> > But then are there no devices which the user could create on a floppy
> > while inserted into his own laptop, owned by his own uid, then insert
> > into this machine, and use the device under the auto-mounted /dev/floppy
> > to gain inappropriate access?
> 
> I assume, that the floppy and cdrom are already mounted with
> nosuid,nodev.

Yeah, of course, what I'm saying is no different whether the upper mount
is a user mount or not.  You're right.

> The problem case is I think is if a sysadmin does some mounting in the
> initial namespace, and this is propagated into the fully user-mounted
> namespace (or chroot), so that a mount with suid binaries slips in.
> Which is bad, because the user may be able rearange the namespace, to
> trick the suid program to something it should not do.

And really this shouldn't be an issue at all - the usermount chroot
would be set up under something like /share/hallyn/root, so the admin
would have to purposely set up propagation into that tree, so this
won't be happening by accident.

> OTOH, a mount with devices can't be abused this way, since it is not
> possible to gain privileges to files/devices just by rearanging the
> mounts.

Thanks for humoring me,

-serge