Hierarchical jails

Jamie Gritton jamie at FreeBSD.org
Thu May 14 19:12:48 UTC 2009

Jilles Tjoelker wrote:
> On Thu, May 14, 2009 at 11:12:50AM -0600, Jamie Gritton wrote:
>> There's still a change to offer your input on the new jails before they
>> go in!  OK, given the lack of response so far, it's less "still a
>> chance" than "please?".  Current plans are to have this in place for
>> 8.0, with connections to the ongoing Vimage work.  Hopefully the silence
>> is approval, and commits will likely be appearing soon.
> I have not tried this, but I think this patch may allow jailed roots to
> escape. The problem is that there is only one fd_jdir. The escape would
> go like: jailed root creates a new jail in a subdirectory, opens its /
> and sends the fd to a process in the new jail via a unix domain socket.
> When the process calls fchdir on the fd, it will be able to access ..
> normally.
> With nested chroot, or chroot in jail, this is not possible, because
> fd_jdir always contains the first jail or chroot done and will not allow
> escaping from it; however, root in a level 2 chroot can escape back to
> level 1 using chroot.

Indeed - considering how that was a major design point of jails, I'm not
sure how I missed it.  ".." processing will need to run up the jail
tree.  No big deal on performance and easily done, but embarrassing not
have had that in place already.

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