Re: [PATCH 0/1] shiftfs: uid/gid shifting filesystem

Djalal Harouni
2016-06-05T21:12:21Z

On Wed, Jun 01, 2016 at 12:41:00PM -0400, James Bottomley wrote:
> On Wed, 2016-06-01 at 18:21 +0200, Michał Zegan wrote:
> > As I sent a reply in a ... wrong way, I do it again. my question was:
> > Why isn't it done at the vfs layer when you mount the fs in different
> > userns, instead of using a separate filesystem for it?
>
> Well, that is what this patch does:
>
> http://thread.gmane.org/gmane.linux.kernel/2214882
>
> However, the reason it doesn't work for me is that I want to be able to
> unpack the image into a subdirectory (so I'm not dedicating a whole
> filesystem for this). This is primarily for a docker hack IBM is
> working on to allow each container instance to use a separate uid/gid
> range, so I need something that behaves much more like a bind mount.
I thought that you were using a loop device ? that's precisely one of
the main case that's solved with that solution... mount the portable fs
image into a loop device, set the shift which will be only active into
that subdirectory...

> > I believe it could be useful to be able to mount all filesystems in
> > userns with autoshifted uids, although I do not know security
> > implications for that usage.
>
> As long as you don't need to subdivide the volume, it works nicely.
> However, from a security point of view, that entire volume is now
> effectively freely writeable by anyone who can set up a userns. If you
> follow the shiftfs route, you can break off writeable subdirectories
> for each namespace shift, but they can't cross over into writing
> subdirectories that belong to other user namespaces (assuming the uids
> are fully segregated).

As said in the other email, I'm not really sure about the use case at
all... but I give you this quick test with:
https://gist.githubusercontent.com/tixxdz/6b84c2c3bd6cb987c82255602ec70f23/raw/97c9ab76878f9d7415583c00b22ca0e4a948847b/userns_test.c

$ mkdir shifted-fedora-tree && sudo mount -t shiftfs -ouidmap=0:1000000:65536,gidmap=0:1000000:65536 ~/fedora-tree/ shifted-fedora-tree

[tixxdz@fedora-kvm bin]$ sudo ./userns-test -m -U -M "0 1000000 1" /bin/bash
uid=0(root) gid=65534(nfsnobody) groups=65534(nfsnobody) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
[root@fedora-kvm bin]# cat /proc/self/uid_map
0 1000000 1
[root@fedora-kvm bin]# echo "$(id -u)_not_a_sandboxed_app" >> shifted-fedora-tree/etc/fedora-release
[root@fedora-kvm bin]# exit
exit
[tixxdz@fedora-kvm bin]$ sudo ./userns-test -m -U -M "48 1000000 1" /bin/bash
[apache@fedora-kvm bin]$ id
uid=48(apache) gid=65534(nfsnobody) groups=65534(nfsnobody) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
[apache@fedora-kvm bin]$ echo "$(id -u)_not_a_sandboxed_app" >> shifted-fedora-tree/etc/fedora-release
[apache@fedora-kvm bin]$ exit
exit
[tixxdz@fedora-kvm bin]$ sudo ./userns-test -m -U -M "70 1000000 1" /bin/bash
[avahi@fedora-kvm bin]$ id
uid=70(avahi) gid=65534(nfsnobody) groups=65534(nfsnobody) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
[avahi@fedora-kvm bin]$ echo "$(id -u)_not_a_sandboxed_app" >> shifted-fedora-tree/etc/fedora-release
[avahi@fedora-kvm bin]$ exit
exit
[tixxdz@fedora-kvm bin]$ sudo ./userns-test -m -U -M "1000 1000000 1" /bin/bash
[tixxdz@fedora-kvm bin]$ id
uid=1000(tixxdz) gid=65534(nfsnobody) groups=65534(nfsnobody) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
[tixxdz@fedora-kvm bin]$ echo "$(id -u)_not_a_sandboxed_app" >> shifted-fedora-tree/etc/fedora-release
[tixxdz@fedora-kvm bin]$ exit
exit
[tixxdz@fedora-kvm bin]$ cat ~/fedora-tree/etc/fedora-release
Fedora release 23 (Twenty Three)
0_not_a_sandboxed_app
48_not_a_sandboxed_app
70_not_a_sandboxed_app
1000_not_a_sandboxed_app

Apps or services still access to whatever they want... It's
still has some other bugs... however as said I'm not sure if
this is useful or if that's what you are planning...

Just my 2cents.

--
Djalal Harouni