Re: [PATCH v3 0/7] User namespace mount updates

From: Serge E. Hallyn
Date: Thu Nov 19 2015 - 09:58:25 EST

On Wed, Nov 18, 2015 at 03:13:35PM +0000, Al Viro wrote:
> On Wed, Nov 18, 2015 at 09:05:12AM -0600, Seth Forshee wrote:
> > Yes, the host admin. I'm not talking about trusting the admin inside the
> > container at all.
> Then why not have the same host admin just plain mount it when setting the
> container up and be done with that? From the host namespace, before spawning
> the docker instance or whatever framework you are using. IDGI...

fwiw one example use case is building a livecd or vm image from inside
container, on a system where each piece of functionality is
compartmentalized in a separate container.

For specific programs we can engineer them to call out to a helper
in the init user_ns, but that doesn't work for existing tools.

James Bottomley a year or two ago had mentioned the idea of using
seccomp to have the mount syscall in a container trap into a
init_user_ns helper, but we'd have to ptrace the whole container
for its whole lifetime to do that, or use SECCOMP_RET_TRAP with
a LD_PRELOAD that does goes through a proxy to do the remote
request (which becomes infeasible bc LD_PRELOAD won't stick).

If we did enable anything but FUSE I would hope each fs would have
a separate sysctl to enable non-init-userns mounts. Then the admin
could enable them just as they do the automount of whatever garbage
is on a usb stick found lying on the trade floor.

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