Mount remount operations propagating from container to host and other containers.

Serge E. Hallyn serue at
Thu Apr 1 07:50:54 PDT 2010

Quoting Michael H. Warfield (mhw at
> Hey all,
> Been running into an ugly situation with LXC-Tools that seems to be
> pointing up a real serious leakage from containers.  If you have a mount
> inside a container (presumably a bind mount in this case), if the
> container does a mount -o remount (say rw->ro or ro->rw) this propagates
> to the host mount points (all the way to the primary mount point for
> that partition in some cases) and is reflected in other containers.

I think you'll want to mount --make-rslave or --make-rprivate
during container setup.  Distros vary with how they leave /
set up, and if it is all rshared from the start, then yeah
your containers' mount actions will propagate backward.

> This first show up with containers running full VM's running on a
> mounted fs (aot the host / fs) were causing the real mounted fs to
> become ro when they were shut down (the VM was remounting its rootfs as
> ro and it was leaking out of the container).
> I've since confirmed that and encountered it trying to have a shared ro
> mounted fs in a container using bind mounts (bind mounts since 2.6.26
> have allowed setting the ro flag on individual mount points) and
> discovering that one container could make it rw and then all the other
> containers would see it as rw as well!  If a container made a mount
> point ro, all the other containers would see it as ro and the mount
> point for the entire real fs in the host would become ro!  This is very
> not good.  That's a pretty serious leakage from the containers out to
> the host.
> Is this a problem with the container isolation or some problem in
> creating the container?
> I'm running and testing on a Fedora 12 system with a 2.6.32 kernel.  Not
> related (I don't think) but I have also noted that linux-utils-ng on F16
> seems to also have a bug irt something similar here.  If I mount a
> directory from a mounted partition onto another location and then make
> that other location ro, the entire partition becomes ro.  BUT!  If I
> then make the partition rw, that does not propagate back up and the bind
> mount remains ro.
> What should work is this:
> Partition /export
> Directory /export/readonly
> mount --bind /export/readonly /srv/readonly
> At this point, /export and /srv/readonly are both rw
> mount -o remount,ro /srv/readonly
> Now. both /export and /srv/readonly are ro!  This is wrong.
> Only /srv/readonly is suppose to be ro!
> Now, running...
> mount -o remount,rw /export
> now, /export is rw and /srv/readonly is readonly.
> Back to containers...
> If I have /srv/readonly mounted in several containers (same mount point)
> it's ro in the host and in the containers...
> Running this in one container:
> mount -o remount,rw /srv/readonly
> (I seriously wish this would NOT WORK AT ALL, but it does.  I don't want
> the container to be able to write to that partition at all, like the
> media was RO.  Anybody have any ideas on that one?)
> Now /srv/readonly is rw in the host and all the containers!
> Running this in one container:
> mount -o remount,ro /srv/readonly
> NOW /srv/readonly is ro in all the containers and /export is ro in the
> host.  NOT GOOD.
> Thoughts?
> Regards,
> Mike
> -- 
> Michael H. Warfield (AI4NB) | (770) 985-6132 |  mhw at
>    /\/\|=mhw=|\/\/          | (678) 463-0932 |
>    NIC whois: MHW9          | An optimist believes we live in the best of all
>  PGP Key: 0x674627FF        | possible worlds.  A pessimist is sure of it!

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