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Date: Wed, 18 Jun 2014 20:15:10 +1000
From: David Jorm <>
Subject: Re: docker VMM breakout

Thanks for reporting this issue, Sebastian. Could a CVE ID please be 
assigned to this issue, given it affects Docker 0.11?

Trevor Jay from Red Hat did some analysis, I am replying on his behalf 
as he is not currently subscribed to oss-security:

Starting on the 23rd of May and continuing through around the 28th, 
docker transitioned from a model where container "types" (i.e. 
libcontainer or LXC) had associated with them a list of capabilities to 
drop to the opposite: all capabilities are dropped and each container 
type has a list of capabilities to add back. Additionally, some checks 
were added so that docker would only try to drop/keep capabilities that 
actually exist on the kernel under which it's running (what a great 
idea!). To my reading of the linux kernel, there are some capabilities 
that you can't ever get back once you renounce, so these will not be 
added back after being dropped. Funnily enough, CAP_DAC_OVERRIDE, seems 
to be such a capability. This is---I suspect---why the post mid-may 0.11 
tests I ran never show a process having this capability. This, *despite* 
the fact that CAP_DAC_OVERRIDE is actually included on the "add back" 
list for libcontainer. The current docker codebase tries to reactivate 
CAP_DAC_OVERRIDE after dropping it, which I don't think should ever 
work. I'd love to get someone with more kernel experience than me to 
confirm this though.

I tested libvirt via virsh and by default both CAP_DAC_READ_SEARCH and 
CAP_DAC_OVERRIDE are available (and thus the PoC does run). However, 
this default is well documented as is the general insecurity of libvirt 
in regards to DAC, so I don't think a CVE ID is required for libvirt.


On 06/18/2014 05:36 PM, Sebastian Krahmer wrote:
> Hi
> As per list policy, here is the public forward.
> The link has been redacted to reflect public location.
> Its fixed in docker 1.0 since CAP_DAC_READ_SEARCH is no
> longer available.
> Other FS-related threats to container based VMM's
> that have been discussed:
> - subvolume related FS operations (snapshots etc)
> - FS ioctl's that accept FS-handles as well (XFS)
> - CAP_DAC_READ_SEARCH also defeats chroot and other
>    bind-mount containers (privileged LXC)
> - CAP_MKNOD might be a problem too (still available in docker 1.0)
>    depending on the drivers available in the kernel
> ----- Forwarded message from Sebastian Krahmer -----
> Subject: [vs] docker VMM breakout
> Date: Mon, 16 Jun 2014 10:56:16 +0200
> Hi
> I dont know if this really belongs here, but better safe
> than sorry. There seem to be distributors that actually ship
> docker as a 'solution' (to whatever problem :).
> I already contacted upstream, they say its not working
> anymore for them in version 1.0. I cannot test this, as I only
> have docker 0.11 running (on a 3.11 kernel). I am not really
> sure they really fixed the problem, they just told me that
> they changed "something" in the capability handling.
> In 0.11 the problem is that the apps that run in the container
> have CAP_DAC_READ_SEARCH and CAP_DAC_OVERRIDE which allows the
> containered app to access files not just by pathname (which would be
> impossible due to the bind mount of the rootfs) but also by handles via
> open_by_handle_at(). Handles are mostly 64bit values and can be kind
> of pre-computed as they are inode-based and the inode of / is 2.
> So you can go ahead and walk / by passing a handle of 2 and
> search the FS until you find the inode# of the file you want
> to access. Even though you are containered somewhere in /var/lib.
> A PoC is available here:
> Note that some FS (namely XFS) have their own fhandle ioctls,
> so if you share the XFS fs structs with the container, there
> might be different attack vectors despite of capability handling.
> Since docker is heavily LXC based, any other LXC based container
> solution might be at risk too. (I think file handles are a generic problem
> for container based VMMs).
> I request a CRD of June 30th. If within the next 2 days no other distributor
> objects, I will already disclose this on June 18th, because the overall
> issue is not really a critical thing.
> Sebastian

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