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Date: Tue, 14 Oct 2014 14:31:01 +0000
From: Fiedler Roman <>
To: "" <>
Subject: Multiple disputed issues in util-vserver


While fixing a bug, I noticed some strange behavior in linux vserver
virtualization, that I would call a security problems, but project
developers see it differently. Since the util-vserver packages and patched
kernel were or are included in some Linux distros, I would be interested in
the communities' opinion on that.

Issue 1: When calling util-vserver tool on the host to execute a job within
the guest, e.g. to install updates, the host process (in host PID ns) might
end up being the child of a guest process (with PID only in guest ns), thus
the parent PID of the host process pointing to a guest ns PID. If the host
process wants to signal the parent process or some other tool operates using
the ppid, a host process might interact with another arbitrary host process
on error (see [1]). Compared to issue 2-3, I'm not sure for myself if it is
really a bug and what the correct behavior of kernel with pid namespaces
would be. At least it breaks bash process handling (gets stuck) when calling
"vserver exec" in a certain way, start-stop-daemon or upstart might not like
it also.

Issue 2: When entering the container from the host or executing commands
within the container, e.g. to perform common administrative tasks, a
malicious login shell inside the container might overwrite the
/usr/sbin/vcontext on the host, thus allowing on to execute arbitrary code
on the host with root privileges next time vcontext is invoked. See [3].
Feedback from developer: " Yes, vlogin is known to have several security
issues. It's a maintenance backdoor, much like the iLO or iDRAC on hardware.
If you can find ways to improve it, patches would be accepted, but I doubt
it will ever be possible to do what it does securely." Project documentation
does not strike out those restrictions (or at least I did not find that or
the list of "several known security issues" online), other sources, e.g.
container vs system virtualization comparison strike out the importance of
the feature to enter a guest from the host easily for maintenance, so I
guess that those tools were not useful just for me alone. This issue I would
rate a killer for production use, e.g. for mass hosting.

Issue 3: It seems that handling of open tty FDs on enter, that allows to
inject arbitrary keyboard input to be read by the parent process, also
affects the tool to start the guest container. This seems to be the same
issue with "vserver start" as reported in [2] for vserver enter, which was
classified as less relevant back than. My rating would be little lower than
2 but still quite high for mass hosting: manual restart, e.g. during
maintenance, seems quite common to me.

>From my point of view, those issues might be expected behavior as claimed by
the developers, but if so it should be at least stated more clearly in

a) never use any tools except vserver stop (to terminate the container) to
interact with a running and possibly compromised container from the host
b) only use network/socket-based tools to connect to processes inside a
possibly compromised guest, e.g. SSH. 
c) never start a possibly compromised container from interactive shell to
avoid injection of shell commands

Regarding documentation I would even vote for a solution d), that all those
tools get a mandatory argument like
'--i-know-entering-insecure-container-may-kill-my-host' so that it is not
very likely, that someone will use those tools for something else then
testing or nice-world administration.

Opinions to issues 1-3?

What about solutions?

Kind regards,

[3] Guest -> Host escape POC: C-code to be put as /bin/bash replacement in
guest, will overwrite /usr/sbin/vcontext on host. Available on request

DI Roman Fiedler
Safety & Security Department
Assistive Healthcare Information Technology

AIT Austrian Institute of Technology GmbH
Reininghausstraße 13/1  |  8020 Graz  |  Austria
T +43(0) 50550 2957  |  M +43(0) 664 8561599  |  F +43(0) 50550 2950 |

FN: 115980 i HG Wien  |  UID: ATU14703506

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