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Date: Mon, 1 Aug 2016 09:26:17 +0000
From: "Reshetova, Elena" <>
To: ""
CC: ""
	<>, ""
	<>, "" <>,
	"" <>, "Schaufler, Casey"
	<>, "Leibowitz, Michael"
	<>, "Roberts, William C"
Subject: RE: [RFC] [PATCH 4/5] invoke path_chroot() LSM
 hook on mntns_install()

On Sun, Jul 31, 2016 at 10:39:04AM +0000, Reshetova, Elena wrote:
> On Fri, Jul 29, 2016 at 10:34:39AM +0300, Elena Reshetova wrote:
> > This adds an additional invocation of the security_path_chroot LSM 
> > hook inside mntns_install().
> > Currently only capabilities are checked at this point, while process 
> > root actually changes.
> >Are you aware that unprivileged user namespace creation doesn't work 
> >in a
> chrooted process? See the invocation of current_chrooted() in 
> create_user_ns(). This means that for this new LSM hook to make any 
> sense, a namespace admin has to attempt >to sandbox himself with chroot().
> I am not sure I understand you fully here. It is possible to create 
> new mount namespace without creating new user namespace, and when this 
> happens, if I understand the code right, there is no check like 
> current_chrooted() or smth like this.
> So, how does it relate to user namespace? 

>You can only create a new mount namespace if you're privileged relative to
your current namespace. See the second and third invocation of ns_capable()
in mntns_install(). To get those privileges, you have to either be
privileged already or unshare the user namespace to get new namespaced
An unprivileged, chrooted process doesn't have existing privileges and can't
unshare the user namespace to get new ones, so it can't unshare other
namespaces (like the mount namespace) anymore.

Yes, all true, I just got confused that you said that checks for chroot are
done. So, yes, you need to have CAP_SYS_ADMIN in current namespace, and then
you can get your new mount ns. 
After thinking more on this, I think I will get rid of this altogether. Yes,
there is a case when a process first enters a new user ns, then enters
chroot and forgets to drop its caps. Then it runs for a while, gets
exploited and now it tries to get out of chroot. It can create a new mount
ns since it has CAP_SYS_ADMIN in his user ns, but what is the outcome?
Probably not much, it certainly doesn't help it much from chroot
perspective. And I can't think of a reason why a legitimate process would
want to create a new namespace inside the chroot.... 

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