linux-kernel - Re: CLONE_NEWNS and bind mounts to make "chroot" jail

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Date:	Sat, 1 Mar 2008 20:26:56 -0600
From:	serge@...lyn.com
To:	"Leibowitz, Michael" <michael.leibowitz@...el.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: CLONE_NEWNS and bind mounts to make "chroot" jail

Quoting Leibowitz, Michael (michael.leibowitz@...el.com):
> I have been trying to use the combination of CLONE_NEWNS and bind mounts
> to create a better (than) chroot jail.  I wish to have the ability to
> bind (ro will be possible in the future, I understand) certain
> directories into the jail (perhaps /bin, /lib, /usr), but not have
> parallel directories in the jail (no /etc, ..., /home). 
> 
> I have heard that this should be possible, but have yet to get a working
> solution.
> 
> I have tried something analogous to:

Try a few more things.  Since you had entered /jail, you can view '/' by
looking at .. .  But if you look at /, you dereference your
task->fsroot.  You never changed that, so it points to the original
mount.  If however you 'ls ..', you should see your 'jail' directory.
However it won't have the /bin and /lib mounted because you didn't
	mount --rbind /jail /
What you really want to do is
	mount --bind /jail /jail
to make sure it's a mountpoint, then set up the new /jail using bind
mounts like you're doing (and likely some rbinds in some places), then
use pivot_root() to change your root.  Then umount2("/old_root",
MNT_DETACH).

-serge

> chdir("/jail");
> unshare(CLONE_NEWNS);
> /* mount(8) syntax given for simplicity, but mount(2) used below */
> mount --bind / /jail/old_root
> mount --bind /jail/old_root/bin /jail/bin
> mount --bind /jail/old_root/usr /jail/usr
> mount --bind /jail/old_root/lib /jail/lib
> mount --bind /jail /               # does nothing?
> umount2("/old_root", MNT_DETACH);  # never happens.
> exec("bin/sh");
> 
> When bin/sh runs, I can still see old_root from /jail and the bind of
> /jail over / seems to have not done anything.  
> 
> Is it possible to create such a jail with bind mounts?  Is there a
> recommended method for doing so?  Thank you for your time.
> 
> --
> Michael Leibowitz
> Software Engineer, UMG
> Intel Corporation
> michael.leibowitz at intel.com
> 
> 
> --
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