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

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Date:	Tue, 4 Mar 2008 15:45:00 -0600
From:	serge@...lyn.com
To:	"Leibowitz, Michael" <michael.leibowitz@...el.com>
Cc:	serge@...lyn.com, linux-kernel@...r.kernel.org
Subject: Re: CLONE_NEWNS and bind mounts to make "chroot" jail

Quoting Leibowitz, Michael (michael.leibowitz@...el.com):
> If I understand correctly, the following should accomplish what I'm
> looking for.  However, pivot_root gives me EBUSY.  I played around with
> moving the mount --bind /jail /jail to before the unshared, as well as
> making old_root a bind mount to itself.  However, pivot_root always
> seems to fail.  Is there something obvious that I'm doing wrong?  The

Yes, you
	cd /jail
	mount --bind /jail /jail
	pivot_root . old_root

but . is now mounted over.

-serge

> following is my test code (error checking has been removed for clarity,
> except for pivot_root).  
> 
>   char *newargv[]= { "sh", NULL };
> 
>   chdir("/jail");
>   unshare(CLONE_NEWNS));
>   mount("/jail", "/jail", NULL, MS_BIND, NULL));
>   mount("/bin", "bin", NULL, MS_BIND, NULL));
>   mount("/usr", "usr", NULL, MS_BIND, NULL));
>   mount("/lib", "lib", NULL, MS_BIND, NULL));
>   if (pivot_root(".", "old_root")) perror("pivot_root . old_root");
>   exec("./bash-static"); /* copied to /jail prior to running */
> 
> Thanks. 
> 
> >Serge replies:
>  [snip...snip]
> >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
> 
> --
> Michael Leibowitz
> Software Engineer, UMG
> Intel Corporation
> michael.leibowitz at intel.com
> 
> 
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