Subject: Re: [vserver] Re: Nested mount namespaces
From: Grzegorz Nosek <grzegorz.nosek@gmail.com>
Date: Fri, 10 Jun 2011 13:09:33 +0200

W dniu 10.06.2011 12:23, Herbert Poetzl pisze:
> might have several reasons:
>
>   - unshare actually fails despite reporting success

It does succeed (a new namespace and nsproxy are created).

>   - the two namespaces used in a typical guest setup
>     (with AFAIK shared mounts) cause unwanted behaviour

OK, I think I get it now. After making ~/foo a mountpoint by itself (via 
mount --bind foo foo before starting the test) I finally get the 
expected result. So apparently util-vserver started to use shared mounts 
between r2772 and r2926. Sigh.

>   - the test process is flawed

The test process is as follows:
shell1:~# mkdir -p foo/bar/baz
shell2:~# find foo
foo/
foo/bar/
foo/bar/baz/
shell1:~# ./ns_exec -m /bin/bash # unshare(CLONE_NEWNS) && exec()
[ns]shell1:~# mount --bind foo/bar/baz foo
[ns]shell1:~# find foo
foo/
shell2:~# find foo # <- *** PROBLEM ***
foo/
[ns]shell1:~# umount foo
shell2:~# find foo
foo/
foo/bar/
foo/bar/baz/

>> VServer version is 2.6.35.8-vs2.3.0.36.33, util-vserver
>> 0.30.216-pre2926.
>
> updating to a more recent kernel would be advised

Is that general advice or are there any serious known bugs with that 
patch version? The machine is in a _very_ remote DC and in case the 
upgrade fails for some reason it's going to be a PITA to bring back up.

Anyway, what's the _recommended_ (as in, at least somewhat proven) 
kernel+patch combo now?

> that behaviour is correct, but in recent kernels/patches
> you can set the umask (unshare mask) which controls what
> namespaces can be unshared inside a guest without actually
> having CAP_SYS_ADMIN assigned to the guest

How?

>> Or, if they worked the other way around, CAP_SYS_ADMIN without
>> VXC_NAMESPACE should prevent unshare but does not.
>> Am I missing something?
>
> the VXC checks are usually done by vx_capable()
> which looks like this:
>
> #define vx_capable(b, c) (capable(b) || \
>          (cap_raised(current_cap(), b)&&  vx_ccaps(c)))
>
> which in turn means, either you _have_ the bcap
> inside the guest, or you _would_ have it without
> the masking and the ccap is enabled
>
> now what does that mean in your case?
>
>            guest has               user has       vx_capable()
> CAP_SYS_ADMIN	VXC_NAMESPACE	CAP_SYS_ADMIN     with both
> --------------------------------------------------------------
>     whatever	   whatever	     no		     no
>       no 	     no		     yes	     no
>       yes	   whatever	     yes	     yes
>     whatever	     yes	     yes	     yes
>
> but VXC_NAMESPACE is probably a misnomer nowadays, as it is
> only checked in do_change_type() which affects mount point
> changes and nothing else

What I'm seeing as root in the guest with the following caps (shell 
after vserver enter):
CapInh:	0000000000000000
CapPrm:	ffffffffffffffff
CapEff:	ffffffffffffffff
CapBnd:	ffffffffffffffff

is:

  CAP_SYS_ADMIN VXC_NAMESPACE unshare(CLONE_NEWNS)==0
  yes           whatever      yes
  no            whatever      no

Best regards,
  Grzegorz Nosek