Re: [PATCH 0/24] kernel: add a netlink interface to get information about processes (v2)

[Andrew Vagin <avagin@odin.com>]

On Tue, Jul 07, 2015 at 08:56:37AM -0700, Andy Lutomirski wrote:
> On Tue, Jul 7, 2015 at 8:43 AM, Andrew Vagin <avagin@odin.com> wrote:
> > On Mon, Jul 06, 2015 at 10:10:32AM -0700, Andy Lutomirski wrote:
> >> On Mon, Jul 6, 2015 at 1:47 AM, Andrey Vagin <avagin@openvz.org> wrote:
> >> > Currently we use the proc file system, where all information are
> >> > presented in text files, what is convenient for humans.  But if we need
> >> > to get information about processes from code (e.g. in C), the procfs
> >> > doesn't look so cool.
> >> >
> >> > From code we would prefer to get information in binary format and to be
> >> > able to specify which information and for which tasks are required. Here
> >> > is a new interface with all these features, which is called task_diag.
> >> > In addition it's much faster than procfs.
> >> >
> >> > task_diag is based on netlink sockets and looks like socket-diag, which
> >> > is used to get information about sockets.
> >>
> >> I think I like this in principle, but I have can see a few potential
> >> problems with using netlink for this:
> >>
> >> 1. Netlink very naturally handles net namespaces, but it doesn't
> >> naturally handle any other kind of namespace.  In fact, the taskstats
> >> code that you're building on has highly broken user and pid namespace
> >> support.  (Look for some obviously useless init_user_ns and
> >> init_pid_ns references.  But that's only the obvious problem.  That
> >> code calls current_user_ns() and task_active_pid_ns(current) from
> >> .doit, which is, in turn, called from sys_write, and looking at
> >> current's security state from sys_write is a big no-no.)
> >>
> >> You could partially fix it by looking at f_cred's namespaces, but that
> >> would be a change of what it means to create a netlink socket, and I'm
> >> not sure that's a good idea.
> >
> > If I don't miss something, all problems around pidns and userns are
> > related with multicast functionality. task_diag is using
> > request/response scheme and doesn't send multicast packets.
> 
> It has nothing to do with multicast.  task_diag needs to know what
> pidns and userns to use for a request, but netlink isn't set up to
> give you any reasonably way to do that.  A netlink socket is
> fundamentally tied to a *net* ns (it's a socket, after all).  But you
> can send it requests using write(2), and calling current_user_ns()
> from write(2) is bad.  There's a long history of bugs and
> vulnerabilities related to thinking that current_cred() and similar
> are acceptable things to use in write(2) implementations.
> 

As far as I understand, socket_diag doesn't have this problem, becaus
each socket has a link on a namespace where it was created.

What if we will pin the current pidns and credentials to a task_diag
socket in a moment when it's created.

Thanks,
Andrew