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

[Andy Lutomirski]

On Wed, Jul 8, 2015 at 9:10 AM, Andrew Vagin <avagin@xxxxxxxx> wrote:
> 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@xxxxxxxx> 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@xxxxxxxxxx> 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.

That's certainly doable.  OTOH, if anything does:

socket(AF_NETLINK, ...);
unshare(CLONE_PID);
fork();

then they now have a (minor) security problem.

--Andy
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/