Ted Toth <txtoth@xxxxxxxxx> writes:
>
> On Wed, Aug 31, 2022 at 9:55 AM Christian Göttsche
> <cgzones@xxxxxxxxxxxxxx> wrote:
>>
>> On Wed, 31 Aug 2022 at 02:47, Paul Moore <paul@xxxxxxxxxxxxxx> wrote:
>> >
>> > On Tue, Aug 30, 2022 at 6:04 PM Ted Toth <txtoth@xxxxxxxxx> wrote:
>> > > On Mon, Aug 29, 2022 at 4:22 PM Paul Moore <paul@xxxxxxxxxxxxxx> wrote:
>> > > >
>> > > > On Thu, Aug 25, 2022 at 9:22 AM Ted Toth <txtoth@xxxxxxxxx> wrote:
>> > > > > I asked on the systemd-devel list about enabling systemd to set the
>> > > > > context of a socket and got the answer I've included below. I don't
>> > > > > know how a transition rule can be written to transition tcp sockets to
>> > > > > multiple different target contexts, is this possible and if so how?
>>
>> What do you mean by "multiple different target contexts"?
>
> Basically what I meant was that you cannot do the following since the
> source and target type are the same and there is no way to specify the
> socket other than if it were a UDS (a socket file):
> type_transition init_t init_t:tcp_socket app1_socket_t;
> type_transition init_t init_t:tcp_socket app2_socket_t;
>
>
>> How should they be different and how should systemd know?
>>
>> Socket unit configurations are normally paired with service unit
>> configurations (e.g. dovecot.socket <-> dovecot.service).
>> To handle incoming traffic the service unit configuration should
>> contain an ExecStart= directive, to start a program to handle the
>> data.
>> By default systemd tries at socket creation to predict the context of
>> the started program (via security_compute_create_raw(3) in
>> src/shared/selinux-util.c:mac_selinux_get_create_label_from_exe()),
>> see src/core/socket.c:socket_determine_selinux_label().
>>
>> For example if the service unit contains ExecStart=/usr/bin/myapp and
>> /usr/bin/myapp has the context myapp_exec_t and the policy contains
>> `type_transition init_t myapp_exec_t:process myapp_t` systemd should
>> assign the context myapp_t to the socket specified in the socket unit
>> configuration.
>
> I'll look at the code you reference but my experience is that the
> socket systemd is listening on is labeled init_t despite, as in your
> example above, the executable being labeled properly and transitioning
> to the type that I've specified, in the type_transition rule in the
> apps policy module, when it is run by systemd.
I am confident that, if were talking about socket activation, this is
not the case. systemd will create, and listen on the socket with the context of the
domain that will "accept" the connection.
for example i have a mpd instance that is socket activated:
root@brutus:~# ss -antlpZ | grep 6600
LISTEN 0 5 *:6600 *:*
users:(("systemd",pid=968,proc_ctx=wheel.id:wheel.role:user.systemd.subj:s0,fd=33))
systemd is listening on behalf of mpd.
if i query the policy:
root@brutus:~# sesearch -A -s user.systemd.subj -t user.systemd.subj -c
tcp_socket
... nothing returns. systemd is not allowed to create tcp_socket with
its own domain type or listen on them. Yes it is still listening on
tcp:6600
this is because:
root@brutus:~# sesearch -A -s user.systemd.subj -t user.mpd.subj -c tcp_socket
allow user.systemd.subj user.systemd.socketactivated.tcp.typeattr:tcp_socket { append bind connect create getattr getopt ioctl listen read setattr setopt shutdown write };
this systemd created a tcp_socket with type user.mpd.subj (on behalf of
mpd) and listens for connections on that tcp_socket. Once a connection
comes in then mpd with accept it (not that user.systemd.subj is not
allowed to accept tcp_socket on behalf of mpd (or any tcp_socket for
that matter:
root@brutus:~# sesearch -A -s user.systemd.subj -t user.mpd.subj -c
tcp_socket -p accept
... nothing returned.
>
>>
>> > > >
>> > > > Ignoring setsockcreatecon(3) as that really isn't an option here,
>> > >
>> > > If we determine that policy can't be written to accomplish the
>> > > transition then maybe systemd will reconsider not wanting to set the
>> > > socket context using a .socket file option.
>> >
>> > I think the challenge is going to be having enough information when
>> > the socket is created to do any useful type transition. I'm open to
>> > suggestions, but I'm skeptical there is anything we can do beyond the
>> > current approach.
>> >
>> > > > sockets created via socket(2) do check to see if there is a type
>> > > > transition defined in the policy. In the case of a TCP socket the
>> > > > type transition would look something like this:
>> > > >
>> > > > type_transition <domain> <domain>:tcp_socket <new_socket_type>
>> > > >
>> > > > ... so you can see there is not much one can select on other than the
>> > > > socket's object class. The reason is that the socket(2) call itself
>> > > > is rather spartan, with not even any clue as to if this is a client or
>> > > > server socket in the case of TCP.
>> > >
>> > > Having written many policy modules, some of which use the
>> > > type_transition statement for tcp_socket objects, I do not see how it
>> > > can be used to transition sockets created by systemd. And under this
>> > > circumstance I see that the selinux socket create hook would not be
>> > > able query the policy database for the port context since the port is
>> > > not known until the bind occurs but what about having the bind hook
>> > > set the socket context if it finds a sid for the port?
>> >
>> > The problem with waiting until the connect()/bind() is that you are
>> > effectively doing a relabel operation, which is a big no-no (but you
>> > already know that). *Maybe* you could justify it in the special case
>> > of stream sockets, as I'm pretty sure there is no way to do anything
>> > useful with them as a data sink/source until they are either connected
>> > to a remote peer or bound to a local port, however, we would all need
>> > to think on that for a bit (it is still a relabel, and thus nasty) and
>> > probably spend some time staring at the code to make sure there is no
>> > way to do something sneaky with an unconnected or unbound stream
>> > socket.
>> >
>> > > > Taking a step back, what are you trying to do? Perhaps there is
>> > > > another approach that would get you where you want to go.
>> > >
>> > > I want to create socket activation services using systemd and to have
>> > > the type of the socket being listened on be one that I've defined so
>> > > that I can write policy to control which source types can connect to
>> > > it.
>> >
>> > --
>> > paul-moore.com
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Dominick Grift
