On Tue, Oct 26, 2021 at 12:47 AM Xin Long [off-list ref] wrote:

On Tue, Oct 26, 2021 at 5:51 AM Paul Moore [off-list ref] wrote:

quoted

On Mon, Oct 25, 2021 at 10:11 AM Xin Long [off-list ref] wrote:

quoted

On Mon, Oct 25, 2021 at 8:08 PM Ondrej Mosnacek [off-list ref] wrote:

quoted

On Mon, Oct 25, 2021 at 12:51 PM Xin Long [off-list ref] wrote:

quoted

On Mon, Oct 25, 2021 at 4:17 PM Ondrej Mosnacek [off-list ref] wrote:

quoted

On Fri, Oct 22, 2021 at 8:36 AM Xin Long [off-list ref] wrote:

quoted

Different from selinux_inet_conn_established(), it also gives the
secid to asoc->peer_secid in selinux_sctp_assoc_established(),
as one UDP-type socket may have more than one asocs.

Note that peer_secid in asoc will save the peer secid for this
asoc connection, and peer_sid in sksec will just keep the peer
secid for the latest connection. So the right use should be do
peeloff for UDP-type socket if there will be multiple asocs in
one socket, so that the peeloff socket has the right label for
its asoc.

Hm... this sounds like something we should also try to fix (if
possible). In access control we can't trust userspace to do the right
thing - receiving from multiple peers on one SOCK_SEQPACKET socket
shouldn't cause checking against the wrong peer_sid. But that can be
addressed separately. (And maybe it's even already accounted for
somehow - I didn't yet look at the code closely.)

There are a couple of things we need to worry about here: the
per-packet access controls (e.g. can this packet be received by this
socket?) and the userspace peer label queries (e.g. SO_GETPEERSEC and
IP_CMSG_PASSSEC).

The per-packet access controls work by checking the individual
packet's security label against the corresponding sock label on the
system (sk->sk_security->sid).  Because of this it is important that
the sock's label is correct.  For unconnected sockets this is fairly
straightforward as it follows the usual inherit-from-parent[1]
behavior we see in other areas of SELinux.  For connected stream
sockets this can be a bit more complicated.  However, since we are
only discussing the client side things aren't too bad with the
behavior essentially the same, inherit-from-parent, with the only
interesting piece worth noting being the sksec->peer_sid
(sk->sk_security->peer_sid) that we record from the packet passed to
the LSM/SELinux hook (using selinux_skb_peerlbl_sid()).  The
sksec->peer_sid is recorded primarily so that the kernel can correctly
respond to SO_GETPEERSEC requests from userspace; it shouldn't be used
in any access control decisions.

Hi, Paul

Understand now, the issue reported seems caused by when
doing peel-off the peel-off socket gets the uninitialised sid
from 'ep' on the client, though it should be "asoc".

Hi Xin Long,

Yes, that is my understanding.  I got the impression from the thread
that there was some confusion about the different labels and what they
were used for in SELinux, I was trying to provide some background in
the text above.  If you are already familiar with how things should
work you can disregard it :)

quoted

In the case of SCTP, I would expect things to behave similarly: the
sksec->peer_sid should match the packet label of the traffic which
acknowledged/accepted the new connection, e.g. the other end of the
connected socket.  You will have to forgive me some of the details,
it's been a while since I last looked at the SCTP bits, but I would
expect that if a client created a new connection and/or spun-off a new
socket the new socket's sksec->peer_sid would have the same property,
it would represent the security label of the other end of the
connection/association.

In SCTP, a socket doesn't represent a peer connection, it's more an
object binding some addresses and receiving incoming connecting
request, then creates 'asoc' to represent the connection, so asoc->
peer_secid represents the security label of the other end of the
connection/association.

As mentioned previously the asoc->peer_secid *should* be the security
label of the remote end, so I think we are okay here.  My concern
remains the asoc->secid label as I don't believe it is being set to
the correct value (more on that below).

After doing peel-off, it makes one asoc 'bind' to one new socket,
and this socket is used for userspace to control this asoc (conection),
so naturally we set sksec->peer_sid to asoc->secid for access control
in socket.

The sksec->peer_sid represents the security label of the remote end so
it should be set to the asoc->peer_secid and *not* the asoc->secid
value.  Yes, they are presently the same value in your patches, but I
believe that is a mistake; I believe the asoc->secid value should be
set to that of the parent (see the prior inherit-from-parent
discussion) which in this case would likely be either the parent
association or the client process, I'm not entirely clear on which is
correct in the SCTP case.  The initial SCTP client association would
need to take it's label from the parent process so perhaps that is the
right answer for all SCTP client associations[2].

[1] I would expect server side associations to follow the more
complicated selinux_conn_sid() labeling, just as we do for TCP/stream
connections today.

[2] I'm guessing the client associations might also want to follow the
setsockcreatecon(3) behavior, see selinux_sockcreate_sid() for more
info.

-- 
paul moore
www.paul-moore.com