sctp_sf_do_5_2_4_dupcook() currently calls security_sctp_assoc_request()
on new_asoc, but as it turns out, this association is always discarded
and the LSM labels never get into the final association (asoc).

This can be reproduced by having two SCTP endpoints try to initiate an
association with each other at approximately the same time and then peel
off the association into a new socket, which exposes the unitialized
labels and triggers SELinux denials.

Fix it by calling security_sctp_assoc_request() on asoc instead of
new_asoc. Xin Long also suggested limit calling the hook only to cases
A, B, and D, since in cases C and E the COOKIE ECHO chunk is discarded
and the association doesn't enter the ESTABLISHED state, so rectify that
as well.

One related caveat with SELinux and peer labeling: When an SCTP
connection is set up simultaneously in this way, we will end up with an
association that is initialized with security_sctp_assoc_request() on
both sides, so the MLS component of the security context of the
association will get swapped between the peers, instead of just one side
setting it to the other's MLS component. However, at that point
security_sctp_assoc_request() had already been called on both sides in
sctp_sf_do_unexpected_init() (on a temporary association) and thus if
the exchange didn't fail before due to MLS, it won't fail now either
(most likely both endpoints have the same MLS range).

Tested by:
 - reproducer from https://src.fedoraproject.org/tests/selinux/pull-request/530
 - selinux-testsuite (https://github.com/SELinuxProject/selinux-testsuite/)
 - sctp-tests (https://github.com/sctp/sctp-tests) - no tests failed
   that wouldn't fail also without the patch applied

Fixes: c081d53f ("security: pass asoc to sctp_assoc_request and sctp_sk_clone")
Suggested-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Acked-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Paul Moore <paul@paul-moore.com> (LSM/SELinux)
Link: https://patch.msgid.link/20240826130711.141271-1-omosnace@redhat.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>