[PATCH net] sctp: fix association labeling in the duplicate COOKIE-ECHO case

[Xin Long]

On Mon, Aug 26, 2024 at 9:07 AM Ondrej Mosnacek <omosnace at redhat.com> wrote:
>
> 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: c081d53f97a1 ("security: pass asoc to sctp_assoc_request and sctp_sk_clone")
> Suggested-by: Xin Long <lucien.xin at gmail.com>
> Signed-off-by: Ondrej Mosnacek <omosnace at redhat.com>

Acked-by: Xin Long <lucien.xin at gmail.com>