sctp: stream: fully roll back denied add-stream state

cve CVE-2026-52929
commit-author Wyatt Feng <bronzed_45_vested@icloud.com>
commit a5f8a90

When ADD_OUT_STREAMS is denied, SCTP only shrinks the queued chunks and
then lowers outcnt. That leaves removed stream metadata behind, so a
later re-add can reuse a stale ext and hit a null-pointer dereference in
the scheduler get path.

Fix the rollback by tearing down the removed stream state the same way
other stream resizes do. Unschedule the current scheduler state, drop
the removed stream ext state with sctp_stream_outq_migrate(), and then
reschedule the remaining streams.

This keeps scheduler-private RR/FC/PRIO lists consistent while fully
rolling back denied outgoing stream additions.

Fixes: 637784a ("sctp: introduce priority based stream scheduler")
	Cc: stable@kernel.org
	Reported-by: Yuan Tan <yuantan098@gmail.com>
	Reported-by: Yifan Wu <yifanwucs@gmail.com>
	Reported-by: Juefei Pu <tomapufckgml@gmail.com>
	Reported-by: Zhengchuan Liang <zcliangcn@gmail.com>
	Reported-by: Xin Liu <bird@lzu.edu.cn>
	Signed-off-by: Wyatt Feng <bronzed_45_vested@icloud.com>
	Signed-off-by: Ren Wei <n05ec@lzu.edu.cn>
	Acked-by: Xin Long <lucien.xin@gmail.com>
Link: https://patch.msgid.link/d78954ecd94954653ee299400e98d74a03a6f7d3.1780603399.git.bronzed_45_vested@icloud.com
	Signed-off-by: Jakub Kicinski <kuba@kernel.org>
(cherry picked from commit a5f8a90)
	Signed-off-by: Shreeya Patel <spatel@ciq.com>

Author: shreeya-patel98

net/sctp/stream.c

@@ -1038,6 +1038,7 @@ struct sctp_chunk *sctp_process_strreset_resp(
 			stsn, rtsn, GFP_ATOMIC);
 	} else if (req->type == SCTP_PARAM_RESET_ADD_OUT_STREAMS) {
 		struct sctp_strreset_addstrm *addstrm;
+		const struct sctp_sched_ops *sched;
 		__u16 number;
 
 		addstrm = (struct sctp_strreset_addstrm *)req;
@@ -1048,7 +1049,10 @@ struct sctp_chunk *sctp_process_strreset_resp(
 			for (i = number; i < stream->outcnt; i++)
 				SCTP_SO(stream, i)->state = SCTP_STREAM_OPEN;
 		} else {
-			sctp_stream_shrink_out(stream, number);
+			sched = sctp_sched_ops_from_stream(stream);
+			sched->unsched_all(stream);
+			sctp_stream_outq_migrate(stream, NULL, number);
+			sched->sched_all(stream);
 			stream->outcnt = number;
 		}