High [CVE-2026-52929] fully roll back denied add-stream state

In the Linux kernel, the following vulnerability has been resolved: sctp: stream: fully roll back denied add-stream state 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. When transient outgoing streams are denied and torn down, the kernel fails to reclaim scheduler-specific metadata before an algorithm switch (e.g., WFQ to PRIO).

The orphaned state - including internal accounting fields - becomes reachable through the new scheduler's accessors, allowing an unprivileged local attacker to read arbitrary kernel memory and corrupt kernel objects via SCTP_STREAM_SCHEDULER_VALUE socket options. This can be leveraged for local privilege escalation to full root capabilities.

A failed exploitation attempt may also crash the kernel, resulting in a Denial of Service (DoS).