CVE-2025-71066

In the Linux kernel, the following vulnerability has been resolved:

net/sched: ets: Always remove class from active list before deleting in ets_qdisc_change

[email protected] says:

The vulnerability is a race condition between ets_qdisc_dequeue and ets_qdisc_change. It leads to UAF on struct Qdisc object. Attacker requires the capability to create new user and network namespace in order to trigger the bug. See my additional commentary at the end of the analysis.

Analysis:

static int ets_qdisc_change(struct Qdisc *sch, struct nlattr *opt, struct netlink_ext_ack *extack) { ...

  // (1) this lock is preventing .change handler (`ets_qdisc_change`)
  //to race with .dequeue handler (`ets_qdisc_dequeue`)
  sch_tree_lock(sch);

  for (i = nbands; i < oldbands; i++) {
          if (i >= q->nstrict && q->classes[i].qdisc->q.qlen)
                  list_del_init(&q->classes[i].alist);
          qdisc_purge_queue(q->classes[i].qdisc);
  }

  WRITE_ONCE(q->nbands, nbands);
  for (i = nstrict; i < q->nstrict; i++) {
          if (q->classes[i].qdisc->q.qlen) {
	      // (2) the class is added to the q->active
                  list_add_tail(&q->classes[i].alist, &q->active);
                  q->classes[i].deficit = quanta[i];
          }
  }
  WRITE_ONCE(q->nstrict, nstrict);
  memcpy(q->prio2band, priomap, sizeof(priomap));

  for (i = 0; i < q->nbands; i++)
          WRITE_ONCE(q->classes[i].quantum, quanta[i]);

  for (i = oldbands; i < q->nbands; i++) {
          q->classes[i].qdisc = queues[i];
          if (q->classes[i].qdisc != &noop_qdisc)
                  qdisc_hash_add(q->classes[i].qdisc, true);
  }

  // (3) the qdisc is unlocked, now dequeue can be called in parallel
  // to the rest of .change handler
  sch_tree_unlock(sch);

  ets_offload_change(sch);
  for (i = q->nbands; i < oldbands; i++) {
      // (4) we're reducing the refcount for our class's qdisc and
      //  freeing it
          qdisc_put(q->classes[i].qdisc);
      // (5) If we call .dequeue between (4) and (5), we will have
      // a strong UAF and we can control RIP
          q->classes[i].qdisc = NULL;
          WRITE_ONCE(q->classes[i].quantum, 0);
          q->classes[i].deficit = 0;
          gnet_stats_basic_sync_init(&q->classes[i].bstats);
          memset(&q->classes[i].qstats, 0, sizeof(q->classes[i].qstats));
  }
  return 0;