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;
}

Comment:
This happens because some of the classes have their qdiscs assigned to
NULL, but remain in the active list. This commit fixes this issue by always
removing the class from the active list before deleting and freeing its
associated qdisc

Reproducer Steps
(trimmed version of what was sent by [email protected])

```
DEV="${DEV:-lo}"
ROOT_HANDLE="${ROOT_HANDLE:-1:}"
BAND2_HANDLE="${BAND2_HANDLE:-20:}"   # child under 1:2
PING_BYTES="${PING_BYTES:-48}"
PING_COUNT="${PING_COUNT:-200000}"
PING_DST="${PING_DST:-127.0.0.1}"

SLOW_TBF_RATE="${SLOW_TBF_RATE:-8bit}"
SLOW_TBF_BURST="${SLOW_TBF_BURST:-100b}"
SLOW_TBF_LAT="${SLOW_TBF_LAT:-1s}"

cleanup() {
  tc qdisc del dev "$DEV" root 2>/dev/null
}
trap cleanup EXIT

ip link set "$DEV" up

tc qdisc del dev "$DEV" root 2>/dev/null || true

tc qdisc add dev "$DEV" root handle "$ROOT_HANDLE" ets bands 2 strict 2

tc qdisc add dev "$DEV" parent 1:2 handle "$BAND2_HANDLE" \
  tbf rate "$SLOW_TBF_RATE" burst "$SLOW_TBF_BURST" latency "$SLOW_TBF_LAT"

tc filter add dev "$DEV" parent 1: protocol all prio 1 u32 match u32 0 0 flowid 1:2
tc -s qdisc ls dev $DEV

ping -I "$DEV" -f -c "$PING_COUNT" -s "$PING_BYTES" -W 0.001 "$PING_DST" \
  >/dev/null 2>&1 &
tc qdisc change dev "$DEV" root handle "$ROOT_HANDLE" ets bands 2 strict 0
tc qdisc change dev "$DEV" root handle "$ROOT_HANDLE" ets bands 2 strict 2
tc -s qdisc ls dev $DEV
tc qdisc del dev "$DEV" parent 
---truncated---

https://git.kernel.org/stable/c/06bfb66a7c8b45e3fed01351a4b087410ae5ef39

https://git.kernel.org/stable/c/45466141da3c98a0c5fa88be0bc14b4b6a4bd75c

https://git.kernel.org/stable/c/9987cda315c08f63a02423fa2f9a1f6602c861a0

https://git.kernel.org/stable/c/a75d617a4ef08682f5cfaadc01d5141c87e019c9

https://git.kernel.org/stable/c/ce052b9402e461a9aded599f5b47e76bc727f7de