CVE-2025-21702

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

pfifo_tail_enqueue: Drop new packet when sch->limit == 0

Expected behaviour:
In case we reach scheduler's limit, pfifo_tail_enqueue() will drop a
packet in scheduler's queue and decrease scheduler's qlen by one.
Then, pfifo_tail_enqueue() enqueue new packet and increase
scheduler's qlen by one. Finally, pfifo_tail_enqueue() return
`NET_XMIT_CN` status code.

Weird behaviour:
In case we set `sch->limit == 0` and trigger pfifo_tail_enqueue() on a
scheduler that has no packet, the 'drop a packet' step will do nothing.
This means the scheduler's qlen still has value equal 0.
Then, we continue to enqueue new packet and increase scheduler's qlen by
one. In summary, we can leverage pfifo_tail_enqueue() to increase qlen by
one and return `NET_XMIT_CN` status code.

The problem is:
Let's say we have two qdiscs: Qdisc_A and Qdisc_B.
 - Qdisc_A's type must have '->graft()' function to create parent/child relationship.
   Let's say Qdisc_A's type is `hfsc`. Enqueue packet to this qdisc will trigger `hfsc_enqueue`.
 - Qdisc_B's type is pfifo_head_drop. Enqueue packet to this qdisc will trigger `pfifo_tail_enqueue`.
 - Qdisc_B is configured to have `sch->limit == 0`.
 - Qdisc_A is configured to route the enqueued's packet to Qdisc_B.

Enqueue packet through Qdisc_A will lead to:
 - hfsc_enqueue(Qdisc_A) -> pfifo_tail_enqueue(Qdisc_B)
 - Qdisc_B->q.qlen += 1
 - pfifo_tail_enqueue() return `NET_XMIT_CN`
 - hfsc_enqueue() check for `NET_XMIT_SUCCESS` and see `NET_XMIT_CN` => hfsc_enqueue() don't increase qlen of Qdisc_A.

The whole process lead to a situation where Qdisc_A->q.qlen == 0 and Qdisc_B->q.qlen == 1.
Replace 'hfsc' with other type (for example: 'drr') still lead to the same problem.
This violate the design where parent's qlen should equal to the sum of its childrens'qlen.

Bug impact: This issue can be used for user->kernel privilege escalation when it is reachable.