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 schedulers limit, pfifo_tail_enqueue() will drop a packet in schedulers queue and decrease schedulers qlen by one. Then, pfifo_tail_enqueue() enqueue new packet and increase schedulers 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 schedulers qlen still has value equal 0. Then, we continue to enqueue new packet and increase schedulers 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: Lets say we have two qdiscs: Qdisc_A and Qdisc_B.

• Qdisc_As type must have ->graft() function to create parent/child relationship. Lets say Qdisc_As type is hfsc. Enqueue packet to this qdisc will trigger hfsc_enqueue.
• Qdisc_Bs 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 enqueueds 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() dont 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 parents qlen should equal to the sum of its childrensqlen.

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

Affected Software

References

• https://git.kernel.org/stable/c/020ecb76812a0526f4130ab5aeb6dc7c773e7ab9
• https://git.kernel.org/stable/c/647cef20e649c576dff271e018d5d15d998b629d
• https://git.kernel.org/stable/c/78285b53266d6d51fa4ff504a23df03852eba84e
• https://git.kernel.org/stable/c/79a955ea4a2e5ddf4a36328959de0de496419888
• https://git.kernel.org/stable/c/7a9723ec27aff5674f1fd4934608937f1d650980
• https://git.kernel.org/stable/c/a56a6e8589a9b98d8171611fbcc1e45a15fd2455
• https://git.kernel.org/stable/c/b6a079c3b6f95378f26e2aeda520cb3176f7067b
• https://git.kernel.org/stable/c/e40cb34b7f247fe2e366fd192700d1b4f38196ca
• https://lists.debian.org/debian-lts-announce/2025/05/msg00030.html
• https://lists.debian.org/debian-lts-announce/2025/05/msg00045.html