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.

The default scheduler is pfifo_fast (and fd_codel for rhel8). This bug in BFIFO (sch_fifo) scheduler is not easy triggerable (requires usage of two qdiscs with special configuration). Both cannot trigger it before sch_fifo enabled. Rated to be Moderate.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

Mitigation for this issue is either not available or the currently available options don't meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base or stability.