CVE-2025-21702 pfifo_tail_enqueue: Drop new packet when sch->limit == 0
A logic error in the Linux kernel packet scheduler allows local attackers to escalate privileges by manipulating queue lengths under specific configurations.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2025-21702?
CVE-2025-21702 is a high-severity vulnerability in Linux kernel, published on 18 Feb 2025. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
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.
Which versions are affected by CVE-2025-21702?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2025-21702?
Apply the vendor's patch.
Is CVE-2025-21702 being exploited?
There is no known exploitation of CVE-2025-21702 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as BELL-CVE-2025-21702, DEBIAN-CVE-2025-21702, GHSA-c82f-pmfx-x3vv, UBUNTU-CVE-2025-21702.