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CVE-2025-21702— pfifo_tail_enqueue: Drop new packet when sch->limit == 0

Quick assessment

Affected
Linux Linux
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

Linux kernel是美国Linux基金会的开源操作系统Linux所使用的内核。 Linux kernel存在安全漏洞，该漏洞源于pfifo_tail_enqueue函数在处理队列时未正确验证输入。攻击者利用该漏洞可以导致拒绝服务（DoS）或权限提升。

CVSS 7.8 · High EPSS 0.27% · P18

Possible ATT&CK Techniques 1 AI

T1068 · Exploitation for Privilege Escalation

Affected Version Matrix 18

VendorProduct Version RangeStatus
Linux Linux 57dbb2d83d100ea601c54fe129bfde0678db5dee< 78285b53266d6d51fa4ff504a23df03852eba84e affected
57dbb2d83d100ea601c54fe129bfde0678db5dee< 7a9723ec27aff5674f1fd4934608937f1d650980 affected
57dbb2d83d100ea601c54fe129bfde0678db5dee< a56a6e8589a9b98d8171611fbcc1e45a15fd2455 affected
57dbb2d83d100ea601c54fe129bfde0678db5dee< 020ecb76812a0526f4130ab5aeb6dc7c773e7ab9 affected
57dbb2d83d100ea601c54fe129bfde0678db5dee< 79a955ea4a2e5ddf4a36328959de0de496419888 affected
57dbb2d83d100ea601c54fe129bfde0678db5dee< e40cb34b7f247fe2e366fd192700d1b4f38196ca affected
57dbb2d83d100ea601c54fe129bfde0678db5dee< b6a079c3b6f95378f26e2aeda520cb3176f7067b affected
57dbb2d83d100ea601c54fe129bfde0678db5dee< 647cef20e649c576dff271e018d5d15d998b629d affected
… +10 more rows
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I. Basic Information for CVE-2025-21702

Vulnerability Information
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Vulnerability Title
pfifo_tail_enqueue: Drop new packet when sch->limit == 0
Source: CVE Program / CVE List V5
Vulnerability 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.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Source: CVE Program / CVE List V5
Vulnerability Type
N/A
Source: CVE Program / CVE List V5
Vulnerability Title
Linux kernel 安全漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
Linux kernel是美国Linux基金会的开源操作系统Linux所使用的内核。 Linux kernel存在安全漏洞，该漏洞源于pfifo_tail_enqueue函数在处理队列时未正确验证输入。攻击者利用该漏洞可以导致拒绝服务（DoS）或权限提升。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
Linux Linux 57dbb2d83d100ea601c54fe129bfde0678db5dee ~ 78285b53266d6d51fa4ff504a23df03852eba84e -
Linux Linux 2.6.34 -

II. Public POCs for CVE-2025-21702

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III. Intelligence Information for CVE-2025-21702

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Patches & Fixes for CVE-2025-21702 (4)

Other References for CVE-2025-21702 (4)

IV. Related Vulnerabilities

Same product: Linux
Same vendor: Linux

V. Comments for CVE-2025-21702

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