pfifo_tail_enqueue: Drop new packet when sch->limit == 0
Key facts
- Attack vector
- Local
- Privileges required
- Low
- User interaction
- None
- Attack complexity
- Low
- Impact type
- —
- Confidentiality · integrity · availability
- High · High · High
- Fix status
- Partial Red Hat: 15 fixed, 3 without a fix
Attack profile from CVSS 3.1 · CNA. Fix status from CNA and Red Hat.
Summary
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.
Why it matters now
SSVC decision points
Not available: CISA has not published SSVC decision points for this record.
Affected products
| Vendor | Product | Affected versions | Fixed in | Status | Source |
|---|---|---|---|---|---|
| Linux | Linux | 2.6.34 and later | 5.4.291, 5.10.235, 5.15.179, 6.1.130, 6.6.83, 6.12.14, 6.13.3, 6.14 | Patch available | CNA |
- Linux VendorLinux Affected 2.6.34 and later Fixed in 5.4.291, 5.10.235, 5.15.179, 6.1.130, 6.6.83, 6.12.14, 6.13.3, 6.14 Patch available CNA
CPE configurations NVD · 2
| CPE | Versions | Role | Vendor |
|---|---|---|---|
| cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:* | 2.6.34 to before 5.4.291; 5.5 to before 5.10.235; 5.11 to before 5.15.179; 5.16 to before 6.1.130; 6.2 to before 6.6.83; 6.7 to before 6.12.14; 6.13 to before 6.13.3 | Vulnerable | Linux |
| cpe:2.3:o:linux:linux_kernel:6.14:rc1:*:*:*:*:*:* | 6.14 rc1 | Vulnerable | Linux |
- cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:* Versions 2.6.34 to before 5.4.291; 5.5 to before 5.10.235; 5.11 to before 5.15.179; 5.16 to before 6.1.130; 6.2 to before 6.6.83; 6.7 to before 6.12.14; 6.13 to before 6.13.3 Vulnerable ·Linux
- cpe:2.3:o:linux:linux_kernel:6.14:rc1:*:*:*:*:*:* Versions 6.14 rc1 Vulnerable ·Linux
Remediation
- Fix · CNA Linux (Linux) Fixed in 5.4.291, 5.10.235, 5.15.179, 6.1.130, 6.6.83, 6.12.14, 6.13.3, 6.14
Red Hat 3 advisories
- RHSA-2026:94438 Released 2026-11-03 · Red Hat rating Criticalkernel security, bug fix, and enhancement update
- Red Hat CodeReady Linux Builder EUS (v.9.6) kernel-tools-libs-devel-5.14.0-570.154.1.el9_6, libperf-5.14.0-570.154.1.el9_6
- Red Hat Enterprise Linux AppStream EUS (v.9.6) kernel-64k-debug-devel-5.14.0-570.154.1.el9_6, kernel-64k-debug-devel-matched-5.14.0-570.154.1.el9_6, kernel-64k-devel-5.14.0-570.154.1.el9_6 +12
- Red Hat Enterprise Linux BaseOS EUS (v.9.6) kernel-5.14.0-570.154.1.el9_6, kernel-64k-5.14.0-570.154.1.el9_6, kernel-64k-core-5.14.0-570.154.1.el9_6 +28
- Red Hat Enterprise Linux Real Time EUS (v.9.6) kernel-rt-5.14.0-570.154.1.el9_6, kernel-rt-64k-5.14.0-570.154.1.el9_6, kernel-rt-64k-core-5.14.0-570.154.1.el9_6 +21
- Red Hat Enterprise Linux Real Time for NFV EUS (v.9.6) kernel-rt-5.14.0-570.154.1.el9_6, kernel-rt-core-5.14.0-570.154.1.el9_6, kernel-rt-debug-5.14.0-570.154.1.el9_6 +11
- RHSA-2025:20518 Released 2025-11-11 · Red Hat rating Moderatekernel security update
- Red Hat Enterprise Linux AppStream (v. 9) kernel-64k-debug-devel-5.14.0-611.5.1.el9_7, kernel-64k-debug-devel-matched-5.14.0-611.5.1.el9_7, kernel-64k-devel-5.14.0-611.5.1.el9_7 +12
- Red Hat Enterprise Linux BaseOS (v. 9) kernel-5.14.0-611.5.1.el9_7, kernel-64k-5.14.0-611.5.1.el9_7, kernel-64k-core-5.14.0-611.5.1.el9_7 +28
- Red Hat Enterprise Linux CodeReady Linux Builder (v. 9) kernel-tools-libs-devel-5.14.0-611.5.1.el9_7, libperf-5.14.0-611.5.1.el9_7
- Red Hat Enterprise Linux Real Time (v. 9) kernel-rt-5.14.0-611.5.1.el9_7, kernel-rt-64k-5.14.0-611.5.1.el9_7, kernel-rt-64k-core-5.14.0-611.5.1.el9_7 +21
- Red Hat Enterprise Linux Real Time for NFV (v. 9) kernel-rt-5.14.0-611.5.1.el9_7, kernel-rt-core-5.14.0-611.5.1.el9_7, kernel-rt-debug-5.14.0-611.5.1.el9_7 +9
- RHSA-2025:20095 Released 2025-11-11 · Red Hat rating Moderatekernel security update
- Red Hat Enterprise Linux AppStream (v. 10) kernel-64k-debug-devel-6.12.0-124.8.1.el10_1, kernel-64k-debug-devel-matched-6.12.0-124.8.1.el10_1, kernel-64k-devel-6.12.0-124.8.1.el10_1 +12
- Red Hat Enterprise Linux BaseOS (v. 10) kernel-6.12.0-124.8.1.el10_1, kernel-64k-6.12.0-124.8.1.el10_1, kernel-64k-core-6.12.0-124.8.1.el10_1 +29
- Red Hat Enterprise Linux CodeReady Linux Builder (v. 10) kernel-tools-libs-devel-6.12.0-124.8.1.el10_1, libperf-6.12.0-124.8.1.el10_1
- Red Hat Enterprise Linux Real Time (v. 10) kernel-rt-6.12.0-124.8.1.el10_1, kernel-rt-64k-6.12.0-124.8.1.el10_1, kernel-rt-64k-core-6.12.0-124.8.1.el10_1 +21
- Red Hat Enterprise Linux Real Time for NFV (v. 10) kernel-rt-6.12.0-124.8.1.el10_1, kernel-rt-core-6.12.0-124.8.1.el10_1, kernel-rt-debug-6.12.0-124.8.1.el10_1 +9
Vendor assessments
Red Hat
Red Hat rating: Moderate. Red Hat's page for this CVE →
| Product | Status | Plan or fix |
|---|---|---|
| Red Hat Enterprise Linux 7 | Affected | No fix planned: Out of support scope |
| Red Hat Enterprise Linux 8 | Affected | No fix planned: Out of support scope |
| Red Hat Enterprise Linux 9 | Affected | No fix yet |
| Red Hat CodeReady Linux Builder EUS (v.9.6) | Fixed | RHSA-2026:94438 |
| Red Hat Enterprise Linux AppStream (v. 10) | Fixed | RHSA-2025:20095 |
| Red Hat Enterprise Linux AppStream (v. 9) | Fixed | RHSA-2025:20518 |
| Red Hat Enterprise Linux AppStream EUS (v.9.6) | Fixed | RHSA-2026:94438 |
| Red Hat Enterprise Linux BaseOS (v. 10) | Fixed | RHSA-2025:20095 |
| Red Hat Enterprise Linux BaseOS (v. 9) | Fixed | RHSA-2025:20518 |
| Red Hat Enterprise Linux BaseOS EUS (v.9.6) | Fixed | RHSA-2026:94438 |
| Red Hat Enterprise Linux CodeReady Linux Builder (v. 10) | Fixed | RHSA-2025:20095 |
| Red Hat Enterprise Linux CodeReady Linux Builder (v. 9) | Fixed | RHSA-2025:20518 |
| Red Hat Enterprise Linux Real Time (v. 10) | Fixed | RHSA-2025:20095 |
| Red Hat Enterprise Linux Real Time (v. 9) | Fixed | RHSA-2025:20518 |
| Red Hat Enterprise Linux Real Time EUS (v.9.6) | Fixed | RHSA-2026:94438 |
| Red Hat Enterprise Linux Real Time for NFV (v. 10) | Fixed | RHSA-2025:20095 |
| Red Hat Enterprise Linux Real Time for NFV (v. 9) | Fixed | RHSA-2025:20518 |
| Red Hat Enterprise Linux Real Time for NFV EUS (v.9.6) | Fixed | RHSA-2026:94438 |
Not affected: 1 product
- Red Hat Enterprise Linux 6
Timeline
- 2026-10-02 Threatist record created from CVE-2025-21702
- 2026-10-03 Alias added: GHSA-c82f-pmfx-x3vv
- 2026-11-03 Fix available: RHSA-2026:94438 · Red Hat
- 2026-11-03 Fix available: Red Hat CodeReady Linux Builder EUS (v.9.6), Red Hat Enterprise Linux AppStream EUS (v.9.6), Red Hat Enterprise Linux BaseOS EUS (v.9.6) +2 · Red Hat
References
Patch 8
- https://git.kernel.org/stable/c/78285b53266d6d51fa4ff504a23df03852eba84e
- https://git.kernel.org/stable/c/7a9723ec27aff5674f1fd4934608937f1d650980
- https://git.kernel.org/stable/c/a56a6e8589a9b98d8171611fbcc1e45a15fd2455
- https://git.kernel.org/stable/c/020ecb76812a0526f4130ab5aeb6dc7c773e7ab9
- https://git.kernel.org/stable/c/79a955ea4a2e5ddf4a36328959de0de496419888
- https://git.kernel.org/stable/c/e40cb34b7f247fe2e366fd192700d1b4f38196ca
- https://git.kernel.org/stable/c/b6a079c3b6f95378f26e2aeda520cb3176f7067b
- https://git.kernel.org/stable/c/647cef20e649c576dff271e018d5d15d998b629d
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Sources
- CVE-2025-21702 · CVE Program
- CVE-2025-21702 · NVD
- FIRST EPSS
- GHSA-c82f-pmfx-x3vv · GitHub Advisory Database · CC BY 4.0
- CVE-2025-21702 · Red Hat security data · CC BY 4.0
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