CVE-2026-31421
Analyzed Analyzed - Analysis Complete
Null Pointer Dereference in Linux Kernel net/sched cls_fw Module

Publication date: 2026-04-13

Last updated on: 2026-05-20

Assigner: kernel.org

Description
In the Linux kernel, the following vulnerability has been resolved: net/sched: cls_fw: fix NULL pointer dereference on shared blocks The old-method path in fw_classify() calls tcf_block_q() and dereferences q->handle. Shared blocks leave block->q NULL, causing a NULL deref when an empty cls_fw filter is attached to a shared block and a packet with a nonzero major skb mark is classified. Reject the configuration in fw_change() when the old method (no TCA_OPTIONS) is used on a shared block, since fw_classify()'s old-method path needs block->q which is NULL for shared blocks. The fixed null-ptr-deref calling stack: KASAN: null-ptr-deref in range [0x0000000000000038-0x000000000000003f] RIP: 0010:fw_classify (net/sched/cls_fw.c:81) Call Trace: tcf_classify (./include/net/tc_wrapper.h:197 net/sched/cls_api.c:1764 net/sched/cls_api.c:1860) tc_run (net/core/dev.c:4401) __dev_queue_xmit (net/core/dev.c:4535 net/core/dev.c:4790)
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Meta Information
Published
2026-04-13
Last Modified
2026-05-20
Generated
2026-06-16
AI Q&A
2026-04-13
EPSS Evaluated
2026-06-15
NVD
CVE-2026-31421
EUVD
EUVD-2026-21945
Affected Vendors & Products
Showing 13 associated CPEs
Vendor Product Version / Range
linux linux_kernel 7.0
linux linux_kernel 7.0
linux linux_kernel 7.0
linux linux_kernel 7.0
linux linux_kernel 7.0
linux linux_kernel 7.0
linux linux_kernel From 5.11 (inc) to 5.15.203 (exc)
linux linux_kernel From 6.13 (inc) to 6.18.22 (exc)
linux linux_kernel From 5.16 (inc) to 6.1.168 (exc)
linux linux_kernel From 6.19 (inc) to 6.19.12 (exc)
linux linux_kernel From 6.7 (inc) to 6.12.81 (exc)
linux linux_kernel From 6.2 (inc) to 6.6.134 (exc)
linux linux_kernel From 4.15 (inc) to 5.10.253 (exc)
Helpful Resources
Exploitability
CWE
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KEV
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CWE ID Description
CWE-476 The product dereferences a pointer that it expects to be valid but is NULL.
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Executive Summary

This vulnerability exists in the Linux kernel's network scheduler, specifically in the cls_fw classifier. The issue is a NULL pointer dereference that occurs when using the old method in the fw_classify() function on shared blocks. Shared blocks have a NULL pointer for block->q, but the old method dereferences q->handle, leading to a NULL pointer dereference when an empty cls_fw filter is attached to a shared block and a packet with a nonzero major skb mark is classified.

The fix involves rejecting configurations that use the old method without TCA_OPTIONS on shared blocks, preventing the NULL pointer dereference by ensuring the code does not attempt to access block->q when it is NULL.

Impact Analysis

This vulnerability can cause a NULL pointer dereference in the Linux kernel's network scheduler, which may lead to a kernel crash or denial of service. Such a crash can disrupt network traffic processing and potentially affect system stability or availability.

Detection Guidance

This vulnerability involves a NULL pointer dereference in the Linux kernel's cls_fw classifier when using shared blocks with the old method in fw_classify(). Detection would involve checking for the presence of empty cls_fw filters attached to shared blocks and monitoring for kernel crashes or KASAN reports indicating null pointer dereferences in fw_classify.

Since the vulnerability is triggered by specific configurations in the traffic control (tc) subsystem, you can inspect your traffic control filters using commands like:

  • tc filter show dev <interface>
  • Look specifically for cls_fw filters attached to shared blocks without TCA_OPTIONS.

Additionally, monitoring kernel logs for messages related to KASAN null pointer dereferences in fw_classify can help detect exploitation attempts.

Mitigation Strategies

The vulnerability is fixed by rejecting configurations that use the old method (no TCA_OPTIONS) on shared blocks in fw_change(). Therefore, immediate mitigation steps include:

  • Avoid using empty cls_fw filters attached to shared blocks with the old method.
  • Update your Linux kernel to a version that includes the fix for this vulnerability.
  • Review and modify any traffic control configurations that may use the old method on shared blocks to ensure they comply with the fixed behavior.
Compliance Impact

The provided information about CVE-2026-31421 does not include any details regarding its impact on compliance with common standards and regulations such as GDPR or HIPAA.

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