Linux Kernel NULL pointer deref in cls_fw module (cve-2026-31421)
CVE-2026-31421 Published on April 13, 2026
net/sched: cls_fw: fix NULL pointer dereference on shared blocks
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)
Products Associated with CVE-2026-31421
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Affected Versions
Linux:- Version 1abf272022cf1d18469405f47b4ec49c6a3125db and below d6d5bd62a09650856e1e2010eb09853eba0d64e1 is affected.
- Version 1abf272022cf1d18469405f47b4ec49c6a3125db and below febf64ca79a2d6540ab6e5e197fa0f4f7e84473e is affected.
- Version 1abf272022cf1d18469405f47b4ec49c6a3125db and below 3d41f9a314afa94b1c7c7c75405920123220e8cd is affected.
- Version 1abf272022cf1d18469405f47b4ec49c6a3125db and below 18328eff2f97d1a6adcdb6d4a0f42f2f83a31e28 is affected.
- Version 1abf272022cf1d18469405f47b4ec49c6a3125db and below 5cf41031922c154aa5ccda8bcdb0f5e6226582ec is affected.
- Version 1abf272022cf1d18469405f47b4ec49c6a3125db and below 3cb055df9e8625ce699a259d8178d67b37f2b160 is affected.
- Version 1abf272022cf1d18469405f47b4ec49c6a3125db and below 96426c348def662b06bfdc65be3002905604927a is affected.
- Version 1abf272022cf1d18469405f47b4ec49c6a3125db and below faeea8bbf6e958bf3c00cb08263109661975987c is affected.
- Version 4.15 is affected.
- Before 4.15 is unaffected.
- Version 5.10.253, <= 5.10.* is unaffected.
- Version 5.15.203, <= 5.15.* is unaffected.
- Version 6.1.168, <= 6.1.* is unaffected.
- Version 6.6.134, <= 6.6.* is unaffected.
- Version 6.12.81, <= 6.12.* is unaffected.
- Version 6.18.22, <= 6.18.* is unaffected.
- Version 6.19.12, <= 6.19.* is unaffected.
- Version 7.0, <= * is unaffected.
Exploit Probability
EPSS (Exploit Prediction Scoring System) scores estimate the probability that a vulnerability will be exploited in the wild within the next 30 days. The percentile shows you how this score compares to all other vulnerabilities.