Bug 2457824 (CVE-2026-31421)

Summary: CVE-2026-31421 kernel: net/sched: cls_fw: fix NULL pointer dereference on shared blocks
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
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Priority: medium    
Version: unspecifiedKeywords: Security
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OS: Linux   
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A flaw was found in the Linux kernel's `cls_fw` network scheduler component. This vulnerability, a null pointer dereference, occurs when an empty `cls_fw` filter is attached to a shared block and a specially crafted network packet with a specific mark is processed. An attacker with network access could potentially exploit this to cause a system crash, leading to a Denial of Service (DoS).
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Description OSIDB Bzimport 2026-04-13 14:01:29 UTC 
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)
Comment 1 Keith Grant 2026-04-13 17:02:18 UTC 
Upstream advisory:
https://lore.kernel.org/linux-cve-announce/2026041355-CVE-2026-31421-78d7@gregkh/T