Bug 2457834 (CVE-2026-31422)

Summary: CVE-2026-31422 kernel: net/sched: cls_flow: fix NULL pointer dereference on shared blocks
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
Status: NEW --- QA Contact:
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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 networking scheduler (net/sched) component, specifically within the flow classifier (cls_flow). A local user could exploit this vulnerability by creating a flow filter without a fully qualified baseclass on a shared block. This action causes a null pointer dereference, which can lead to a system crash, resulting in a Denial of Service (DoS).
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Description OSIDB Bzimport 2026-04-13 14:02:08 UTC 
In the Linux kernel, the following vulnerability has been resolved:

net/sched: cls_flow: fix NULL pointer dereference on shared blocks

flow_change() calls tcf_block_q() and dereferences q->handle to derive
a default baseclass.  Shared blocks leave block->q NULL, causing a NULL
deref when a flow filter without a fully qualified baseclass is created
on a shared block.

Check tcf_block_shared() before accessing block->q and return -EINVAL
for shared blocks.  This avoids the null-deref shown below:

=======================================================================
KASAN: null-ptr-deref in range [0x0000000000000038-0x000000000000003f]
RIP: 0010:flow_change (net/sched/cls_flow.c:508)
Call Trace:
 tc_new_tfilter (net/sched/cls_api.c:2432)
 rtnetlink_rcv_msg (net/core/rtnetlink.c:6980)
 [...]
=======================================================================
Comment 1 Keith Grant 2026-04-13 17:03:34 UTC 
Upstream advisory:
https://lore.kernel.org/linux-cve-announce/2026041356-CVE-2026-31422-04eb@gregkh/T