CVE-2026-31421
MediumCVSS 5.5Exploitation Probability (EPSS)
Low risk2th percentile - higher than 2% of all known CVEs
Summary
In the Linux kernel, a NULL pointer dereference was found in the cls_fw classifier when using filters on shared blocks. The issue occurs when an empty cls_fw filter is attached to a shared block and a packet with a nonzero skb mark is classified, causing a system crash.
Risk Assessment
An attacker can trigger a kernel panic by sending a crafted packet, leading to a denial of service (DoS). The vulnerability affects systems using shared blocks in the Traffic Control (TC) subsystem.
Recommendation
Immediately update the Linux kernel to a version containing the fix (commit patching cls_fw). If an update is not possible, temporarily avoid using empty cls_fw filters on shared blocks.
Original NVD description (English source)
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)