CVE-2026-31421

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)