CVE-2026-31421net/sched: cls_fw: fix NULL pointer dereference on shared blocks

Official description

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)

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/d6d5bd62a09650856e1e2010eb09853eba0d64e1
• https://git.kernel.org/stable/c/febf64ca79a2d6540ab6e5e197fa0f4f7e84473e
• https://git.kernel.org/stable/c/3d41f9a314afa94b1c7c7c75405920123220e8cd
• https://git.kernel.org/stable/c/18328eff2f97d1a6adcdb6d4a0f42f2f83a31e28
• https://git.kernel.org/stable/c/5cf41031922c154aa5ccda8bcdb0f5e6226582ec
• https://git.kernel.org/stable/c/3cb055df9e8625ce699a259d8178d67b37f2b160
• https://git.kernel.org/stable/c/96426c348def662b06bfdc65be3002905604927a
• https://git.kernel.org/stable/c/faeea8bbf6e958bf3c00cb08263109661975987c
• https://cert-portal.siemens.com/productcert/html/ssa-082556.html
• https://cert-portal.siemens.com/productcert/html/ssa-019113.html
• https://cert-portal.siemens.com/productcert/html/ssa-457936.html