CVE-2025-21857: net/sched: cls_api: fix error handling causing NULL dereference

Severity: medium (CVSS 5.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2025-03-12 · updated 2026-06-17
Weakness: CWE-476

Description

In the Linux kernel, the following vulnerability has been resolved: net/sched: cls_api: fix error handling causing NULL dereference tcf_exts_miss_cookie_base_alloc() calls xa_alloc_cyclic() which can return 1 if the allocation succeeded after wrapping. This was treated as an error, with value 1 returned to caller tcf_exts_init_ex() which sets exts->actions to NULL and returns 1 to caller fl_change(). fl_change() treats err == 1 as success, calling tcf_exts_validate_ex() which calls tcf_action_init() with exts->actions as argument, where it is dereferenced. Example trace: BUG: kernel NULL pointer dereference, address: 0000000000000000 CPU: 114 PID: 16151 Comm: handler114 Kdump: loaded Not tainted 5.14.0-503.16.1.el9_5.x86_64 #1 RIP: 0010:tcf_action_init+0x1f8/0x2c0 Call Trace: tcf_action_init+0x1f8/0x2c0 tcf_exts_validate_ex+0x175/0x190 fl_change+0x537/0x1120 [cls_flower]

Affected products

Linux kernel: from 6.3 before 6.6.80; from 6.7 before 6.12.17; from 6.13 before 6.13.5; 6.14-rc1, 6.14-rc2, 6.14-rc3 — fixed in 6.12.17, 6.13.5, 6.6.80

How to fix

Upgrade to a fixed version: Linux kernel 6.12.17, 6.13.5, 6.6.80. Check the vendor advisory for mitigations.

References

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