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

Affected: Linux

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]

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