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CVE-2025-21857 is a NULL-pointer dereference in the Linux kernel traffic-control classifier extensions API in net/sched. The allocation helper tcf_exts_miss_cookie_base_alloc() invokes xa_alloc_cyclic(), which may return 1 to indicate successful allocation after cyclic allocation wraps. This successful return value was incorrectly handled as an error by tcf_exts_init_ex(), which clears the extensions actions pointer and returns 1. The Flower classifier change path treats that return value as success and proceeds to tcf_exts_validate_ex() and tcf_action_init(), where the NULL actions pointer is dereferenced.
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