CVE-2026-89663
HighCVSS 8.8Exploitation Probability (EPSS)
Low risk34th percentile - higher than 34% of all known CVEs
Summary
A use-after-free vulnerability was found in the Linux kernel nfsd subsystem related to copy-notify stateids. The _free_cpntf_state_locked() function only unlinked an entry once its refcount reached zero, so revoke paths (parent-stid drain, OFFLOAD_CANCEL, and laundromat expiry) could drop a reference without unlinking, leaving the entry discoverable to a concurrent reader. The fix adds revoke_cpntf_state_locked(), which unhashes the entry from the IDR and sc_cp_list first and defers the final free to any holder.
Risk Assessment
The vulnerability can cause a KASAN-detectable use-after-free in the kernel, potentially leading to a kernel panic and denial of service on NFS servers. In the worst case, a kernel memory corruption bug could be exploited for privilege escalation.
Recommendation
Update the Linux kernel to a version containing the fix (the commit introducing revoke_cpntf_state_locked()) and reboot the system. If updating is not possible, restrict NFS service access to trusted clients.
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Original NVD description (English source)
In the Linux kernel, the following vulnerability has been resolved: nfsd: revoke copy-notify stateids before dropping their reference Copy-notify stateids live in the s2s_cp_stateids IDR and on their parent stid's sc_cp_list, pinned by a single membership reference. _free_cpntf_state_locked() only unlinks an entry once its refcount reaches zero, so any revoke path that runs while a concurrent find_cpntf_state()/manage_cpntf_state() holder has elevated cs_count drops the reference without unlinking, leaving the entry discoverable with its membership reference already consumed. A second revoke or a laundromat tick then frees it while the reader still holds the pointer -- a KASAN-detectable use-after-free at the reader's nfs4_put_cpntf_state(). This affected all three revoke paths: - The parent-stid drain (nfs4_free_cpntf_statelist()) repeatedly called _free_cpntf_state_locked() on the first list entry; a holder that had bumped cs_count made it return early, so the next iteration re-decremented and burned the holder's reference. - OFFLOAD_CANCEL (manage_cpntf_state()) and laundromat expiry likewise used _free_cpntf_state_locked() and could drop 2->1 without unlinking. Add revoke_cpntf_state_locked(), which unhashes the entry from the IDR and sc_cp_list first (deferring the final free to any holder), and use it from all three revoke paths. The drain now walks with list_for_each_entry_safe() and revokes each entry unconditionally, so it terminates in one pass per entry regardless of cs_count. The unhash is gated on !list_empty(&cps->cp_list); the idr_remove() gate matters because idr_alloc_cyclic() may have recycled the so_id by then. Keep _free_cpntf_state_locked() for the reference-holder put path only, where a concurrent revoke may already have unlinked the entry (its list_del_init() then a no-op).
Vulnerability data from NVD (NIST) · CISA KEV · EPSS