CVE-2026-89659
CriticalCVSS 9.8Exploitation Probability (EPSS)
Low risk6th percentile - higher than 6% of all known CVEs
Summary
In the Linux kernel, the NFSD module has a use-after-free vulnerability during delegation revocation. The nfs4_laundromat() function unhashes an expired delegation from cl_delegations, drops deleg_lock, and then revoke_delegation() attempts to use a client that may have already been freed. The lack of client pinning via cl_rpc_users enables a race with client teardown.
Risk Assessment
It is possible to cause a kernel panic or potentially execute code in kernel context by triggering the race during delegation revocation. This affects stability and security of an NFS server.
Recommendation
Update the Linux kernel to a version with the fix that pins the client via cl_rpc_users during delegation revocation. Alternatively, restrict the ability of untrusted clients to trigger delegation operations.
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Original NVD description (English source)
In the Linux kernel, the following vulnerability has been resolved: NFSD: Prevent client use-after-free during delegation revoke A delegation stateid holds only a bare pointer to its owning nfs4_client and does not keep it alive. The client survives its stateids only because __destroy_client() drains cl_delegations and cl_revoked before free_client() runs. nfs4_laundromat() breaks that invariant: it unhashes an expired delegation from cl_delegations, drops deleg_lock, then revoke_delegation() relinks it onto cl_revoked under cl_lock. In that window the delegation is on neither list, so client_has_state() can report no remaining state. Every teardown path first requires cl_rpc_users to be zero, but the laundromat holds no such reference. A client whose recalled delegation has just timed out can therefore reach free_client() while revoke_delegation() is still about to dereference cl_lock, a use-after-free. Pin the client with cl_rpc_users across the revoke so teardown blocks until it completes, then reap the delegation from cl_revoked. A client already expiring reaps its own, so skip it and leave the delegation on del_recall_lru.