CVE-2026-89659

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.