In the Linux kernel, the following vulnerability has been resolved:

NFSD: Prevent client use-after-free during admin state revocation

A stateid holds only a bare pointer to its nfs4_client; a stateid
reference does not pin it.  The client survives only because
__destroy_client() drains its stateids before free_client() runs.

nfsd4_revoke_states() drops nn->client_lock across revoke_one_stid(),
which dereferences the client to revoke a stateid and read
clp->cl_minorversion.  A teardown racing the dropped lock can free
the client first.

Pinning cl_rpc_users under client_lock blocks the DESTROY_CLIENTID and
EXCHANGE_ID teardown, which refuses while cl_rpc_users is non-zero.
force_expire_client() ignores it: once its wait for cl_rpc_users to
reach zero has passed, a later pin goes unnoticed.

Under client_lock, skip a client whose cl_time is already zero --
force_expire_client() clears it there before waiting -- otherwise pin
cl_rpc_users before dropping the lock.  The walk then either sees the
expiry and skips, or pins in time for that wait to cover the revoke.