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.
