[PATCH v2 2/6] NFSD: Prevent client use-after-free during delegation revoke

A delegation stateid stores only a bare pointer to its owning
nfs4_client; a reference on the stateid does not keep the client
alive.  The client outlives its stateids solely because
__destroy_client() drains every delegation from cl_delegations and
cl_revoked before free_client() runs.

nfs4_laundromat() breaks that invariant.  It unhashes an expired
delegation from cl_delegations under deleg_lock, drops the lock, and
calls revoke_delegation(), which reacquires clp->cl_lock and links the
delegation onto clp->cl_revoked.  Between the unhash and the revoke the
delegation is on neither client-reachable list, so client_has_state()
can report no remaining state.

Every path that can free a client holding delegations first requires
cl_rpc_users to be zero: DESTROY_CLIENTID and a superseding EXCHANGE_ID
gate on mark_client_expired_locked(), and the admin "expire" write
waits in force_expire_client().  The laundromat holds no such
reference.  A client whose recalled delegation has just timed out -- a
rebooted client whose new incarnation supersedes the old one, say --
can therefore run __destroy_client() to completion and free the client
while revoke_delegation() is still about to dereference clp->cl_lock
and clp->cl_revoked, a use-after-free.

Pin the client with cl_rpc_users across the revoke so the teardown
paths block until it completes and then reap it from cl_revoked
themselves.  A client already expiring reaps its own delegations, so
skip it and leave the delegation on del_recall_lru for
__destroy_client() to handle.

Fixes: 3bd64a5ba171 ("nfsd4: implement SEQ4_STATUS_RECALLABLE_STATE_REVOKED")
Signed-off-by: Chuck Lever <[email protected]>
---
 fs/nfsd/netns.h     |  6 ++++--
 fs/nfsd/nfs4state.c | 23 +++++++++++++++++++++++
 2 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/fs/nfsd/netns.h b/fs/nfsd/netns.h
index 03724bef10a7..a7bd7b67fa4f 100644
--- a/fs/nfsd/netns.h
+++ b/fs/nfsd/netns.h
@@ -115,7 +115,8 @@ struct nfsd_net {
 	struct list_head client_lru;
 	struct list_head close_lru;
 
-	/* protects del_recall_lru and delegation hash/unhash */
+	/* protects del_recall_lru and delegation hash/unhash;
+	 * nests outside client_lock */
 	spinlock_t deleg_lock ____cacheline_aligned;
 	struct list_head del_recall_lru;
 
@@ -124,7 +125,8 @@ struct nfsd_net {
 
 	struct delayed_work laundromat_work;
 
-	/* client_lock protects the client lru list and session hash table */
+	/* client_lock protects the client lru list and session hash
+	 * table; nests inside deleg_lock */
 	spinlock_t client_lock;
 
 	/* protects blocked_locks_lru */
diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
index e000ed3e96e9..efeb2a2e9c8f 100644
--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c
@@ -7457,6 +7457,7 @@ nfs4_laundromat(struct nfsd_net *nn)
 		.new_timeo = nn->nfsd4_lease
 	};
 	struct nfs4_cpntf_state *cps;
+	struct nfs4_client *clp;
 	copy_stateid_t *cps_t;
 	int i;
 
@@ -7485,6 +7486,18 @@ nfs4_laundromat(struct nfsd_net *nn)
 		dp = list_entry (pos, struct nfs4_delegation, dl_recall_lru);
 		if (!state_expired(&lt, dp->dl_time))
 			break;
+		clp = dp->dl_stid.sc_client;
+		spin_lock(&nn->client_lock);
+		if (is_client_expired(clp)) {
+			spin_unlock(&nn->client_lock);
+			continue;
+		}
+		/*
+		 * Pin without reviving: get_client_locked() would
+		 * flip a courtesy client back to NFSD4_ACTIVE.
+		 */
+		atomic_inc(&clp->cl_rpc_users);
+		spin_unlock(&nn->client_lock);
 		refcount_inc(&dp->dl_stid.sc_count);
 		unhash_delegation_locked(dp, SC_STATUS_REVOKED);
 		list_add(&dp->dl_recall_lru, &reaplist);
@@ -7493,8 +7506,18 @@ nfs4_laundromat(struct nfsd_net *nn)
 	while (!list_empty(&reaplist)) {
 		dp = list_first_entry(&reaplist, struct nfs4_delegation,
 					dl_recall_lru);
+		clp = dp->dl_stid.sc_client;
 		list_del_init(&dp->dl_recall_lru);
 		revoke_delegation(dp);
+		/*
+		 * Unpin without renewing: put_client_renew() would
+		 * renew the reaped client's lease.
+		 */
+		if (atomic_dec_and_lock(&clp->cl_rpc_users, &nn->client_lock)) {
+			if (is_client_expired(clp))
+				wake_up_all(&expiry_wq);
+			spin_unlock(&nn->client_lock);
+		}
 	}
 
 	spin_lock(&nn->client_lock);

-- 
2.54.0