nfsd: Fix race between FREE_STATEID and LOCK

When running LTP's nfslock01 test, the Linux client can send a LOCK
and a FREE_STATEID request at the same time. The outcome is:

Frame 324    R OPEN stateid [2,O]

Frame 115004 C LOCK lockowner_is_new stateid [2,O] offset 672000 len 64
Frame 115008 R LOCK stateid [1,L]
Frame 115012 C WRITE stateid [0,L] offset 672000 len 64
Frame 115016 R WRITE NFS4_OK
Frame 115019 C LOCKU stateid [1,L] offset 672000 len 64
Frame 115022 R LOCKU NFS4_OK
Frame 115025 C FREE_STATEID stateid [2,L]
Frame 115026 C LOCK lockowner_is_new stateid [2,O] offset 672128 len 64
Frame 115029 R FREE_STATEID NFS4_OK
Frame 115030 R LOCK stateid [3,L]
Frame 115034 C WRITE stateid [0,L] offset 672128 len 64
Frame 115038 R WRITE NFS4ERR_BAD_STATEID

In other words, the server returns stateid L in a successful LOCK
reply, but it has already released it. Subsequent uses of stateid L
fail.

To address this, protect the generation check in nfsd4_free_stateid
with the st_mutex. This should guarantee that only one of two
outcomes occurs: either LOCK returns a fresh valid stateid, or
FREE_STATEID returns NFS4ERR_LOCKS_HELD.

Reported-by: Alexey Kodanev <[email protected]>
Fix-suggested-by: Jeff Layton <[email protected]>
Signed-off-by: Chuck Lever <[email protected]>
Tested-by: Alexey Kodanev <[email protected]>
Cc: [email protected]
Signed-off-by: J. Bruce Fields <[email protected]>

fs/nfsd/nfs4state.c

@@ -4903,14 +4903,39 @@ nfsd4_test_stateid(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
 	return nfs_ok;
 }
 
+static __be32
+nfsd4_free_lock_stateid(stateid_t *stateid, struct nfs4_stid *s)
+{
+	struct nfs4_ol_stateid *stp = openlockstateid(s);
+	__be32 ret;
+
+	mutex_lock(&stp->st_mutex);
+
+	ret = check_stateid_generation(stateid, &s->sc_stateid, 1);
+	if (ret)
+		goto out;
+
+	ret = nfserr_locks_held;
+	if (check_for_locks(stp->st_stid.sc_file,
+			    lockowner(stp->st_stateowner)))
+		goto out;
+
+	release_lock_stateid(stp);
+	ret = nfs_ok;
+
+out:
+	mutex_unlock(&stp->st_mutex);
+	nfs4_put_stid(s);
+	return ret;
+}
+
 __be32
 nfsd4_free_stateid(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
 		   struct nfsd4_free_stateid *free_stateid)
 {
 	stateid_t *stateid = &free_stateid->fr_stateid;
 	struct nfs4_stid *s;
 	struct nfs4_delegation *dp;
-	struct nfs4_ol_stateid *stp;
 	struct nfs4_client *cl = cstate->session->se_client;
 	__be32 ret = nfserr_bad_stateid;
 
@@ -4929,18 +4954,9 @@ nfsd4_free_stateid(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
 		ret = nfserr_locks_held;
 		break;
 	case NFS4_LOCK_STID:
-		ret = check_stateid_generation(stateid, &s->sc_stateid, 1);
-		if (ret)
-			break;
-		stp = openlockstateid(s);
-		ret = nfserr_locks_held;
-		if (check_for_locks(stp->st_stid.sc_file,
-				    lockowner(stp->st_stateowner)))
-			break;
-		WARN_ON(!unhash_lock_stateid(stp));
+		atomic_inc(&s->sc_count);
 		spin_unlock(&cl->cl_lock);
-		nfs4_put_stid(s);
-		ret = nfs_ok;
+		ret = nfsd4_free_lock_stateid(stateid, s);
 		goto out;
 	case NFS4_REVOKED_DELEG_STID:
 		dp = delegstateid(s);