NFSv4: Fix races in the legacy idmapper upcall

nfs_idmap_instantiate() will cause the process that is waiting in
request_key_with_auxdata() to wake up and exit. If there is a second
process waiting for the idmap->idmap_mutex, then it may wake up and
start a new call to request_key_with_auxdata(). If the call to
idmap_pipe_downcall() from the first process has not yet finished
calling nfs_idmap_complete_pipe_upcall_locked(), then we may end up
triggering the WARN_ON_ONCE() in nfs_idmap_prepare_pipe_upcall().

The fix is to ensure that we clear idmap->idmap_upcall_data before
calling nfs_idmap_instantiate().

Fixes: e9ab41b620e4 ("NFSv4: Clean up the legacy idmapper upcall")
Signed-off-by: Trond Myklebust <[e-mail]>

fs/nfs/nfs4idmap.c

···
 	return true;
 }
 
-static void
-nfs_idmap_complete_pipe_upcall_locked(struct idmap *idmap, int ret)
+static void nfs_idmap_complete_pipe_upcall(struct idmap_legacy_upcalldata *data,
+					   int ret)
 {
-	struct key *authkey = idmap->idmap_upcall_data->authkey;
-
-	kfree(idmap->idmap_upcall_data);
-	idmap->idmap_upcall_data = NULL;
-	complete_request_key(authkey, ret);
-	key_put(authkey);
+	complete_request_key(data->authkey, ret);
+	key_put(data->authkey);
+	kfree(data);
 }
 
-static void
-nfs_idmap_abort_pipe_upcall(struct idmap *idmap, int ret)
+static void nfs_idmap_abort_pipe_upcall(struct idmap *idmap,
+					struct idmap_legacy_upcalldata *data,
+					int ret)
 {
-	if (idmap->idmap_upcall_data != NULL)
-		nfs_idmap_complete_pipe_upcall_locked(idmap, ret);
+	if (cmpxchg(&idmap->idmap_upcall_data, data, NULL) == data)
+		nfs_idmap_complete_pipe_upcall(data, ret);
 }
 
 static int nfs_idmap_legacy_upcall(struct key *authkey, void *aux)
···
 
 	ret = rpc_queue_upcall(idmap->idmap_pipe, msg);
 	if (ret < 0)
-		nfs_idmap_abort_pipe_upcall(idmap, ret);
+		nfs_idmap_abort_pipe_upcall(idmap, data, ret);
 
 	return ret;
 out2:
···
 	struct request_key_auth *rka;
 	struct rpc_inode *rpci = RPC_I(file_inode(filp));
 	struct idmap *idmap = (struct idmap *)rpci->private;
+	struct idmap_legacy_upcalldata *data;
 	struct key *authkey;
 	struct idmap_msg im;
 	size_t namelen_in;
···
 	 * will have been woken up and someone else may now have used
 	 * idmap_key_cons - so after this point we may no longer touch it.
 	 */
-	if (idmap->idmap_upcall_data == NULL)
+	data = xchg(&idmap->idmap_upcall_data, NULL);
+	if (data == NULL)
 		goto out_noupcall;
 
-	authkey = idmap->idmap_upcall_data->authkey;
+	authkey = data->authkey;
 	rka = get_request_key_auth(authkey);
 
 	if (mlen != sizeof(im)) {
···
 	if (namelen_in == 0 || namelen_in == IDMAP_NAMESZ) {
 		ret = -EINVAL;
 		goto out;
-}
+	}
 
-	ret = nfs_idmap_read_and_verify_message(&im,
-			&idmap->idmap_upcall_data->idmap_msg,
-			rka->target_key, authkey);
+	ret = nfs_idmap_read_and_verify_message(&im, &data->idmap_msg,
+						rka->target_key, authkey);
 	if (ret >= 0) {
 		key_set_timeout(rka->target_key, nfs_idmap_cache_timeout);
 		ret = mlen;
 	}
 
 out:
-	nfs_idmap_complete_pipe_upcall_locked(idmap, ret);
+	nfs_idmap_complete_pipe_upcall(data, ret);
 out_noupcall:
 	return ret;
 }
···
 	struct idmap *idmap = data->idmap;
 
 	if (msg->errno)
-		nfs_idmap_abort_pipe_upcall(idmap, msg->errno);
+		nfs_idmap_abort_pipe_upcall(idmap, data, msg->errno);
 }
 
 static void
···
 {
 	struct rpc_inode *rpci = RPC_I(inode);
 	struct idmap *idmap = (struct idmap *)rpci->private;
+	struct idmap_legacy_upcalldata *data;
 
-	nfs_idmap_abort_pipe_upcall(idmap, -EPIPE);
+	data = xchg(&idmap->idmap_upcall_data, NULL);
+	if (data)
+		nfs_idmap_complete_pipe_upcall(data, -EPIPE);
 }
 
 int nfs_map_name_to_uid(const struct nfs_server *server, const char *name, size_t namelen, kuid_t *uid)