[PATCH 1/3] SUNRPC: fix races on PipeFS UMOUNT notifications

[Stanislav Kinsbursky <skinsbursky@parallels.com>]

Below are races, when RPC client can be created without PiepFS dentries

CPU#0					CPU#1
-----------------------------		-----------------------------
rpc_new_client				rpc_fill_super
rpc_setup_pipedir
mutex_lock(&sn->pipefs_sb_lock)
rpc_get_sb_net == NULL
(no per-net PipeFS superblock)
					sn->pipefs_sb = sb;
					notifier_call_chain(MOUNT)
					(client is not in the list)
rpc_register_client
(client without pipes dentries)

To fix this patch:
1) makes PipeFS mount notification call with pipefs_sb_lock being held.
2) releases pipefs_sb_lock on new SUNRPC client creation only after
registration.

Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Cc: stable@vger.kernel.org
---
 net/sunrpc/clnt.c     |    7 ++++++-
 net/sunrpc/rpc_pipe.c |    3 +++
 2 files changed, 9 insertions(+), 1 deletions(-)

diff --git a/net/sunrpc/clnt.c b/net/sunrpc/clnt.c
index 5a750b9..cf5b226 100644
--- a/net/sunrpc/clnt.c
+++ b/net/sunrpc/clnt.c
@@ -170,7 +170,10 @@ rpc_setup_pipedir(struct rpc_clnt *clnt, const char *dir_name)
 	if (!pipefs_sb)
 		return 0;
 	dentry = rpc_setup_pipedir_sb(pipefs_sb, clnt, dir_name);
-	rpc_put_sb_net(net);
+	/*
+	 * PipeFS superblock will be put after client registration to prevent
+	 * races with PipeFS mount call.
+	 */
 	if (IS_ERR(dentry))
 		return PTR_ERR(dentry);
 	clnt->cl_dentry = dentry;
@@ -369,11 +372,13 @@ static struct rpc_clnt * rpc_new_client(const struct rpc_create_args *args, stru
 	/* save the nodename */
 	rpc_clnt_set_nodename(clnt, utsname()->nodename);
 	rpc_register_client(clnt);
+	rpc_put_sb_net(xprt->xprt_net);
 	return clnt;
 
 out_no_auth:
 	rpc_clnt_remove_pipedir(clnt);
 out_no_path:
+	rpc_put_sb_net(xprt->xprt_net);
 	kfree(clnt->cl_principal);
 out_no_principal:
 	rpc_free_iostats(clnt->cl_metrics);
diff --git a/net/sunrpc/rpc_pipe.c b/net/sunrpc/rpc_pipe.c
index e7ce4b3..c512448 100644
--- a/net/sunrpc/rpc_pipe.c
+++ b/net/sunrpc/rpc_pipe.c
@@ -1126,6 +1126,7 @@ rpc_fill_super(struct super_block *sb, void *data, int silent)
 		return -ENOMEM;
 	dprintk("RPC:       sending pipefs MOUNT notification for net %p%s\n",
 		net, NET_NAME(net));
+	mutex_lock(&sn->pipefs_sb_lock);
 	sn->pipefs_sb = sb;
 	err = blocking_notifier_call_chain(&rpc_pipefs_notifier_list,
 					   RPC_PIPEFS_MOUNT,
@@ -1133,6 +1134,7 @@ rpc_fill_super(struct super_block *sb, void *data, int silent)
 	if (err)
 		goto err_depopulate;
 	sb->s_fs_info = get_net(net);
+	mutex_unlock(&sn->pipefs_sb_lock);
 	return 0;
 
 err_depopulate:
@@ -1141,6 +1143,7 @@ err_depopulate:
 					   sb);
 	sn->pipefs_sb = NULL;
 	__rpc_depopulate(root, files, RPCAUTH_lockd, RPCAUTH_RootEOF);
+	mutex_unlock(&sn->pipefs_sb_lock);
 	return err;
 }