[Devel] [PATCH v5 2/2] nfs: protect callback execution against per-net callback thread shutdown

[Stanislav Kinsburskiy]

From: Stanislav Kinsburskiy <skinsbursky at parallels.com>

Here is the race:

CPU #0				CPU#1

cleanup_mnt			nfs41_callback_svc (get xprt from the list)
nfs_callback_down		...
...				...
svc_close_net			...
...				...
svc_xprt_free			...
svc_bc_sock_free		bc_svc_process
kfree(xprt)			svc_process_common
				rqstp->rq_xprt->xpt_ops (use after free)

The problem is that per-net SUNRPC transports shutdown is done regardless
current callback execution. This is a race leading to transport use-after-free
in callback handler.
This patch fixes it in stright-forward way. I.e. it protects callback
execution with the same mutex used for per-net data creation and destruction.
Hopefully, it won't slow down NFS client significantly.

https://jira.sw.ru/browse/PSBM-75751

v5: destroy all per-net backchannel requests before transports on in
nfs_callback_down_net

v4: use another mutex to protect callback execution agains per-net transports
shutdown.
This guarantees, that transports won't be destroyed by shutdown callback while
execution is in progress and vice versa.

v3: Fix mutex deadlock, when shutdown callback waits for thread to exit (with
mutex taken), while thread wait for the mutex to take.
The idea is to simply check if thread has to exit, if mutex lock has failed.
This is a busy loop, but it shouldn't happend often and for long.

Signed-off-by: Stanislav Kinsburskiy <skinsbursky at virtuozzo.com>
---
 fs/nfs/callback.c |   17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/fs/nfs/callback.c b/fs/nfs/callback.c
index 0beb275..e18d774 100644
--- a/fs/nfs/callback.c
+++ b/fs/nfs/callback.c
@@ -99,6 +99,8 @@ nfs4_callback_up(struct svc_serv *serv)
 }
 
 #if defined(CONFIG_NFS_V4_1)
+static DEFINE_MUTEX(nfs41_callback_mutex);
+
 /*
  * The callback service for NFSv4.1 callbacks
  */
@@ -117,6 +119,12 @@ nfs41_callback_svc(void *vrqstp)
 		if (try_to_freeze())
 			continue;
 
+		mutex_lock(&nfs41_callback_mutex);
+		if (kthread_should_stop()) {
+			mutex_unlock(&nfs41_callback_mutex);
+			return 0;
+		}
+
 		prepare_to_wait(&serv->sv_cb_waitq, &wq, TASK_INTERRUPTIBLE);
 		spin_lock_bh(&serv->sv_cb_lock);
 		if (!list_empty(&serv->sv_cb_list)) {
@@ -129,8 +137,10 @@ nfs41_callback_svc(void *vrqstp)
 			error = bc_svc_process(serv, req, rqstp);
 			dprintk("bc_svc_process() returned w/ error code= %d\n",
 				error);
+			mutex_unlock(&nfs41_callback_mutex);
 		} else {
 			spin_unlock_bh(&serv->sv_cb_lock);
+			mutex_unlock(&nfs41_callback_mutex);
 			schedule();
 			finish_wait(&serv->sv_cb_waitq, &wq);
 		}
@@ -242,6 +252,7 @@ static void nfs_callback_down_net(u32 minorversion, struct svc_serv *serv, struc
 		return;
 
 	dprintk("NFS: destroy per-net callback data; net=%p\n", net);
+	bc_svc_flush_queue_net(serv, net);
 	svc_shutdown_net(serv, net);
 }
 
@@ -377,7 +388,13 @@ void nfs_callback_down(int minorversion, struct net *net)
 	struct nfs_callback_data *cb_info = &nfs_callback_info[minorversion];
 
 	mutex_lock(&nfs_callback_mutex);
+#if defined(CONFIG_NFS_V4_1)
+	mutex_lock(&nfs41_callback_mutex);
+	nfs_callback_down_net(minorversion, cb_info->serv, net);
+	mutex_unlock(&nfs41_callback_mutex);
+#else
 	nfs_callback_down_net(minorversion, cb_info->serv, net);
+#endif
 	cb_info->users--;
 	if (cb_info->users == 0 && cb_info->task != NULL) {
 		kthread_stop(cb_info->task);