NFSD: grab nfsd_mutex in nfsd_nl_rpc_status_get_dumpit()

Grab nfsd_mutex lock in nfsd_nl_rpc_status_get_dumpit routine and remove
nfsd_nl_rpc_status_get_start() and nfsd_nl_rpc_status_get_done(). This
patch fix the syzbot log reported below:

INFO: task syz-executor.1:17770 blocked for more than 143 seconds.
      Not tainted 6.10.0-rc3-syzkaller-00022-gcea2a26553ac #0
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
task:syz-executor.1  state:D stack:23800 pid:17770 tgid:17767 ppid:11381  flags:0x00000006
Call Trace:
 <TASK>
 context_switch kernel/sched/core.c:5408 [inline]
 __schedule+0x17e8/0x4a20 kernel/sched/core.c:6745
 __schedule_loop kernel/sched/core.c:6822 [inline]
 schedule+0x14b/0x320 kernel/sched/core.c:6837
 schedule_preempt_disabled+0x13/0x30 kernel/sched/core.c:6894
 __mutex_lock_common kernel/locking/mutex.c:684 [inline]
 __mutex_lock+0x6a4/0xd70 kernel/locking/mutex.c:752
 nfsd_nl_listener_get_doit+0x115/0x5d0 fs/nfsd/nfsctl.c:2124
 genl_family_rcv_msg_doit net/netlink/genetlink.c:1115 [inline]
 genl_family_rcv_msg net/netlink/genetlink.c:1195 [inline]
 genl_rcv_msg+0xb16/0xec0 net/netlink/genetlink.c:1210
 netlink_rcv_skb+0x1e5/0x430 net/netlink/af_netlink.c:2564
 genl_rcv+0x28/0x40 net/netlink/genetlink.c:1219
 netlink_unicast_kernel net/netlink/af_netlink.c:1335 [inline]
 netlink_unicast+0x7ec/0x980 net/netlink/af_netlink.c:1361
 netlink_sendmsg+0x8db/0xcb0 net/netlink/af_netlink.c:1905
 sock_sendmsg_nosec net/socket.c:730 [inline]
 __sock_sendmsg+0x223/0x270 net/socket.c:745
 ____sys_sendmsg+0x525/0x7d0 net/socket.c:2585
 ___sys_sendmsg net/socket.c:2639 [inline]
 __sys_sendmsg+0x2b0/0x3a0 net/socket.c:2668
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f24ed27cea9
RSP: 002b:00007f24ee0080c8 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 00007f24ed3b3f80 RCX: 00007f24ed27cea9
RDX: 0000000000000000 RSI: 0000000020000100 RDI: 0000000000000005
RBP: 00007f24ed2ebff4 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000

Fixes: 1bd773b ("nfsd: hold nfsd_mutex across entire netlink operation")
Fixes: bd9d6a3 ("NFSD: add rpc_status netlink support")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

Author: LorenzoBianconi

Documentation/netlink/specs/nfsd.yaml

@@ -123,8 +123,6 @@ operations:
       doc: dump pending nfsd rpc
       attribute-set: rpc-status
       dump:
-        pre: nfsd-nl-rpc-status-get-start
-        post: nfsd-nl-rpc-status-get-done
         reply:
           attributes:
             - xid

fs/nfsd/netlink.c

@@ -44,9 +44,7 @@ static const struct nla_policy nfsd_listener_set_nl_policy[NFSD_A_SERVER_SOCK_AD
 static const struct genl_split_ops nfsd_nl_ops[] = {
 	{
 		.cmd	= NFSD_CMD_RPC_STATUS_GET,
-		.start	= nfsd_nl_rpc_status_get_start,
 		.dumpit	= nfsd_nl_rpc_status_get_dumpit,
-		.done	= nfsd_nl_rpc_status_get_done,
 		.flags	= GENL_CMD_CAP_DUMP,
 	},
 	{

fs/nfsd/netlink.h

@@ -15,9 +15,6 @@
 extern const struct nla_policy nfsd_sock_nl_policy[NFSD_A_SOCK_TRANSPORT_NAME + 1];
 extern const struct nla_policy nfsd_version_nl_policy[NFSD_A_VERSION_ENABLED + 1];
 
-int nfsd_nl_rpc_status_get_start(struct netlink_callback *cb);
-int nfsd_nl_rpc_status_get_done(struct netlink_callback *cb);
-
 int nfsd_nl_rpc_status_get_dumpit(struct sk_buff *skb,
 				  struct netlink_callback *cb);
 int nfsd_nl_threads_set_doit(struct sk_buff *skb, struct genl_info *info);

fs/nfsd/nfsctl.c

@@ -1460,28 +1460,6 @@ static int create_proc_exports_entry(void)
 
 unsigned int nfsd_net_id;
 
-/**
- * nfsd_nl_rpc_status_get_start - Prepare rpc_status_get dumpit
- * @cb: netlink metadata and command arguments
- *
- * Return values:
- *   %0: The rpc_status_get command may proceed
- *   %-ENODEV: There is no NFSD running in this namespace
- */
-int nfsd_nl_rpc_status_get_start(struct netlink_callback *cb)
-{
-	struct nfsd_net *nn = net_generic(sock_net(cb->skb->sk), nfsd_net_id);
-	int ret = -ENODEV;
-
-	mutex_lock(&nfsd_mutex);
-	if (nn->nfsd_serv)
-		ret = 0;
-	else
-		mutex_unlock(&nfsd_mutex);
-
-	return ret;
-}
-
 static int nfsd_genl_rpc_status_compose_msg(struct sk_buff *skb,
 					    struct netlink_callback *cb,
 					    struct nfsd_genl_rqstp *rqstp)
@@ -1558,8 +1536,16 @@ static int nfsd_genl_rpc_status_compose_msg(struct sk_buff *skb,
 int nfsd_nl_rpc_status_get_dumpit(struct sk_buff *skb,
 				  struct netlink_callback *cb)
 {
-	struct nfsd_net *nn = net_generic(sock_net(skb->sk), nfsd_net_id);
 	int i, ret, rqstp_index = 0;
+	struct nfsd_net *nn;
+
+	mutex_lock(&nfsd_mutex);
+
+	nn = net_generic(sock_net(skb->sk), nfsd_net_id);
+	if (!nn->nfsd_serv) {
+		ret = -ENODEV;
+		goto out_unlock;
+	}
 
 	rcu_read_lock();
 
@@ -1636,22 +1622,10 @@ int nfsd_nl_rpc_status_get_dumpit(struct sk_buff *skb,
 	ret = skb->len;
 out:
 	rcu_read_unlock();
-
-	return ret;
-}
-
-/**
- * nfsd_nl_rpc_status_get_done - rpc_status_get dumpit post-processing
- * @cb: netlink metadata and command arguments
- *
- * Return values:
- *   %0: Success
- */
-int nfsd_nl_rpc_status_get_done(struct netlink_callback *cb)
-{
+out_unlock:
 	mutex_unlock(&nfsd_mutex);
 
-	return 0;
+	return ret;
 }
 
 /**