CIFS: Do not reconnect TCP session in add_credits()

When executing add_credits() we currently call cifs_reconnect()
if the number of credits is zero and there are no requests in
flight. In this case we may call cifs_reconnect() recursively
twice and cause memory corruption given the following sequence
of functions:

mid1.callback() -> add_credits() -> cifs_reconnect() ->
-> mid2.callback() -> add_credits() -> cifs_reconnect().

Fix this by avoiding to call cifs_reconnect() in add_credits()
and checking for zero credits in the demultiplex thread.

Cc: <stable@vger.kernel.org>
Signed-off-by: Pavel Shilovsky <pshilov@microsoft.com>
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/connect.c

@@ -720,6 +720,21 @@ server_unresponsive(struct TCP_Server_Info *server)
 	return false;
 }
 
+static inline bool
+zero_credits(struct TCP_Server_Info *server)
+{
+	int val;
+
+	spin_lock(&server->req_lock);
+	val = server->credits + server->echo_credits + server->oplock_credits;
+	if (server->in_flight == 0 && val == 0) {
+		spin_unlock(&server->req_lock);
+		return true;
+	}
+	spin_unlock(&server->req_lock);
+	return false;
+}
+
 static int
 cifs_readv_from_socket(struct TCP_Server_Info *server, struct msghdr *smb_msg)
 {
@@ -732,6 +747,12 @@ cifs_readv_from_socket(struct TCP_Server_Info *server, struct msghdr *smb_msg)
 	for (total_read = 0; msg_data_left(smb_msg); total_read += length) {
 		try_to_freeze();
 
+		/* reconnect if no credits and no requests in flight */
+		if (zero_credits(server)) {
+			cifs_reconnect(server);
+			return -ECONNABORTED;
+		}
+
 		if (server_unresponsive(server))
 			return -ECONNABORTED;
 		if (cifs_rdma_enabled(server) && server->smbd_conn)

fs/cifs/smb2ops.c

@@ -34,6 +34,7 @@
 #include "cifs_ioctl.h"
 #include "smbdirect.h"
 
+/* Change credits for different ops and return the total number of credits */
 static int
 change_conf(struct TCP_Server_Info *server)
 {
@@ -41,17 +42,15 @@ change_conf(struct TCP_Server_Info *server)
 	server->oplock_credits = server->echo_credits = 0;
 	switch (server->credits) {
 	case 0:
-		return -1;
+		return 0;
 	case 1:
 		server->echoes = false;
 		server->oplocks = false;
-		cifs_dbg(VFS, "disabling echoes and oplocks\n");
 		break;
 	case 2:
 		server->echoes = true;
 		server->oplocks = false;
 		server->echo_credits = 1;
-		cifs_dbg(FYI, "disabling oplocks\n");
 		break;
 	default:
 		server->echoes = true;
@@ -64,14 +63,15 @@ change_conf(struct TCP_Server_Info *server)
 		server->echo_credits = 1;
 	}
 	server->credits -= server->echo_credits + server->oplock_credits;
-	return 0;
+	return server->credits + server->echo_credits + server->oplock_credits;
 }
 
 static void
 smb2_add_credits(struct TCP_Server_Info *server, const unsigned int add,
 		 const int optype)
 {
-	int *val, rc = 0;
+	int *val, rc = -1;
+
 	spin_lock(&server->req_lock);
 	val = server->ops->get_credits_field(server, optype);
 
@@ -101,8 +101,26 @@ smb2_add_credits(struct TCP_Server_Info *server, const unsigned int add,
 	}
 	spin_unlock(&server->req_lock);
 	wake_up(&server->request_q);
-	if (rc)
-		cifs_reconnect(server);
+
+	if (server->tcpStatus == CifsNeedReconnect)
+		return;
+
+	switch (rc) {
+	case -1:
+		/* change_conf hasn't been executed */
+		break;
+	case 0:
+		cifs_dbg(VFS, "Possible client or server bug - zero credits\n");
+		break;
+	case 1:
+		cifs_dbg(VFS, "disabling echoes and oplocks\n");
+		break;
+	case 2:
+		cifs_dbg(FYI, "disabling oplocks\n");
+		break;
+	default:
+		cifs_dbg(FYI, "add %u credits total=%d\n", add, rc);
+	}
 }
 
 static void