[PATCH v4] usb: gadget: u_serial: Add null pointer check in gserial_resume

[Prashanth K <quic_prashk@quicinc.com>]

Consider a case where gserial_disconnect has already cleared
gser->ioport. And if a wakeup interrupt triggers afterwards,
gserial_resume gets called, which will lead to accessing of
gser->ioport and thus causing null pointer dereference.Add
a null pointer check to prevent this.

Added a static spinlock to prevent gser->ioport from becoming
null after the newly added check.

Fixes: aba3a8d01d62 ("usb: gadget: u_serial: add suspend resume callbacks")
Signed-off-by: Prashanth K <quic_prashk@quicinc.com>
---
v4: Added short comment about serial_port_lock.

 drivers/usb/gadget/function/u_serial.c | 27 +++++++++++++++++++++++----
 1 file changed, 23 insertions(+), 4 deletions(-)

diff --git a/drivers/usb/gadget/function/u_serial.c b/drivers/usb/gadget/function/u_serial.c
index 840626e..3a6adf2 100644
--- a/drivers/usb/gadget/function/u_serial.c
+++ b/drivers/usb/gadget/function/u_serial.c
@@ -82,6 +82,13 @@
 #define WRITE_BUF_SIZE		8192		/* TX only */
 #define GS_CONSOLE_BUF_SIZE	8192
 
+/*
+ * Since all the spinlocks used in this driver is part of struct gs_port (port_lock).
+ * Added a new static spinlock to prevent race conditions where gs_port is dereferenced
+ * after marking as null.
+ */
+static DEFINE_SPINLOCK(serial_port_lock);
+
 /* console info */
 struct gs_console {
 	struct console		console;
@@ -1375,8 +1382,10 @@ void gserial_disconnect(struct gserial *gser)
 	if (!port)
 		return;
 
+	spin_lock_irqsave(&serial_port_lock, flags);
+
 	/* tell the TTY glue not to do I/O here any more */
-	spin_lock_irqsave(&port->port_lock, flags);
+	spin_lock(&port->port_lock);
 
 	gs_console_disconnect(port);
 
@@ -1391,7 +1400,8 @@ void gserial_disconnect(struct gserial *gser)
 			tty_hangup(port->port.tty);
 	}
 	port->suspended = false;
-	spin_unlock_irqrestore(&port->port_lock, flags);
+	spin_unlock(&port->port_lock);
+	spin_unlock_irqrestore(&serial_port_lock, flags);
 
 	/* disable endpoints, aborting down any active I/O */
 	usb_ep_disable(gser->out);
@@ -1425,10 +1435,19 @@ EXPORT_SYMBOL_GPL(gserial_suspend);
 
 void gserial_resume(struct gserial *gser)
 {
-	struct gs_port *port = gser->ioport;
+	struct gs_port *port;
 	unsigned long	flags;
 
-	spin_lock_irqsave(&port->port_lock, flags);
+	spin_lock_irqsave(&serial_port_lock, flags);
+	port = gser->ioport;
+
+	if (!port) {
+		spin_unlock_irqrestore(&serial_port_lock, flags);
+		return;
+	}
+
+	spin_lock(&port->port_lock);
+	spin_unlock(&serial_port_lock);
 	port->suspended = false;
 	if (!port->start_delayed) {
 		spin_unlock_irqrestore(&port->port_lock, flags);
-- 
2.7.4