[PATCH] USB: serial: option: add interface-number sanity check to flag handling

[Johan Hovold <johan@kernel.org>]

Add an interface-number sanity check before testing the device flags to
avoid relying on undefined behaviour when left shifting in case a device
uses an interface number greater than or equal to BITS_PER_LONG (i.e. 64
or 32).

Reported-by: syzbot+8881b478dad0a7971f79@syzkaller.appspotmail.com
Fixes: c3a65808f04a ("USB: serial: option: reimplement interface masking")
Cc: stable@vger.kernel.org
Signed-off-by: Johan Hovold <johan@kernel.org>
---
 drivers/usb/serial/option.c | 23 +++++++++++++++++++++--
 1 file changed, 21 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/serial/option.c b/drivers/usb/serial/option.c
index 2a3bfd6f867e..c5908c4f2046 100644
--- a/drivers/usb/serial/option.c
+++ b/drivers/usb/serial/option.c
@@ -561,6 +561,9 @@ static void option_instat_callback(struct urb *urb);
 
 /* Device flags */
 
+/* Highest interface number which can be used with NCTRL() and RSVD() */
+#define FLAG_IFNUM_MAX	7
+
 /* Interface does not support modem-control requests */
 #define NCTRL(ifnum)	((BIT(ifnum) & 0xff) << 8)
 
@@ -2089,6 +2092,14 @@ static struct usb_serial_driver * const serial_drivers[] = {
 
 module_usb_serial_driver(serial_drivers, option_ids);
 
+static bool iface_is_reserved(unsigned long device_flags, u8 ifnum)
+{
+	if (ifnum > FLAG_IFNUM_MAX)
+		return false;
+
+	return device_flags & RSVD(ifnum);
+}
+
 static int option_probe(struct usb_serial *serial,
 			const struct usb_device_id *id)
 {
@@ -2105,7 +2116,7 @@ static int option_probe(struct usb_serial *serial,
 	 * the same class/subclass/protocol as the serial interfaces.  Look at
 	 * the Windows driver .INF files for reserved interface numbers.
 	 */
-	if (device_flags & RSVD(iface_desc->bInterfaceNumber))
+	if (iface_is_reserved(device_flags, iface_desc->bInterfaceNumber))
 		return -ENODEV;
 
 	/*
@@ -2121,6 +2132,14 @@ static int option_probe(struct usb_serial *serial,
 	return 0;
 }
 
+static bool iface_no_modem_control(unsigned long device_flags, u8 ifnum)
+{
+	if (ifnum > FLAG_IFNUM_MAX)
+		return false;
+
+	return device_flags & NCTRL(ifnum);
+}
+
 static int option_attach(struct usb_serial *serial)
 {
 	struct usb_interface_descriptor *iface_desc;
@@ -2136,7 +2155,7 @@ static int option_attach(struct usb_serial *serial)
 
 	iface_desc = &serial->interface->cur_altsetting->desc;
 
-	if (!(device_flags & NCTRL(iface_desc->bInterfaceNumber)))
+	if (!iface_no_modem_control(device_flags, iface_desc->bInterfaceNumber))
 		data->use_send_setup = 1;
 
 	if (device_flags & ZLP)
-- 
2.26.2