[PATCH v2] usb: Fix null-ptr-deref in qt2_process_read_urb()

This patch addresses a null-ptr-deref in qt2_process_read_urb() due to
an incorrect bounds check in the following:

""
       if (newport > serial->num_ports) {
               dev_err(&port->dev,
                       "%s - port change to invalid port: %i\n",
                       __func__, newport);
               break;
       }
""

The condition doesn't account for the valid range of the serial->port
buffer, which is from 0 to serial->num_ports - 1. When newport is equal
to serial->num_ports, the assignment of "port" in the
following code is out-of-bounds and NULL:

""
       serial_priv->current_port = newport;
       port = serial->port[serial_priv->current_port];

""

The fix checks if newport is greater than or equal to serial->num_ports
indicating it is out-of-bounds.

Reported-by: syzbot <redacted>
Closes: https://syzkaller.appspot.com/bug?extid=506479ebf12fe435d01a
Signed-off-by: Qasim Ijaz <redacted>
---
 drivers/usb/serial/quatech2.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/usb/serial/quatech2.c b/drivers/usb/serial/quatech2.c
index a317bdbd00ad..72fe83a6c978 100644
--- a/drivers/usb/serial/quatech2.c
+++ b/drivers/usb/serial/quatech2.c

@@ -503,7 +503,7 @@ static void qt2_process_read_urb(struct urb *urb)

                                newport = *(ch + 3);

-                               if (newport > serial->num_ports) {
+                               if (newport >= serial->num_ports) {
                                        dev_err(&port->dev,
                                                "%s - port change to invalid port: %i\n",
                                                __func__, newport);

-- 
2.39.5

On Sun, Jan 12, 2025 at 09:55:58AM +0000, Qasim Ijaz wrote:

This patch addresses a null-ptr-deref in qt2_process_read_urb() due to
an incorrect bounds check in the following:

""
       if (newport > serial->num_ports) {
               dev_err(&port->dev,
                       "%s - port change to invalid port: %i\n",
                       __func__, newport);
               break;
       }
""

The condition doesn't account for the valid range of the serial->port
buffer, which is from 0 to serial->num_ports - 1. When newport is equal
to serial->num_ports, the assignment of "port" in the
following code is out-of-bounds and NULL:

""
       serial_priv->current_port = newport;
       port = serial->port[serial_priv->current_port];

""

The fix checks if newport is greater than or equal to serial->num_ports
indicating it is out-of-bounds.

Reported-by: syzbot <redacted>
Closes: https://syzkaller.appspot.com/bug?extid=506479ebf12fe435d01a

Oh wow, syzbot is now starting to fuzz random USB devices?  And this is
the only thing it found for this device?  That's not all that bad, I
expected much worse.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

On Sun, Jan 12, 2025 at 09:55:58AM +0000, Qasim Ijaz wrote:

This patch addresses a null-ptr-deref in qt2_process_read_urb() due to
an incorrect bounds check in the following:

Thanks for the fix. This looks correct, but your patch is whitespace
damaged (tabs replaced with spaces) and does not apply.

Can you please fix your mail setup and resend a v3? (Make sure to send
it to yourself first.)

When doing so, please use the common patch prefix for this driver:

	USB: serial: quatech2:

""

You can drop these (markup?) quotes.

       if (newport > serial->num_ports) {
               dev_err(&port->dev,
                       "%s - port change to invalid port: %i\n",
                       __func__, newport);
               break;
       }
""

The condition doesn't account for the valid range of the serial->port
buffer, which is from 0 to serial->num_ports - 1. When newport is equal
to serial->num_ports, the assignment of "port" in the
following code is out-of-bounds and NULL:

""
       serial_priv->current_port = newport;
       port = serial->port[serial_priv->current_port];

""

The fix checks if newport is greater than or equal to serial->num_ports
indicating it is out-of-bounds.

Reported-by: syzbot <redacted>
Closes: https://syzkaller.appspot.com/bug?extid=506479ebf12fe435d01a
Signed-off-by: Qasim Ijaz <redacted>

And please include a Fixes and CC stable tag:

	Fixes: f7a33e608d9a ("USB: serial: add quatech2 usb to serial driver")
	Cc: stable@vger.kernel.org	# 3.5

Johan