[PATCH] staging: comedi: check validity of wMaxPacketSize of usb endpoints found

[Anant Thazhemadam]

While finding usb endpoints in vmk80xx_find_usb_endpoints(), check if 
wMaxPacketSize = 0 for the endpoints found.

Some devices have isochronous endpoints that have wMaxPacketSize = 0
(as required by the USB-2 spec).
However, since this doesn't apply here, wMaxPacketSize = 0 can be
considered to be invalid.

Reported-by: syzbot+009f546aa1370056b1c2@xxxxxxxxxxxxxxxxxxxxxxxxx
Tested-by: syzbot+009f546aa1370056b1c2@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Anant Thazhemadam <anant.thazhemadam@xxxxxxxxx>
---
The error (as detected by syzbot) is generated in 
vmk80xx_write_packet() (which is called in vmk80xx_reset_device()) when
it tries to assign devpriv->usb_tx_buf[0] = cmd.

This NULL pointer dereference issue arises because
size = usb_endpoint_maxp(devpriv->ep_tx) = 0.

This can be traced back to vmk80xx_find_usb_endpoints(), where the usb 
endpoints are found, and assigned accordingly.
(For some more insight, in vmk80xx_find_usb_endpoints(), 
if one of intf->cur_altsetting->iface_desc->endpoints' desc value = 0, 
and consequently this endpoint is assigned to devpriv->ep_tx,
this issue gets triggered.)

Checking if the wMaxPacketSize of an endpoint is invalid and returning
an error value accordingly, seems to fix the error.

We could also alternatively perform this checking (if the size is 0 or not) 
in vmk80xx_reset_device() itself, but it only seemed like covering up the issue
at that place, rather than fixing it, so I wasn't sure that was any better.

However, if I'm not wrong, this might end up causing the probe to fail, and I'm 
not sure if that's the right thing to do in cases like this, and if it isn't I'd
like some input on what exactly is the required course of action in cases like this.

 drivers/staging/comedi/drivers/vmk80xx.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/staging/comedi/drivers/vmk80xx.c b/drivers/staging/comedi/drivers/vmk80xx.c
index 65dc6c51037e..cb0a965d3c37 100644
--- a/drivers/staging/comedi/drivers/vmk80xx.c
+++ b/drivers/staging/comedi/drivers/vmk80xx.c
@@ -667,6 +667,9 @@ static int vmk80xx_find_usb_endpoints(struct comedi_device *dev)
 	if (!devpriv->ep_rx || !devpriv->ep_tx)
 		return -ENODEV;
 
+	if(!usb_endpoint_maxp(devpriv->ep_rx) || !usb_endpoint_maxp(devpriv->ep_tx))
+		return -EINVAL;
+
 	return 0;
 }
 
-- 
2.25.1