[CVE-2017-8924][PATCH T/Y] USB: serial: io_ti: fix information leak in completion handler

[Po-Hsu Lin]

From: Johan Hovold <johan at kernel.org>

CVE-2017-8924

Add missing sanity check to the bulk-in completion handler to avoid an
integer underflow that can be triggered by a malicious device.

This avoids leaking 128 kB of memory content from after the URB transfer
buffer to user space.

Fixes: 8c209e6782ca ("USB: make actual_length in struct urb field u32")
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable <stable at vger.kernel.org>	# 2.6.30
Signed-off-by: Johan Hovold <johan at kernel.org>
(cherry picked from commit 654b404f2a222f918af9b0cd18ad469d0c941a8e)

Signed-off-by: Po-Hsu Lin <po-hsu.lin at canonical.com>
---
 drivers/usb/serial/io_ti.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/usb/serial/io_ti.c b/drivers/usb/serial/io_ti.c
index 0385bc4..8c2b58a 100644
--- a/drivers/usb/serial/io_ti.c
+++ b/drivers/usb/serial/io_ti.c
@@ -1653,7 +1653,7 @@ static void edge_bulk_in_callback(struct urb *urb)
 
 	port_number = edge_port->port->port_number;
 
-	if (edge_port->lsr_event) {
+	if (urb->actual_length > 0 && edge_port->lsr_event) {
 		edge_port->lsr_event = 0;
 		dev_dbg(dev, "%s ===== Port %u LSR Status = %02x, Data = %02x ======\n",
 			__func__, port_number, edge_port->lsr_mask, *data);
-- 
1.7.9.5