[PATCH 1/5] net: ch9200: fix uninitialised access bug during mii_nway_restart

[Qasim Ijaz <qasdev00@gmail.com>]

In mii_nway_restart() during the line:

        bmcr = mii->mdio_read(mii->dev, mii->phy_id, MII_BMCR);

The code attempts to call mii->mdio_read which is ch9200_mdio_read().

ch9200_mdio_read() utilises a local buffer, which is initialised
with control_read():

        unsigned char buff[2];

However buff is conditionally initialised inside control_read():

        if (err == size) {
                memcpy(data, buf, size);
        }

If the condition of "err == size" is not met, then buff remains
uninitialised. Once this happens the uninitialised buff is accessed
and returned during ch9200_mdio_read():

        return (buff[0] | buff[1] << 8);

The problem stems from the fact that ch9200_mdio_read() ignores the
return value of control_read(), leading to uinit-access of buff.

To fix this we should check the return value of control_read()
and return early on error. We return 0 on control_read() failure here 
because returning a negative may trigger the "bmcr & BMCR_ANENABLE" 
check within mii_nway_restart.

Reported-by: syzbot <syzbot+3361c2d6f78a3e0892f9@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=3361c2d6f78a3e0892f9
Tested-by: syzbot <syzbot+3361c2d6f78a3e0892f9@syzkaller.appspotmail.com>
Fixes: 4a476bd6d1d9 ("usbnet: New driver for QinHeng CH9200 devices")
Cc: stable@vger.kernel.org
Signed-off-by: Qasim Ijaz <qasdev00@gmail.com>
---
 drivers/net/usb/ch9200.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/net/usb/ch9200.c b/drivers/net/usb/ch9200.c
index f69d9b902da0..4f29ecf2240a 100644
--- a/drivers/net/usb/ch9200.c
+++ b/drivers/net/usb/ch9200.c
@@ -178,6 +178,7 @@ static int ch9200_mdio_read(struct net_device *netdev, int phy_id, int loc)
 {
 	struct usbnet *dev = netdev_priv(netdev);
 	unsigned char buff[2];
+	int ret;
 
 	netdev_dbg(netdev, "%s phy_id:%02x loc:%02x\n",
 		   __func__, phy_id, loc);
@@ -185,8 +186,10 @@ static int ch9200_mdio_read(struct net_device *netdev, int phy_id, int loc)
 	if (phy_id != 0)
 		return -ENODEV;
 
-	control_read(dev, REQUEST_READ, 0, loc * 2, buff, 0x02,
-		     CONTROL_TIMEOUT_MS);
+	ret = control_read(dev, REQUEST_READ, 0, loc * 2, buff, 0x02,
+			   CONTROL_TIMEOUT_MS);
+	if (ret < 0)
+		return 0;
 
 	return (buff[0] | buff[1] << 8);
 }
-- 
2.39.5