net: ch9200: fix uninitialised access during mii_nway_restart

jira VULN-71594
cve CVE-2025-38086
commit-author Qasim Ijaz <[email protected]>
commit 9ad0452

In mii_nway_restart() the code attempts to call
mii->mdio_read which is ch9200_mdio_read(). ch9200_mdio_read()
utilises a local buffer called "buff", which is initialised
with control_read(). 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.

	Reported-by: syzbot <[email protected]>
Closes: https://syzkaller.appspot.com/bug?extid=3361c2d6f78a3e0892f9
	Tested-by: syzbot <[email protected]>
Fixes: 4a476bd ("usbnet: New driver for QinHeng CH9200 devices")
	Cc: [email protected]
	Signed-off-by: Qasim Ijaz <[email protected]>
Link: https://patch.msgid.link/[email protected]
	Signed-off-by: Jakub Kicinski <[email protected]>
(cherry picked from commit 9ad0452)
	Signed-off-by: Brett Mastbergen <[email protected]>

Author: bmastbergen

drivers/net/usb/ch9200.c

@@ -178,15 +178,18 @@ 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);
 
 	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 ret;
 
 	return (buff[0] | buff[1] << 8);
 }