net: usb: dm9601: fix uninitialized variable use in dm9601_mdio_read

syzbot has found an uninit-value bug triggered by the dm9601 driver [1].

This error happens because the variable res is not updated if the call
to dm_read_shared_word returns an error. In this particular case -EPROTO
was returned and res stayed uninitialized.

This can be avoided by checking the return value of dm_read_shared_word
and propagating the error if the read operation failed.

[1] https://syzkaller.appspot.com/bug?extid=1f53a30781af65d2c955

Cc: stable@vger.kernel.org
Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Reported-and-tested-by: syzbot+1f53a30781af65d2c955@syzkaller.appspotmail.com
Acked-by: Peter Korsgaard <peter@korsgaard.com>
Fixes: d0374f4f9c35cdfbee0 ("USB: Davicom DM9601 usbnet driver")
Link: https://lore.kernel.org/r/20231009-topic-dm9601_uninit_mdio_read-v2-1-f2fe39739b6c@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/drivers/net/usb/dm9601.c b/drivers/net/usb/dm9601.c
index 48d7d27..99ec1d4 100644 (file)
--- a/drivers/net/usb/dm9601.c
+++ b/drivers/net/usb/dm9601.c
@@ -222,13 +222,18 @@ static int dm9601_mdio_read(struct net_device *netdev, int phy_id, int loc)
        struct usbnet *dev = netdev_priv(netdev);
 
        __le16 res;
+       int err;
 
        if (phy_id) {
                netdev_dbg(dev->net, "Only internal phy supported\n");
                return 0;
        }
 
-       dm_read_shared_word(dev, 1, loc, &res);
+       err = dm_read_shared_word(dev, 1, loc, &res);
+       if (err < 0) {
+               netdev_err(dev->net, "MDIO read error: %d\n", err);
+               return err;
+       }
 
        netdev_dbg(dev->net,
                   "dm9601_mdio_read() phy_id=0x%02x, loc=0x%02x, returns=0x%04x\n",