[PATCH] net: usb: dm9601: fix uninitialized variable use in dm9601_mdio_read

[Javier Carrasco <javier.carrasco.cruz@gmail.com>]

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 or if no data is read (see
__usbnet_read_cmd()). 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 returning an error if the read operation failed or no data was read.

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

Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Reported-and-tested-by: syzbot+1f53a30781af65d2c955@syzkaller.appspotmail.com
---
 drivers/net/usb/dm9601.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/net/usb/dm9601.c b/drivers/net/usb/dm9601.c
index 48d7d278631e..e223daa93229 100644
--- a/drivers/net/usb/dm9601.c
+++ b/drivers/net/usb/dm9601.c
@@ -222,13 +222,20 @@ 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) {
+		if (err == 0)
+			err = -ENODATA;
+		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",

---
base-commit: 94f6f0550c625fab1f373bb86a6669b45e9748b3
change-id: 20231009-topic-dm9601_uninit_mdio_read-a15918ffbd6f

Best regards,
-- 
Javier Carrasco <javier.carrasco.cruz@gmail.com>