[PATCH v2] usbip: usbip_host: fix null pointer dereference in

[Jeffin Philip <jeffinphilip14@gmail.com>]

rebind_store calls do_rebind which dereferences udev without
checking if it is NULL. If busid is registered using match_busid
but the device is never bound to the driver or it is never present
in the first place, it triggers a null pointer dereference when we
attempt to rebind the device. Fix this by checking explicitly for
udev first and returning -ENODEV if udev is NULL. Add usb_get_dev
in addition to the null check to get a reference to udev to prevent
udev from becoming NULL after the check. Drop the reference after
using it in do_rebind().

Reported-by: syzbot+af76b01c9a0f0ab60fb0@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=af76b01c9a0f0ab60fb0
Fixes: 4bfb141bc013 ("usbip: usbip_host: fix to hold parent lock for device_attach() calls")
Cc: stable@vger.kernel.org
Signed-off-by: Jeffin Philip <jeffinphilip14@gmail.com>
---
Changes in v2:
 - Addressed concerns raised by the maintainer in v1 discussion
   - Added usb_get_dev() to get a reference to udev preventing
     it from becoming null after the null check. Drop the reference
     after using it in do_rebind()
---
 drivers/usb/usbip/stub_main.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/usb/usbip/stub_main.c b/drivers/usb/usbip/stub_main.c
index 79110a69d697..c911626427dc 100644
--- a/drivers/usb/usbip/stub_main.c
+++ b/drivers/usb/usbip/stub_main.c
@@ -256,12 +256,21 @@ static ssize_t rebind_store(struct device_driver *dev, const char *buf,
 	if (!bid)
 		return -ENODEV;
 
+	if (!bid->udev) {
+		put_busid_priv(bid);
+		return -ENODEV;
+	}
+
+	/* get a reference to udev to prevent it from becoming NULL */
+	usb_get_dev(bid->udev);
 	/* mark the device for deletion so probe ignores it during rescan */
 	bid->status = STUB_BUSID_OTHER;
 	/* release the busid lock */
 	put_busid_priv(bid);
 
 	ret = do_rebind((char *) buf, bid);
+	/* we finished using udev and don't need the reference anymore, drop it */
+	usb_put_dev(bid->udev);
 	if (ret < 0)
 		return ret;
 
-- 
2.55.0