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From:   Pavel Skripkin <paskripkin@gmail.com>
To:     wg@grandegger.com, mkl@pengutronix.de, davem@davemloft.net
Cc:     linux-can@vger.kernel.org, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        linux-kernel-mentees@lists.linuxfoundation.org,
        Pavel Skripkin <paskripkin@gmail.com>
Subject: [PATCH] net: can: fix use-after-free in ems_usb_disconnect
Date:   Thu, 17 Jun 2021 21:51:30 +0300
Message-Id: <20210617185130.5834-1-paskripkin@gmail.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

In ems_usb_disconnect() dev pointer, which is
netdev private data, is used after free_candev() call:

	if (dev) {
		unregister_netdev(dev->netdev);
		free_candev(dev->netdev);

		unlink_all_urbs(dev);

		usb_free_urb(dev->intr_urb);

		kfree(dev->intr_in_buffer);
		kfree(dev->tx_msg_buffer);
	}

Fix it by simply moving free_candev() at the end of
the block.

Fail log:
 BUG: KASAN: use-after-free in ems_usb_disconnect
 Read of size 8 at addr ffff88804e041008 by task kworker/1:2/2895

 CPU: 1 PID: 2895 Comm: kworker/1:2 Not tainted 5.13.0-rc5+ #164
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a-rebuilt.opensuse.4
 Workqueue: usb_hub_wq hub_event
 Call Trace:
     dump_stack (lib/dump_stack.c:122)
     print_address_description.constprop.0.cold (mm/kasan/report.c:234)
     kasan_report.cold (mm/kasan/report.c:420 mm/kasan/report.c:436)
     ems_usb_disconnect (drivers/net/can/usb/ems_usb.c:683 drivers/net/can/usb/ems_usb.c:1058)

Fixes: 702171adeed3 ("ems_usb: Added support for EMS CPC-USB/ARM7 CAN/USB interface")
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
---
 drivers/net/can/usb/ems_usb.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/net/can/usb/ems_usb.c b/drivers/net/can/usb/ems_usb.c
index 5af69787d9d5..0a37af4a3fa4 100644
--- a/drivers/net/can/usb/ems_usb.c
+++ b/drivers/net/can/usb/ems_usb.c
@@ -1053,7 +1053,6 @@ static void ems_usb_disconnect(struct usb_interface *intf)
 
 	if (dev) {
 		unregister_netdev(dev->netdev);
-		free_candev(dev->netdev);
 
 		unlink_all_urbs(dev);
 
@@ -1061,6 +1060,8 @@ static void ems_usb_disconnect(struct usb_interface *intf)
 
 		kfree(dev->intr_in_buffer);
 		kfree(dev->tx_msg_buffer);
+
+		free_candev(dev->netdev);
 	}
 }
 
-- 
2.32.0