Re: [PATCH] net: usb: fix possible use-after-free in smsc75xx_bind

On Tue, Jun 15, 2021 at 12:00 AM Pavel Skripkin [off-list ref] wrote:

On Mon, 14 Jun 2021 23:37:12 +0800
Dongliang Mu [off-list ref] wrote:

quoted

The commit 46a8b29c6306 ("net: usb: fix memory leak in smsc75xx_bind")
fails to clean up the work scheduled in smsc75xx_reset->
smsc75xx_set_multicast, which leads to use-after-free if the work is
scheduled to start after the deallocation. In addition, this patch
also removes one dangling pointer - dev->data[0].

This patch calls cancel_work_sync to cancel the schedule work and set
the dangling pointer to NULL.

Fixes: 46a8b29c6306 ("net: usb: fix memory leak in smsc75xx_bind")
Signed-off-by: Dongliang Mu <redacted>
---
 drivers/net/usb/smsc75xx.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/net/usb/smsc75xx.c b/drivers/net/usb/smsc75xx.c
index b286993da67c..f81740fcc8d5 100644
--- a/drivers/net/usb/smsc75xx.c
+++ b/drivers/net/usb/smsc75xx.c

@@ -1504,7 +1504,10 @@ static int smsc75xx_bind(struct usbnet *dev,

struct usb_interface *intf) return 0;

 err:
+     cancel_work_sync(&pdata->set_multicast);
      kfree(pdata);
+     pdata = NULL;
+     dev->data[0] = 0;
      return ret;
 }

Hi, Dongliang!

Just my thougth about this patch:

INIT_WORK(&pdata->set_multicast, smsc75xx_deferred_multicast_write);
does not queue anything, it just initalizes list structure and assigns
callback function. The actual work sheduling happens in
smsc75xx_set_multicast() which is smsc75xx_netdev_ops member.

Yes, you are right. However, as written in the commit message,
smsc75xx_set_multicast will be called by smsc75xx_reset [1].

If smsc75xx_set_multicast is called before any check failure occurs,
this work(set_multicast) will be queued into the global list with

schedule_work(&pdata->set_multicast); [2]

At last, if the pdata or dev->data[0] is freed before the
set_multicast really executes, it may lead to a UAF. Is this correct?

BTW, even if the above is true, I don't know if I call the API
``cancel_work_sync(&pdata->set_multicast)'' properly if the
schedule_work is not called.

[1] https://elixir.bootlin.com/linux/latest/source/drivers/net/usb/smsc75xx.c#L1322

[2] https://elixir.bootlin.com/linux/latest/source/drivers/net/usb/smsc75xx.c#L583

In case of any error in smsc75xx_bind() the device registration fails
and smsc75xx_netdev_ops won't be registered, so, i guess, there is no
chance of UAF.


Am I missing something? :)



With regards,
Pavel Skripkin