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

[Dongliang Mu]

On Tue, Jun 15, 2021 at 9:31 PM Pavel Skripkin <paskripkin@xxxxxxxxx> wrote:
>
> On Tue, 15 Jun 2021 07:01:13 +0800
> Dongliang Mu <mudongliangabcd@xxxxxxxxx> wrote:
>
> > On Tue, Jun 15, 2021 at 12:00 AM Pavel Skripkin
> > <paskripkin@xxxxxxxxx> wrote:
> > >
> > > On Mon, 14 Jun 2021 23:37:12 +0800
> > > Dongliang Mu <mudongliangabcd@xxxxxxxxx> wrote:
> > >
> > > > 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 <mudongliangabcd@xxxxxxxxx>
> > > > ---
> > > >  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]
>
> Ah, I missed it, sorry :)
>
> Maybe, small optimization for error handling path like:
>
> cancel_work:
>         cancel_work_sync(&pdata->set_multicast);
>         dev->data[0] = 0;
> free_pdata:
>         kfree(pdata);
>         return ret;
>
>
> is suitbale here.

I agree with this style of error handling. However, I need to adjust
the location of dev->data[0] = 0 after kfree(pdata) because if there
still leaves a dangling pointer it directly goes to free_pdata.

>
> >
> > 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.
> >
>
> Yeah, it will be ok.

Thanks for the confirmation. I've tested it under the previous kernel
crash. It works fine.

I will send a v2 patch quickly.

>
> > [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
>
>
>
>
> With regards,
> Pavel Skripkin