Re: Possible null pointer dereference in adutux.ko

[Anton Volkov]

On 15.08.2017 18:58, Oliver Neukum wrote:
> Am Dienstag, den 15.08.2017, 16:38 +0300 schrieb Anton Volkov:
> > On 15.08.2017 16:20, Oliver Neukum wrote:
> > > Am Dienstag, den 15.08.2017, 15:59 +0300 schrieb Anton Volkov:
> > > > Hello.
> > > >
> > > > While searching for races in the Linux kernel I've come across
> > > > "drivers/usb/misc/adutux.ko" module. Here is a question that I came up
> > > > with while analyzing results. Lines are given using the info from Linux
> > > > v4.12.
> > > >
> > > > Consider the following case:
> > > >
> > > > Thread 1:                   Thread 2:
> > > > adu_release
> > > > ->adu_release_internal      adu_disconnect
> > > >        <READ &dev->udev->dev>    dev->udev = NULL
> > > >        (adutux.c: line 298)      (adutux.c: line 771)
> > > >                                  usb_deregister_dev
> > > >
> > > > Comments in the source code point at the possibility of adu_release()
> > > > being called separately from adu_disconnect(). adu_release() and
> > > > adu_disconnect() acquire different mutexes, so they are not protected
> > > > from one another. If adu_disconnect() changes dev->udev before its value
> > > > is read in adu_release_internal() there will be a NULL pointer
> > > > dereference on a read attempt. Is this case feasible from your point of
> > > > view?
> > > >
> > > > Thank you for your time.
> > >
> > > Hi,
> > >
> > > your analysis seems correct to me. In fact it looks like
> > >
> > > 66d4bc30d128e7c7ac4cf64aa78cb76e971cec5b
> > > USB: adutux: remove custom debug macro
> > >
> > > more or less broke disconnect on this driver
> > > (the URBs can also finish after dev->udev = NULL)
> > >
> > > Do you want to do a fix or do you want me to do it?
> > >
> > > 	Regards
> > > 		Oliver
> >
> > Hello, Oliver.
> >
> > I am not sure about the best way to solve this problem. If you have any
> > ideas about it then it would probably be better if you could handle the
> > fix. Or if you share the ideas I can prepare a patch.
>
> Hi,
>
> given the age of the drivers I would suggest to simply remove the debugging statements
>
> 	Regards
> 		Oliver

Hello, Oliver.

Looks like deletion of lots of debug print won't solve the race problem 
because there are other places that could potentially try to dereference 
dev->udev when disconnect has already poisoned it. For example in 
adu_open there are calls to usb_fill_int_urb with dev->udev as a 
parameter to be dereferenced inside the function.

There are other possible solutions, if I understand correctly:
1) although it is described that adutux_mutex should be used to protect 
only open_count, it usually protects the whole body of a function, so we 
could probably place it before the locking of dev->mtx;
2) move poisoning of dev->udev after usb_deregister_dev in order to wait 
for all other callbacks to finish.

What do you think?

Regards,
Anton

-- Anton Volkov
Linux Verification Center, ISPRAS
web: http://linuxtesting.org
e-mail: avolkov@xxxxxxxxx