Re: Possible null pointer dereference in adutux.ko

[Oliver Neukum]

Am Freitag, den 18.08.2017, 18:04 +0300 schrieb 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.

Yes, you are right.

> 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;

It seems to me that disconnect, open and release must take both
mutexes.

> 2) move poisoning of dev->udev after usb_deregister_dev in order to wait 
> for all other callbacks to finish.

That would defeat the purpose of poisoning.

	Regards
		Oliver