Re: [PATCH v4 3/5] tee: generic TEE subsystem

[Dmitry Torokhov <dmitry.torokhov@gmail.com>]

On Wed, Jul 08, 2015 at 03:33:25PM -0700, Greg Kroah-Hartman wrote:
> On Wed, Jul 08, 2015 at 04:26:49PM -0600, Jason Gunthorpe wrote:
> > On Wed, Jul 08, 2015 at 02:11:29PM -0700, Greg Kroah-Hartman wrote:
> > > > > +       cdev_init(&teedev->cdev, &tee_fops);
> > > > > +       teedev->cdev.owner = teedesc->owner;
> > > > 
> > > > This also needs to set teedev->cdev.kobj.parent.
> > > > I'm guessing:
> > > > 
> > > >  teedev->cdev.kobj.parent = &teedev->dev.kobj;
> > > > 
> > > > TPM had the same mistake..
> > > 
> > > Really?  As of a few years ago, A cdev's kobject should not be touched
> > > by anything other than the cdev core.  It's not a "real" kobject in that
> > > it is never registered in sysfs, and no one sees it.  I keep meaning to
> > 
> > Well, when I looked at it, it looked like it was necessary to maintain
> > the refcount on the memory that is holding cdev.
> > 
> > The basic issue is that cdev_del doesn't seem to be synchronizing.
> > 
> > The use after free race is then something like:
> > 
> >    struct tpm_chip {
> >  	struct device dev;
> > 	struct cdev cdev;
> 
> Oops, right there's your problem.  You can't have two reference counted
> objects trying to manage the memory of a single structure.  No matter
> what you do, it's going to be a pain to deal with this, so don't :)
> 
> > 
> >        CPU0                            CPU1
> > =================             ======================
> > tpm_chip = kalloc
> > cdev_add(&tpm_chip->cdev)
> > device_add(&tpm_chip->dev)
> >                                 chrdev_open
> > 		                 filp->f_op->open
> > cdev_del(&tpm_chip->cdev)
> > device_unregister
> >    (&tpm_chip->dev)
> >  kfree(tpm_chip)
> > 		                  tpm_chip = container_of
> > 				 fput
> > 				  cdev_put(.. cdev)
> > 
> > Ie we need cdev to hold a ref on tpm_chip->dev until cdev_put is
> > called.
> 
> No, separate them, make the cdev a pointer and all should be fine.
> 
> > > just use something else one of these days for that structure, as lots of
> > > people get it wrong.  Or has things changed there?
> > 
> > Not recently, but this is the commit:
> > 
> > commit 2f0157f13f42800aa3d9017ebb0fb80a65f7b2de
> > Author: Dmitry Torokhov <dmitry.torokhov@gmail.com>
> > Date:   Sun Oct 21 17:57:19 2012 -0700
> > 
> >     char_dev: pin parent kobject
> >     
> >     In certain cases (for example when a cdev structure is embedded into
> >     another object whose lifetime is controlled by a separate kobject) it is
> >     beneficial to tie lifetime of another object to the lifetime of
> >     character device so that related object is not freed until after
> >     char_dev object is freed.
> >     
> >     To achieve this let's pin kobject's parent when doing cdev_add() and
> >     unpin when last reference to cdev structure is being released.
> >     
> >     Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
> >     Acked-by: Al Viro <viro@zeniv.linux.org.uk>
> >     Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
> > 
> > It doesn't seem the be the best situation, this is the 3rd time this
> > week I've noticed cdev with a kalloc'd struct being used improperly.
> > 
> > Perhaps cdev_init should accept the module and kref parent as an
> > argument?
> 
> Oh yeah, that commit :(
> 
> If you know _exactly_ what you are doing, you can get away with this,
> but I strongly recommend not doing that.  As proof of that, in some new
> code I'm working on, I did not do this, just because I'm not smart
> enough to ensure it's all working properly...

I know you like to allocate everything separately and access it via
pointers (ala device_create) but cdevs explicitly allow embedding them
into other structures (cdev_init vs cdev_alloc). I do not think there is
anything wrong with this, as well as there is nothing wrong in embedding
a struct device into other structures, but it does require coordinating
lifetime rules and selecting a "master" kobject. I think having
cdev_init accept such "master" kobject would bring author's attention to
the issue and avoid such mistakes in the future.

Thanks.

-- 
Dmitry