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

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Wed, Jul 08, 2015 at 04:28:26PM -0700, Dmitry Torokhov wrote:
> 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.

Embedding cdevs into other structures is great, I like that.  What I
don't like is having two different reference counts for the same
structure based on the lifetime rules of two different embedded
structures.  That's a very difficult thing to get right and I would
argue, is something that should almost never be done.

thanks,

greg k-h