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@xxxxxxxxx>
> 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@xxxxxxxxx>
> Acked-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
>
> 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...
thanks,
greg k-h
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