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

    thanks,

    greg k-h


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