Messages in this thread | | | Date | Wed, 8 Jul 2015 16:26:49 -0600 | From | Jason Gunthorpe <> | Subject | Re: [PATCH v4 3/5] tee: generic TEE subsystem |
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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;
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.
> 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?
Jason
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