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@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?
Jason
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