From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dmitry Torokhov Subject: Re: [PATCH v4 3/5] tee: generic TEE subsystem Date: Wed, 8 Jul 2015 16:28:26 -0700 Message-ID: <20150708232826.GB379@dtor-ws> References: <1436350592-7732-1-git-send-email-jens.wiklander@linaro.org> <1436350592-7732-4-git-send-email-jens.wiklander@linaro.org> <20150708171026.GA11740@obsidianresearch.com> <20150708211129.GA29824@kroah.com> <20150708222649.GA20068@obsidianresearch.com> <20150708223325.GA5843@kroah.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20150708223325.GA5843-U8xfFu+wG4EAvxtiuMwx3w@public.gmane.org> Sender: devicetree-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Greg Kroah-Hartman Cc: Jason Gunthorpe , Jens Wiklander , linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org, devicetree-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Arnd Bergmann , Rob Herring , Herbert Xu , valentin.manea-hv44wF8Li93QT0dZR+AlfA@public.gmane.org, jean-michel.delorme-qxv4g6HH51o@public.gmane.org, emmanuel.michel-qxv4g6HH51o@public.gmane.org, javier-5MUHepqpBA1BDgjK7y7TUQ@public.gmane.org, Mark Rutland , Michal Simek List-Id: devicetree@vger.kernel.org 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 > > 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 > > Acked-by: Al Viro > > Signed-off-by: Linus Torvalds > > > > 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. Thanks. -- Dmitry -- To unsubscribe from this list: send the line "unsubscribe devicetree" in the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org More majordomo info at http://vger.kernel.org/majordomo-info.html