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Date: Fri, 12 Feb 2021 12:59:58 +0200
From: Jarkko Sakkinen <jarkko@...nel.org>
To: Lino Sanfilippo <l.sanfilippo@...bus.com>
Cc: Jason Gunthorpe <jgg@...pe.ca>,
James Bottomley <James.Bottomley@...senpartnership.com>,
Lino Sanfilippo <LinoSanfilippo@....de>, peterhuewe@....de,
stefanb@...ux.vnet.ibm.com, stable@...r.kernel.org,
linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/2] tpm: in tpm2_del_space check if ops pointer is
still valid
On Tue, Feb 09, 2021 at 12:52:17PM +0100, Lino Sanfilippo wrote:
> Hi Jason,
>
> On 05.02.21 18:25, Jason Gunthorpe wrote:
> > On Fri, Feb 05, 2021 at 08:48:11AM -0800, James Bottomley wrote:
> >>> Thanks for pointing this out. I'd strongly support Jason's proposal:
> >>>
> >>> https://lore.kernel.org/linux-integrity/20201215175624.GG5487@ziepe.ca/
> >>>
> >>> It's the best long-term way to fix this.
> >>
> >> Really, no it's not. It introduces extra mechanism we don't need.
> >
> >> To recap the issue: character devices already have an automatic
> >> mechanism which holds a reference to the struct device while the
> >> character node is open so the default is to release resources on final
> >> put of the struct device.
> >
> > The refcount on the struct device only keeps the memory alive, it
> > doesn't say anything about the ops. We still need to lock and check
> > the ops each and every time they are used.
> >
> > The fact cdev goes all the way till fput means we don't need the extra
> > get/put I suggested to Lino at all.
> >
> >> The practical consequence of this model is that if you allocate a chip
> >> structure with tpm_chip_alloc() you have to release it again by doing a
> >> put of *both* devices.
> >
> > The final put of the devs should be directly after the
> > cdev_device_del(), not in a devm. This became all confused because the
> > devs was created during alloc, not register. Having a device that is
> > initialized but will never be added is weird.
> >
> > See sketch below.
> >
> >> Stefan noticed the latter, so we got the bogus patch 8979b02aaf1d
> >> ("tpm: Fix reference count to main device") applied which simply breaks
> >> the master/slave model by not taking a reference on the master for the
> >> slave. I'm not sure why I didn't notice the problem with this fix at
> >> the time, but attention must have been elsewhere.
> >
> > Well, this is sort of OK because we never use the devs in TPM1, so we
> > end up freeing the chip with a positive refcount on the devs, which is
> > weird but not a functional bug.
> >
> > Jason
> >
> > diff --git a/drivers/char/tpm/tpm-chip.c b/drivers/char/tpm/tpm-chip.c
> > index ddaeceb7e10910..e07193a0dd4438 100644
> > --- a/drivers/char/tpm/tpm-chip.c
> > +++ b/drivers/char/tpm/tpm-chip.c
> > @@ -344,7 +344,6 @@ struct tpm_chip *tpm_chip_alloc(struct device *pdev,
> > chip->dev_num = rc;
> >
> > device_initialize(&chip->dev);
> > - device_initialize(&chip->devs);
> >
> > chip->dev.class = tpm_class;
> > chip->dev.class->shutdown_pre = tpm_class_shutdown;
> > @@ -352,29 +351,12 @@ struct tpm_chip *tpm_chip_alloc(struct device *pdev,
> > chip->dev.parent = pdev;
> > chip->dev.groups = chip->groups;
> >
> > - chip->devs.parent = pdev;
> > - chip->devs.class = tpmrm_class;
> > - chip->devs.release = tpm_devs_release;
> > - /* get extra reference on main device to hold on
> > - * behalf of devs. This holds the chip structure
> > - * while cdevs is in use. The corresponding put
> > - * is in the tpm_devs_release (TPM2 only)
> > - */
> > - if (chip->flags & TPM_CHIP_FLAG_TPM2)
> > - get_device(&chip->dev);
> > -
> > if (chip->dev_num == 0)
> > chip->dev.devt = MKDEV(MISC_MAJOR, TPM_MINOR);
> > else
> > chip->dev.devt = MKDEV(MAJOR(tpm_devt), chip->dev_num);
> >
> > - chip->devs.devt =
> > - MKDEV(MAJOR(tpm_devt), chip->dev_num + TPM_NUM_DEVICES);
> > -
> > rc = dev_set_name(&chip->dev, "tpm%d", chip->dev_num);
> > - if (rc)
> > - goto out;
> > - rc = dev_set_name(&chip->devs, "tpmrm%d", chip->dev_num);
> > if (rc)
> > goto out;
> >
> > @@ -382,9 +364,7 @@ struct tpm_chip *tpm_chip_alloc(struct device *pdev,
> > chip->flags |= TPM_CHIP_FLAG_VIRTUAL;
> >
> > cdev_init(&chip->cdev, &tpm_fops);
> > - cdev_init(&chip->cdevs, &tpmrm_fops);
> > chip->cdev.owner = THIS_MODULE;
> > - chip->cdevs.owner = THIS_MODULE;
> >
> > rc = tpm2_init_space(&chip->work_space, TPM2_SPACE_BUFFER_SIZE);
> > if (rc) {
> > @@ -396,7 +376,6 @@ struct tpm_chip *tpm_chip_alloc(struct device *pdev,
> > return chip;
> >
> > out:
> > - put_device(&chip->devs);
> > put_device(&chip->dev);
> > return ERR_PTR(rc);
> > }
> > @@ -445,13 +424,33 @@ static int tpm_add_char_device(struct tpm_chip *chip)
> > }
> >
> > if (chip->flags & TPM_CHIP_FLAG_TPM2) {
> > + device_initialize(&chip->devs);
> > + chip->devs.parent = pdev;
> > + chip->devs.class = tpmrm_class;
> > + rc = dev_set_name(&chip->devs, "tpmrm%d", chip->dev_num);
> > + if (rc)
> > + goto out_put_devs;
> > +
> > + /*
> > + * get extra reference on main device to hold on behalf of devs.
> > + * This holds the chip structure while cdevs is in use. The
> > + * corresponding put is in the tpm_devs_release.
> > + */
> > + get_device(&chip->dev);
> > + chip->devs.release = tpm_devs_release;
> > +
> > + chip->devs.devt =
> > + MKDEV(MAJOR(tpm_devt), chip->dev_num + TPM_NUM_DEVICES);
> > + cdev_init(&chip->cdevs, &tpmrm_fops);
> > + chip->cdevs.owner = THIS_MODULE;
> > +
> > rc = cdev_device_add(&chip->cdevs, &chip->devs);
> > if (rc) {
> > dev_err(&chip->devs,
> > "unable to cdev_device_add() %s, major %d, minor %d, err=%d\n",
> > dev_name(&chip->devs), MAJOR(chip->devs.devt),
> > MINOR(chip->devs.devt), rc);
> > - return rc;
> > + goto out_put_devs;
> > }
> > }
> >
> > @@ -460,6 +459,10 @@ static int tpm_add_char_device(struct tpm_chip *chip)
> > idr_replace(&dev_nums_idr, chip, chip->dev_num);
> > mutex_unlock(&idr_lock);
> >
> > +out_put_devs:
> > + put_device(&chip->devs);
> > +out_del_dev:
> > + cdev_device_del(&chip->cdev);
> > return rc;
> > }
> >
> > @@ -640,8 +643,10 @@ void tpm_chip_unregister(struct tpm_chip *chip)
> > if (IS_ENABLED(CONFIG_HW_RANDOM_TPM))
> > hwrng_unregister(&chip->hwrng);
> > tpm_bios_log_teardown(chip);
> > - if (chip->flags & TPM_CHIP_FLAG_TPM2)
> > + if (chip->flags & TPM_CHIP_FLAG_TPM2) {
> > cdev_device_del(&chip->cdevs, &chip->devs);
> > + put_device(&chip->devs);
> > + }
> > tpm_del_char_device(chip);
> > }
> > EXPORT_SYMBOL_GPL(tpm_chip_unregister);
> >
>
> I tested the solution you scetched and it fixes the issue for me. Will
> you send a (real) patch for this?
One *option*:
1. You take the Jason's patch.
2. https://www.kernel.org/doc/html/v5.10/process/submitting-patches.html#when-to-use-acked-by-cc-and-co-developed-by
Just mentioning this, and spreading the knowledge about co-developed-by.
> Best regards,
> Lino
/Jarkko
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