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    SubjectRe: [PATCH v3 2/2] tpm: in tpm2_del_space check if ops pointer is still valid
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    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?

    Best regards,
    Lino

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