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

[Jason Gunthorpe]

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@xxxxxxxx/
> > 
> > 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);