mmc: block: Fix bug when removing RPMB chardev

I forgot to account for the fact that the device core holds a
reference to a device added with device_initialize() that need
to be released with a corresponding put_device() to reach a 0
refcount at the end of the lifecycle.

This led to a NULL pointer reference when freeing the device
when e.g. unbidning the host device in sysfs.

Fix this and use the device .release() callback to free the
IDA and free:ing the memory used by the RPMB device.

Before this patch:

/sys/bus/amba/drivers/mmci-pl18x$ echo 80114000.sdi4_per2 > unbind
[   29.797332] mmc3: card 0001 removed
[   29.810791] Unable to handle kernel NULL pointer dereference at
               virtual address 00000050
[   29.818878] pgd = de70c000
[   29.821624] [00000050] *pgd=1e70a831, *pte=00000000, *ppte=00000000
[   29.827911] Internal error: Oops: 17 [#1] PREEMPT SMP ARM
[   29.833282] Modules linked in:
[   29.836334] CPU: 1 PID: 154 Comm: sh Not tainted
               4.14.0-rc3-00039-g83318e309566-dirty #736
[   29.844604] Hardware name: ST-Ericsson Ux5x0 platform (Device Tree Support)
[   29.851562] task: de572700 task.stack: de742000
[   29.856079] PC is at kernfs_find_ns+0x8/0x100
[   29.860443] LR is at kernfs_find_and_get_ns+0x30/0x48

After this patch:

/sys/bus/amba/drivers/mmci-pl18x$ echo 80005000.sdi4_per2 > unbind
[   20.623382] mmc3: card 0001 removed

Fixes: 97548575bef3 ("mmc: block: Convert RPMB to a character device")
Reported-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

drivers/mmc/core/block.c

@@ -2277,9 +2277,7 @@ static int mmc_rpmb_chrdev_open(struct inode *inode, struct file *filp)
 
 	get_device(&rpmb->dev);
 	filp->private_data = rpmb;
-	mutex_lock(&open_lock);
-	rpmb->md->usage++;
-	mutex_unlock(&open_lock);
+	mmc_blk_get(rpmb->md->disk);
 
 	return nonseekable_open(inode, filp);
 }
@@ -2290,9 +2288,7 @@ static int mmc_rpmb_chrdev_release(struct inode *inode, struct file *filp)
 						  struct mmc_rpmb_data, chrdev);
 
 	put_device(&rpmb->dev);
-	mutex_lock(&open_lock);
-	rpmb->md->usage--;
-	mutex_unlock(&open_lock);
+	mmc_blk_put(rpmb->md);
 
 	return 0;
 }
@@ -2308,6 +2304,13 @@ static const struct file_operations mmc_rpmb_fileops = {
 #endif
 };
 
+static void mmc_blk_rpmb_device_release(struct device *dev)
+{
+	struct mmc_rpmb_data *rpmb = dev_get_drvdata(dev);
+
+	ida_simple_remove(&mmc_rpmb_ida, rpmb->id);
+	kfree(rpmb);
+}
 
 static int mmc_blk_alloc_rpmb_part(struct mmc_card *card,
 				   struct mmc_blk_data *md,
@@ -2326,8 +2329,10 @@ static int mmc_blk_alloc_rpmb_part(struct mmc_card *card,
 		return devidx;
 
 	rpmb = kzalloc(sizeof(*rpmb), GFP_KERNEL);
-	if (!rpmb)
+	if (!rpmb) {
+		ida_simple_remove(&mmc_rpmb_ida, devidx);
 		return -ENOMEM;
+	}
 
 	snprintf(rpmb_name, sizeof(rpmb_name),
 		 "mmcblk%u%s", card->host->index, subname ? subname : "");
@@ -2338,6 +2343,7 @@ static int mmc_blk_alloc_rpmb_part(struct mmc_card *card,
 	rpmb->dev.bus = &mmc_rpmb_bus_type;
 	rpmb->dev.devt = MKDEV(MAJOR(mmc_rpmb_devt), rpmb->id);
 	rpmb->dev.parent = &card->dev;
+	rpmb->dev.release = mmc_blk_rpmb_device_release;
 	device_initialize(&rpmb->dev);
 	dev_set_drvdata(&rpmb->dev, rpmb);
 	rpmb->md = md;
@@ -2347,7 +2353,7 @@ static int mmc_blk_alloc_rpmb_part(struct mmc_card *card,
 	ret = cdev_device_add(&rpmb->chrdev, &rpmb->dev);
 	if (ret) {
 		pr_err("%s: could not add character device\n", rpmb_name);
-		goto out_remove_ida;
+		goto out_put_device;
 	}
 
 	list_add(&rpmb->node, &md->rpmbs);
@@ -2362,18 +2368,16 @@ static int mmc_blk_alloc_rpmb_part(struct mmc_card *card,
 
 	return 0;
 
-out_remove_ida:
-	ida_simple_remove(&mmc_rpmb_ida, rpmb->id);
-	kfree(rpmb);
+out_put_device:
+	put_device(&rpmb->dev);
 	return ret;
 }
 
 static void mmc_blk_remove_rpmb_part(struct mmc_rpmb_data *rpmb)
+
 {
 	cdev_device_del(&rpmb->chrdev, &rpmb->dev);
-	device_del(&rpmb->dev);
-	ida_simple_remove(&mmc_rpmb_ida, rpmb->id);
-	kfree(rpmb);
+	put_device(&rpmb->dev);
 }
 
 /* MMC Physical partitions consist of two boot partitions and