block_dev: Support checking inode permissions in lookup_bdev()

When looking up a block device by path no permission check is
done to verify that the user has access to the block device inode
at the specified path. In some cases it may be necessary to
check permissions towards the inode, such as allowing
unprivileged users to mount block devices in user namespaces.

Add an argument to lookup_bdev() to optionally perform this
permission check. A value of 0 skips the permission check and
behaves the same as before. A non-zero value specifies the mask
of access rights required towards the inode at the specified
path. The check is always skipped if the user has CAP_SYS_ADMIN.

All callers of lookup_bdev() currently pass a mask of 0, so this
patch results in no functional change. Subsequent patches will
add permission checks where appropriate.
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>

drivers/md/bcache/super.c

@@ -1926,7 +1926,7 @@ static ssize_t register_bcache(struct kobject *k, struct kobj_attribute *attr,
 				  sb);
 	if (IS_ERR(bdev)) {
 		if (bdev == ERR_PTR(-EBUSY)) {
-			bdev = lookup_bdev(strim(path));
+			bdev = lookup_bdev(strim(path), 0);
 			mutex_lock(&bch_register_lock);
 			if (!IS_ERR(bdev) && bch_is_open(bdev))
 				err = "device already registered";

drivers/md/dm-table.c

@@ -380,7 +380,7 @@ int dm_get_device(struct dm_target *ti, const char *path, fmode_t mode,
 	BUG_ON(!t);
 
 	/* convert the path to a device */
-	bdev = lookup_bdev(path);
+	bdev = lookup_bdev(path, 0);
 	if (IS_ERR(bdev)) {
 		dev = name_to_dev_t(path);
 		if (!dev)

drivers/mtd/mtdsuper.c

@@ -176,7 +176,7 @@ struct dentry *mount_mtd(struct file_system_type *fs_type, int flags,
 	/* try the old way - the hack where we allowed users to mount
 	 * /dev/mtdblock$(n) but didn't actually _use_ the blockdev
 	 */
-	bdev = lookup_bdev(dev_name);
+	bdev = lookup_bdev(dev_name, 0);
 	if (IS_ERR(bdev)) {
 		ret = PTR_ERR(bdev);
 		pr_debug("MTDSB: lookup_bdev() returned %d\n", ret);

fs/block_dev.c

@@ -1426,7 +1426,7 @@ struct block_device *blkdev_get_by_path(const char *path, fmode_t mode,
 	struct block_device *bdev;
 	int err;
 
-	bdev = lookup_bdev(path);
+	bdev = lookup_bdev(path, 0);
 	if (IS_ERR(bdev))
 		return bdev;
 
@@ -1736,12 +1736,14 @@ EXPORT_SYMBOL(ioctl_by_bdev);
 /**
  * lookup_bdev  - lookup a struct block_device by name
  * @pathname:	special file representing the block device
+ * @mask:	rights to check for (%MAY_READ, %MAY_WRITE, %MAY_EXEC)
  *
  * Get a reference to the blockdevice at @pathname in the current
  * namespace if possible and return it.  Return ERR_PTR(error)
- * otherwise.
+ * otherwise.  If @mask is non-zero, check for access rights to the
+ * inode at @pathname.
  */
-struct block_device *lookup_bdev(const char *pathname)
+struct block_device *lookup_bdev(const char *pathname, int mask)
 {
 	struct block_device *bdev;
 	struct inode *inode;
@@ -1756,6 +1758,11 @@ struct block_device *lookup_bdev(const char *pathname)
 		return ERR_PTR(error);
 
 	inode = d_backing_inode(path.dentry);
+	if (mask != 0 && !capable(CAP_SYS_ADMIN)) {
+		error = __inode_permission(inode, mask);
+		if (error)
+			goto fail;
+	}
 	error = -ENOTBLK;
 	if (!S_ISBLK(inode->i_mode))
 		goto fail;

fs/quota/quota.c

@@ -733,7 +733,7 @@ static struct super_block *quotactl_block(const char __user *special, int cmd)
 
 	if (IS_ERR(tmp))
 		return ERR_CAST(tmp);
-	bdev = lookup_bdev(tmp->name);
+	bdev = lookup_bdev(tmp->name, 0);
 	putname(tmp);
 	if (IS_ERR(bdev))
 		return ERR_CAST(bdev);

include/linux/fs.h

@@ -2373,7 +2373,7 @@ static inline void unregister_chrdev(unsigned int major, const char *name)
 #define BLKDEV_MAJOR_HASH_SIZE	255
 extern const char *__bdevname(dev_t, char *buffer);
 extern const char *bdevname(struct block_device *bdev, char *buffer);
-extern struct block_device *lookup_bdev(const char *);
+extern struct block_device *lookup_bdev(const char *, int mask);
 extern void blkdev_show(struct seq_file *,off_t);
 
 #else