[PATCH 01/11] block_dev: Support checking inode permissions in lookup_bdev()

[Dongsu Park]

From: Seth Forshee <seth.forshee at canonical.com>

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.

Patch v4 is available: https://patchwork.kernel.org/patch/8943601/

Cc: dm-devel at redhat.com
Cc: linux-bcache at vger.kernel.org
Cc: linux-fsdevel at vger.kernel.org
Cc: linux-mtd at lists.infradead.org
Cc: linux-kernel at vger.kernel.org
Cc: Alexander Viro <viro at zeniv.linux.org.uk>
Cc: Jan Kara <jack at suse.com>
Cc: Serge Hallyn <serge at hallyn.com>
Signed-off-by: Seth Forshee <seth.forshee at canonical.com>
Signed-off-by: Dongsu Park <dongsu at kinvolk.io>
---
 drivers/md/bcache/super.c |  2 +-
 drivers/md/dm-table.c     |  2 +-
 drivers/mtd/mtdsuper.c    |  2 +-
 fs/block_dev.c            | 13 ++++++++++---
 fs/quota/quota.c          |  2 +-
 include/linux/fs.h        |  2 +-
 6 files changed, 15 insertions(+), 8 deletions(-)

diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
index b4d28928..acc9d56c 100644
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -1967,7 +1967,7 @@ 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";
diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
index 88130b5d..bca5eaf4 100644
--- a/drivers/md/dm-table.c
+++ b/drivers/md/dm-table.c
@@ -410,7 +410,7 @@ dev_t dm_get_dev_t(const char *path)
 	dev_t dev;
 	struct block_device *bdev;
 
-	bdev = lookup_bdev(path);
+	bdev = lookup_bdev(path, 0);
 	if (IS_ERR(bdev))
 		dev = name_to_dev_t(path);
 	else {
diff --git a/drivers/mtd/mtdsuper.c b/drivers/mtd/mtdsuper.c
index e43fea89..4a4d40c0 100644
--- a/drivers/mtd/mtdsuper.c
+++ b/drivers/mtd/mtdsuper.c
@@ -180,7 +180,7 @@ 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);
diff --git a/fs/block_dev.c b/fs/block_dev.c
index 4a181fcb..5ca06095 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -1662,7 +1662,7 @@ 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;
 
@@ -2052,12 +2052,14 @@ 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;
@@ -2072,6 +2074,11 @@ 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;
diff --git a/fs/quota/quota.c b/fs/quota/quota.c
index 43612e2a..e5d47955 100644
--- a/fs/quota/quota.c
+++ b/fs/quota/quota.c
@@ -807,7 +807,7 @@ 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);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 2995a271..fce19c49 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2551,7 +2551,7 @@ static inline void unregister_chrdev(unsigned int major, const char *name)
 #define BLKDEV_MAJOR_MAX	512
 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
-- 
2.13.6