Re: [PATCH 01/19] block_dev: Support checking inode permissions in lookup_bdev()

Serge E. Hallyn
2015-12-04T16:26:41Z

On Wed, Dec 02, 2015 at 09:40:01AM -0600, Seth Forshee wrote:
> 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-Z7WLFzj8eWMS+FvcfC7Uqw@public.gmane.org>

Acked-by: Serge Hallyn <serge.hallyn-Z7WLFzj8eWMS+FvcfC7Uqw@public.gmane.org>

> ---
> 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 679a093a3bf6..e8287b0d1dac 100644
> --- a/drivers/md/bcache/super.c
> +++ b/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";
> diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
> index 061152a43730..81c60b2495ed 100644
> --- a/drivers/md/dm-table.c
> +++ b/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)
> diff --git a/drivers/mtd/mtdsuper.c b/drivers/mtd/mtdsuper.c
> index 20c02a3b7417..b5b60e1af31c 100644
> --- a/drivers/mtd/mtdsuper.c
> +++ b/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);
> diff --git a/fs/block_dev.c b/fs/block_dev.c
> index f90d91efa1b4..3ebbde85d898 100644
> --- a/fs/block_dev.c
> +++ b/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;
> diff --git a/fs/quota/quota.c b/fs/quota/quota.c
> index 3746367098fd..a40eaecbd5cc 100644
> --- a/fs/quota/quota.c
> +++ b/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);
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 8a17c5649ef2..879ec382fd88 100644
> --- a/include/linux/fs.h
> +++ b/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
> --
> 1.9.1
>
> --
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