Re: [PATCH 1/5] fs: Verify access of user towards block device file when mounting

[Eric W. Biederman]

Seth Forshee <seth.forshee@xxxxxxxxxxxxx> writes:

> When mounting a filesystem on a block device there is currently
> no verification that the user has appropriate access to the
> device file passed to mount. This has not been an issue so far
> since the user in question has always been root, but this must
> be changed before allowing unprivileged users to mount in user
> namespaces.
>
> To fix this, add an argument to lookup_bdev() to specify the
> required permissions. If the mask of permissions is zero, or
> if the user has CAP_SYS_ADMIN, the permission check is skipped,
> otherwise the lookup fails if the user does not have the
> specified access rights for the inode at the supplied path.
>
> Callers associated with mounting are updated to pass permission
> masks to lookup_bdev() so that these mounts will fail for an
> unprivileged user who lacks permissions for the block device
> inode. All other callers pass 0 to maintain their current
> behaviors.
>

Seth can you split this patch?

One patch to add an argument to lookup_bdev,
and then for each kind of callsite a follow-on patch (if we are ready
for that).

That will separate the logical changes and make things easier to track
via bisect and more importantly easier to review things.

Eric


> Signed-off-by: Seth Forshee <seth.forshee@xxxxxxxxxxxxx>
> ---
>  drivers/md/bcache/super.c |  2 +-
>  drivers/md/dm-table.c     |  2 +-
>  drivers/mtd/mtdsuper.c    |  6 +++++-
>  fs/block_dev.c            | 18 +++++++++++++++---
>  fs/quota/quota.c          |  2 +-
>  include/linux/fs.h        |  2 +-
>  6 files changed, 24 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 e76ed003769e..35bb3ea4cbe2 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..5d7e7705fed8 100644
> --- a/drivers/mtd/mtdsuper.c
> +++ b/drivers/mtd/mtdsuper.c
> @@ -125,6 +125,7 @@ struct dentry *mount_mtd(struct file_system_type *fs_type, int flags,
>  #ifdef CONFIG_BLOCK
>  	struct block_device *bdev;
>  	int ret, major;
> +	int perm;
>  #endif
>  	int mtdnr;
>  
> @@ -176,7 +177,10 @@ 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);
> +	perm = MAY_READ;
> +	if (!(flags & MS_RDONLY))
> +		perm |= MAY_WRITE;
> +	bdev = lookup_bdev(dev_name, perm);
>  	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 26cee058dc02..54d94cd64577 100644
> --- a/fs/block_dev.c
> +++ b/fs/block_dev.c
> @@ -1394,9 +1394,14 @@ struct block_device *blkdev_get_by_path(const char *path, fmode_t mode,
>  					void *holder)
>  {
>  	struct block_device *bdev;
> +	int perm = 0;
>  	int err;
>  
> -	bdev = lookup_bdev(path);
> +	if (mode & FMODE_READ)
> +		perm |= MAY_READ;
> +	if (mode & FMODE_WRITE)
> +		perm |= MAY_WRITE;
> +	bdev = lookup_bdev(path, perm);
>  	if (IS_ERR(bdev))
>  		return bdev;
>  
> @@ -1706,12 +1711,14 @@ 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;
> @@ -1726,6 +1733,11 @@ 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 458ee7b213be..cc18dfb0b98e 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -2388,7 +2388,7 @@ 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
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