Re: [PATCH 4/4] ubifs: Implement new mount option, fscrypt_key_required

[Eric Biggers]

Hi Richard,

On Thu, Mar 14, 2019 at 06:15:59PM +0100, Richard Weinberger wrote:
> Usually fscrypt allows limited access to encrypted files even
> if no key is available.
> Encrypted filenames are shown and based on this names users
> can unlink and move files.

Actually, fscrypt doesn't allow moving files without the key.  It would only be
possible for cross-renames, i.e. renames with the RENAME_EXCHANGE flag.  So for
consistency with regular renames, fscrypt also forbids cross-renames if the key
for either the source or destination directory is missing.

So the main use case for the ciphertext view is *deleting* files.  For example,
deleting a user's home directory after that user has been removed from the
system.  Or the system freeing up space by deleting cache files from a user who
isn't currently logged in.

> 
> This is not always what people expect. The fscrypt_key_required mount
> option disables this feature.
> If no key is present all access is denied with the -ENOKEY error code.

The problem with this mount option is that it allows users to create undeletable
files.  So I'm not really convinced yet this is a good change.  And though the
fscrypt_key_required semantics are easier to implement, we'd still have to
support the existing semantics too, thus increasing the maintenance cost.

> 
> The side benefit of this is that we don't need ->d_revalidate().
> Not having ->d_revalidate() makes an encrypted ubifs usable
> as overlayfs upper directory.
> 

It would be preferable if we could get overlayfs to work without providing a
special mount option.

> Signed-off-by: Richard Weinberger <richard@xxxxxx>
> ---
>  fs/ubifs/crypto.c |  2 +-
>  fs/ubifs/dir.c    | 29 ++++++++++++++++++++++++++---
>  fs/ubifs/super.c  | 15 +++++++++++++++
>  fs/ubifs/ubifs.h  |  1 +
>  4 files changed, 43 insertions(+), 4 deletions(-)
> 

Shouldn't readlink() honor the mount option too?

> diff --git a/fs/ubifs/crypto.c b/fs/ubifs/crypto.c
> index 4aaedf2d7f44..a6a48c5dc058 100644
> --- a/fs/ubifs/crypto.c
> +++ b/fs/ubifs/crypto.c
> @@ -76,7 +76,7 @@ int ubifs_decrypt(const struct inode *inode, struct ubifs_data_node *dn,
>  }
>  
>  const struct fscrypt_operations ubifs_crypt_operations = {
> -	.flags			= FS_CFLG_OWN_PAGES,
> +	.flags			= FS_CFLG_OWN_PAGES | FS_CFLG_OWN_D_OPS,
>  	.key_prefix		= "ubifs:",
>  	.get_context		= ubifs_crypt_get_context,
>  	.set_context		= ubifs_crypt_set_context,
> diff --git a/fs/ubifs/dir.c b/fs/ubifs/dir.c
> index b0cb913697c5..4d029f08b80d 100644
> --- a/fs/ubifs/dir.c
> +++ b/fs/ubifs/dir.c
> @@ -208,6 +208,16 @@ static int dbg_check_name(const struct ubifs_info *c,
>  	return 0;
>  }
>  
> +static void ubifs_set_dentry_ops(struct inode *dir, struct dentry *dentry)
> +{
> +#ifdef CONFIG_FS_ENCRYPTION
> +	struct ubifs_info *c = dir->i_sb->s_fs_info;
> +
> +	if (IS_ENCRYPTED(dir) && !c->fscrypt_key_required)
> +		d_set_d_op(dentry, &fscrypt_d_ops);
> +#endif
> +}
> +
>  static struct dentry *ubifs_lookup(struct inode *dir, struct dentry *dentry,
>  				   unsigned int flags)
>  {
> @@ -224,7 +234,10 @@ static struct dentry *ubifs_lookup(struct inode *dir, struct dentry *dentry,
>  	if (err)
>  		return ERR_PTR(err);
>  
> -	err = fscrypt_setup_filename(dir, &dentry->d_name, 1, &nm);
> +	ubifs_set_dentry_ops(dir, dentry);
> +
> +	err = fscrypt_setup_filename(dir, &dentry->d_name,
> +				     !c->fscrypt_key_required, &nm);
>  	if (err)
>  		return ERR_PTR(err);
>  
> @@ -240,6 +253,11 @@ static struct dentry *ubifs_lookup(struct inode *dir, struct dentry *dentry,
>  	}
>  
>  	if (nm.hash) {
> +		if (c->fscrypt_key_required) {
> +			inode = ERR_PTR(-ENOKEY);
> +			goto done;
> +		}
> +
>  		ubifs_assert(c, fname_len(&nm) == 0);
>  		ubifs_assert(c, fname_name(&nm) == NULL);
>  		dent_key_init_hash(c, &key, dir->i_ino, nm.hash);
> @@ -529,6 +547,9 @@ static int ubifs_readdir(struct file *file, struct dir_context *ctx)
>  		if (err)
>  			return err;
>  
> +		if (c->fscrypt_key_required && !dir->i_crypt_info)
> +			return -ENOKEY;
> +

How about returning -ENOKEY when trying to open the directory in the first
place, rather than allowing getting to readdir()?  That would match the behavior
of regular files.

>  		err = fscrypt_fname_alloc_buffer(dir, UBIFS_MAX_NLEN, &fstr);
>  		if (err)
>  			return err;
> @@ -798,7 +819,8 @@ static int ubifs_unlink(struct inode *dir, struct dentry *dentry)
>  			return err;
>  	}
>  
> -	err = fscrypt_setup_filename(dir, &dentry->d_name, 1, &nm);
> +	err = fscrypt_setup_filename(dir, &dentry->d_name, !c->fscrypt_key_required,
> +				     &nm);
>  	if (err)
>  		return err;
>  
> @@ -908,7 +930,8 @@ static int ubifs_rmdir(struct inode *dir, struct dentry *dentry)
>  			return err;
>  	}
>  
> -	err = fscrypt_setup_filename(dir, &dentry->d_name, 1, &nm);
> +	err = fscrypt_setup_filename(dir, &dentry->d_name,
> +				     !c->fscrypt_key_required, &nm);
>  	if (err)
>  		return err;
>  
> diff --git a/fs/ubifs/super.c b/fs/ubifs/super.c
> index 8dc2818fdd84..e081b00236b6 100644
> --- a/fs/ubifs/super.c
> +++ b/fs/ubifs/super.c
> @@ -445,6 +445,9 @@ static int ubifs_show_options(struct seq_file *s, struct dentry *root)
>  			   ubifs_compr_name(c, c->mount_opts.compr_type));
>  	}
>  
> +	if (c->fscrypt_key_required)
> +		seq_puts(s, ",fscrypt_key_required");
> +
>  	seq_printf(s, ",assert=%s", ubifs_assert_action_name(c));
>  	seq_printf(s, ",ubi=%d,vol=%d", c->vi.ubi_num, c->vi.vol_id);
>  
> @@ -952,6 +955,7 @@ enum {
>  	Opt_assert,
>  	Opt_auth_key,
>  	Opt_auth_hash_name,
> +	Opt_fscrypt_key_required,
>  	Opt_ignore,
>  	Opt_err,
>  };
> @@ -969,6 +973,7 @@ static const match_table_t tokens = {
>  	{Opt_ignore, "ubi=%s"},
>  	{Opt_ignore, "vol=%s"},
>  	{Opt_assert, "assert=%s"},
> +	{Opt_fscrypt_key_required, "fscrypt_key_required"},
>  	{Opt_err, NULL},
>  };
>  
> @@ -1008,6 +1013,7 @@ static int ubifs_parse_options(struct ubifs_info *c, char *options,
>  {
>  	char *p;
>  	substring_t args[MAX_OPT_ARGS];
> +	unsigned int old_fscrypt_key_required = c->fscrypt_key_required;
>  
>  	if (!options)
>  		return 0;
> @@ -1099,6 +1105,15 @@ static int ubifs_parse_options(struct ubifs_info *c, char *options,
>  			if (!c->auth_hash_name)
>  				return -ENOMEM;
>  			break;
> +		case Opt_fscrypt_key_required:
> +			c->fscrypt_key_required = 1;
> +
> +			if (is_remount && (old_fscrypt_key_required != c->fscrypt_key_required)) {
> +				ubifs_err(c, "fscrypt_key_required cannot changed by remount");
> +				return -EINVAL;
> +			}
> +
> +			break;
>  		case Opt_ignore:
>  			break;
>  		default:
> diff --git a/fs/ubifs/ubifs.h b/fs/ubifs/ubifs.h
> index 1ae12900e01d..71b03a6798ae 100644
> --- a/fs/ubifs/ubifs.h
> +++ b/fs/ubifs/ubifs.h
> @@ -1304,6 +1304,7 @@ struct ubifs_info {
>  	unsigned int rw_incompat:1;
>  	unsigned int assert_action:2;
>  	unsigned int authenticated:1;
> +	unsigned int fscrypt_key_required:1;
>  
>  	struct mutex tnc_mutex;
>  	struct ubifs_zbranch zroot;
> -- 
> 2.21.0
>