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

Eric Biggers
2019-03-14T17:49:19Z

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@nod.at>
> ---
> 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
>