[PATCH v3 21/22] fscrypt: add flag allowing partially-encrypted directories

Sweet Tea Dorminy
2022-10-20T17:00:22Z

From: Omar Sandoval <osandov@osandov.com>

Creating several new subvolumes out of snapshots of another subvolume,
each for a different VM's storage, is a important usecase for btrfs. We
would like to give each VM a unique encryption key to use for new writes
to its subvolume, so that secure deletion of the VM's data is as simple
as securely deleting the key; to avoid needing multiple keys in each VM,
we envision the initial subvolume being unencrypted. However, this means
that the snapshot's directories would have a mix of encrypted and
unencrypted files. During lookup with a key, both unencrypted and
encrypted forms of the desired name must be queried.

To allow this, add another FS_CFLG to allow filesystems to opt into
partially encrypted directories.

Signed-off-by: Omar Sandoval <osandov@osandov.com>
Signed-off-by: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
---
fs/crypto/fname.c | 17 ++++++++++++++++-
include/linux/fscrypt.h | 2 ++
2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/fs/crypto/fname.c b/fs/crypto/fname.c
index b3e7e3a66312..16b64e0589e4 100644
--- a/fs/crypto/fname.c
+++ b/fs/crypto/fname.c
@@ -414,6 +414,7 @@ int fscrypt_setup_filename(struct inode *dir, const struct qstr *iname,
fname->usr_fname = iname;

if (!IS_ENCRYPTED(dir) || fscrypt_is_dot_dotdot(iname)) {
+unencrypted:
fname->disk_name.name = (unsigned char *)iname->name;
fname->disk_name.len = iname->len;
return 0;
@@ -448,8 +449,16 @@ int fscrypt_setup_filename(struct inode *dir, const struct qstr *iname,
* user-supplied name
*/

- if (iname->len > FSCRYPT_NOKEY_NAME_MAX_ENCODED)
+ if (iname->len > FSCRYPT_NOKEY_NAME_MAX_ENCODED) {
+ /*
+ * This isn't a valid nokey name, but it could be an unencrypted
+ * name if the filesystem allows partially encrypted
+ * directories.
+ */
+ if (dir->i_sb->s_cop->flags & FS_CFLG_ALLOW_PARTIAL)
+ goto unencrypted;
return -ENOENT;
+ }

fname->crypto_buf.name = kmalloc(FSCRYPT_NOKEY_NAME_MAX, GFP_KERNEL);
if (fname->crypto_buf.name == NULL)
@@ -460,6 +469,12 @@ int fscrypt_setup_filename(struct inode *dir, const struct qstr *iname,
if (ret < (int)offsetof(struct fscrypt_nokey_name, bytes[1]) ||
(ret > offsetof(struct fscrypt_nokey_name, sha256) &&
ret != FSCRYPT_NOKEY_NAME_MAX)) {
+ /* Again, this could be an unencrypted name. */
+ if (dir->i_sb->s_cop->flags & FS_CFLG_ALLOW_PARTIAL) {
+ kfree(fname->crypto_buf.name);
+ fname->crypto_buf.name = NULL;
+ goto unencrypted;
+ }
ret = -ENOENT;
goto errout;
}
diff --git a/include/linux/fscrypt.h b/include/linux/fscrypt.h
index 4143c722ea1b..485471174594 100644
--- a/include/linux/fscrypt.h
+++ b/include/linux/fscrypt.h
@@ -109,6 +109,8 @@ struct fscrypt_nokey_name {
* pages for writes and therefore won't need the fscrypt bounce page pool.
*/
#define FS_CFLG_OWN_PAGES (1U << 1)
+/* The filesystem allows partially encrypted directories/files. */
+#define FS_CFLG_ALLOW_PARTIAL (1U << 2)

/* Crypto operations for filesystems */
struct fscrypt_operations {
--
2.35.1