linux-kernel - [PATCH RFC 2/4] fscrypt: add flag allowing partially-encrypted directories

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Date:   Sat, 23 Jul 2022 20:52:26 -0400
From:   Sweet Tea Dorminy <sweettea-kernel@...miny.me>
To:     "Theodore Y . Ts'o " <tytso@....edu>,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        Eric Biggers <ebiggers@...nel.org>,
        linux-fscrypt@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-btrfs@...r.kernel.org, osandov@...ndov.com,
        kernel-team@...com
Cc:     Sweet Tea Dorminy <sweettea-kernel@...miny.me>
Subject: [PATCH RFC 2/4] fscrypt: add flag allowing partially-encrypted directories

From: Omar Sandoval <osandov@...ndov.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.

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

Signed-off-by: Omar Sandoval <osandov@...ndov.com>
Signed-off-by: Sweet Tea Dorminy <sweettea-kernel@...miny.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 5d5c26d827fd..c5dd19c1d19e 100644
--- a/fs/crypto/fname.c
+++ b/fs/crypto/fname.c
@@ -389,6 +389,7 @@ 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;
@@ -424,8 +425,16 @@ 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)
@@ -436,6 +445,12 @@ 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 6020b738c3b2..fb48961c46f6 100644
--- a/include/linux/fscrypt.h
+++ b/include/linux/fscrypt.h
@@ -102,6 +102,8 @@ 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