fscrypt: handle test_dummy_encryption in more logical way

The behavior of the test_dummy_encryption mount option is that when a
new file (or directory or symlink) is created in an unencrypted
directory, it's automatically encrypted using a dummy encryption policy.
That's it; in particular, the encryption (or lack thereof) of existing
files (or directories or symlinks) doesn't change.

Unfortunately the implementation of test_dummy_encryption is a bit weird
and confusing.  When test_dummy_encryption is enabled and a file is
being created in an unencrypted directory, we set up an encryption key
(->i_crypt_info) for the directory.  This isn't actually used to do any
encryption, however, since the directory is still unencrypted!  Instead,
->i_crypt_info is only used for inheriting the encryption policy.

One consequence of this is that the filesystem ends up providing a
"dummy context" (policy + nonce) instead of a "dummy policy".  In
commit ed318a6cc0b6 ("fscrypt: support test_dummy_encryption=v2"), I
mistakenly thought this was required.  However, actually the nonce only
ends up being used to derive a key that is never used.

Another consequence of this implementation is that it allows for
'inode->i_crypt_info != NULL && !IS_ENCRYPTED(inode)', which is an edge
case that can be forgotten about.  For example, currently
FS_IOC_GET_ENCRYPTION_POLICY on an unencrypted directory may return the
dummy encryption policy when the filesystem is mounted with
test_dummy_encryption.  That seems like the wrong thing to do, since
again, the directory itself is not actually encrypted.

Therefore, switch to a more logical and maintainable implementation
where the dummy encryption policy inheritance is done without setting up
keys for unencrypted directories.  This involves:

- Adding a function fscrypt_policy_to_inherit() which returns the
  encryption policy to inherit from a directory.  This can be a real
  policy, a dummy policy, or no policy.

- Replacing struct fscrypt_dummy_context, ->get_dummy_context(), etc.
  with struct fscrypt_dummy_policy, ->get_dummy_policy(), etc.

- Making fscrypt_fname_encrypted_size() take an fscrypt_policy instead
  of an inode.

Acked-by: Jaegeuk Kim <jaegeuk@kernel.org>
Acked-by: Jeff Layton <jlayton@kernel.org>
Link: https://lore.kernel.org/r/20200917041136.178600-13-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>

1 files changed, 8 insertions, 8 deletions

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index ea425b49b345..aa6f64badf58 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -1104,7 +1104,7 @@ static void ext4_put_super(struct super_block *sb)
 		crypto_free_shash(sbi->s_chksum_driver);
 	kfree(sbi->s_blockgroup_lock);
 	fs_put_dax(sbi->s_daxdev);
-	fscrypt_free_dummy_context(&sbi->s_dummy_enc_ctx);
+	fscrypt_free_dummy_policy(&sbi->s_dummy_enc_policy);
 #ifdef CONFIG_UNICODE
 	utf8_unload(sbi->s_encoding);
 #endif
@@ -1392,10 +1392,9 @@ retry:
 	return res;
 }
 
-static const union fscrypt_context *
-ext4_get_dummy_context(struct super_block *sb)
+static const union fscrypt_policy *ext4_get_dummy_policy(struct super_block *sb)
 {
-	return EXT4_SB(sb)->s_dummy_enc_ctx.ctx;
+	return EXT4_SB(sb)->s_dummy_enc_policy.policy;
 }
 
 static bool ext4_has_stable_inodes(struct super_block *sb)
@@ -1414,7 +1413,7 @@ static const struct fscrypt_operations ext4_cryptops = {
 	.key_prefix		= "ext4:",
 	.get_context		= ext4_get_context,
 	.set_context		= ext4_set_context,
-	.get_dummy_context	= ext4_get_dummy_context,
+	.get_dummy_policy	= ext4_get_dummy_policy,
 	.empty_dir		= ext4_empty_dir,
 	.max_namelen		= EXT4_NAME_LEN,
 	.has_stable_inodes	= ext4_has_stable_inodes,
@@ -1888,12 +1887,13 @@ static int ext4_set_test_dummy_encryption(struct super_block *sb,
 	 * needed to allow it to be set or changed during remount.  We do allow
 	 * it to be specified during remount, but only if there is no change.
 	 */
-	if (is_remount && !sbi->s_dummy_enc_ctx.ctx) {
+	if (is_remount && !sbi->s_dummy_enc_policy.policy) {
 		ext4_msg(sb, KERN_WARNING,
 			 "Can't set test_dummy_encryption on remount");
 		return -1;
 	}
-	err = fscrypt_set_test_dummy_encryption(sb, arg, &sbi->s_dummy_enc_ctx);
+	err = fscrypt_set_test_dummy_encryption(sb, arg,
+						&sbi->s_dummy_enc_policy);
 	if (err) {
 		if (err == -EEXIST)
 			ext4_msg(sb, KERN_WARNING,
@@ -4935,7 +4935,7 @@ failed_mount:
 	for (i = 0; i < EXT4_MAXQUOTAS; i++)
 		kfree(get_qf_name(sb, sbi, i));
 #endif
-	fscrypt_free_dummy_context(&sbi->s_dummy_enc_ctx);
+	fscrypt_free_dummy_policy(&sbi->s_dummy_enc_policy);
 	ext4_blkdev_remove(sbi);
 	brelse(bh);
 out_fail: