fscrypt: make test_dummy_encryption require a keyring key

Currently, the test_dummy_encryption ext4 mount option, which exists
only to test encrypted I/O paths with xfstests, overrides all
per-inode encryption keys with a fixed key.

This change minimizes test_dummy_encryption-specific code path changes
by supplying a fake context for directories which are not encrypted
for use when creating new directories, files, or symlinks.  This
allows us to properly exercise the keyring lookup, derivation, and
context inheritance code paths.

Before mounting a file system using test_dummy_encryption, userspace
must execute the following shell commands:

    mode='\x00\x00\x00\x00'
    raw="$(printf ""\\\\x%02x"" $(seq 0 63))"
    if lscpu | grep "Byte Order" | grep -q Little ; then
        size='\x40\x00\x00\x00'
    else
        size='\x00\x00\x00\x40'
    fi
    key="${mode}${raw}${size}"
    keyctl new_session
    echo -n -e "${key}" | keyctl padd logon fscrypt:4242424242424242 @s

Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/crypto/keyinfo.c

@@ -206,12 +206,16 @@ retry:
 
 	res = inode->i_sb->s_cop->get_context(inode, &ctx, sizeof(ctx));
 	if (res < 0) {
-		if (!fscrypt_dummy_context_enabled(inode))
+		if (!fscrypt_dummy_context_enabled(inode) ||
+		    inode->i_sb->s_cop->is_encrypted(inode))
 			return res;
+		/* Fake up a context for an unencrypted directory */
+		memset(&ctx, 0, sizeof(ctx));
 		ctx.format = FS_ENCRYPTION_CONTEXT_FORMAT_V1;
 		ctx.contents_encryption_mode = FS_ENCRYPTION_MODE_AES_256_XTS;
 		ctx.filenames_encryption_mode = FS_ENCRYPTION_MODE_AES_256_CTS;
-		ctx.flags = 0;
+		memset(ctx.master_key_descriptor, 0x42, FS_KEY_DESCRIPTOR_SIZE);
+		res = sizeof(ctx);
 	} else if (res != sizeof(ctx)) {
 		return -EINVAL;
 	}
@@ -247,12 +251,6 @@ retry:
 	if (!raw_key)
 		goto out;
 
-	if (fscrypt_dummy_context_enabled(inode)) {
-		memset(raw_key, 0x42, keysize/2);
-		memset(raw_key+keysize/2, 0x24, keysize - (keysize/2));
-		goto got_key;
-	}
-
 	res = validate_user_key(crypt_info, &ctx, raw_key,
 			FS_KEY_DESC_PREFIX, FS_KEY_DESC_PREFIX_SIZE);
 	if (res && inode->i_sb->s_cop->key_prefix) {
@@ -270,7 +268,6 @@ retry:
 	} else if (res) {
 		goto out;
 	}
-got_key:
 	ctfm = crypto_alloc_skcipher(cipher_str, 0, 0);
 	if (!ctfm || IS_ERR(ctfm)) {
 		res = ctfm ? PTR_ERR(ctfm) : -ENOMEM;

fs/crypto/policy.c

@@ -198,9 +198,9 @@ EXPORT_SYMBOL(fscrypt_has_permitted_context);
  * @parent: Parent inode from which the context is inherited.
  * @child:  Child inode that inherits the context from @parent.
  * @fs_data:  private data given by FS.
- * @preload:  preload child i_crypt_info
+ * @preload:  preload child i_crypt_info if true
  *
- * Return: Zero on success, non-zero otherwise
+ * Return: 0 on success, -errno on failure
  */
 int fscrypt_inherit_context(struct inode *parent, struct inode *child,
 						void *fs_data, bool preload)
@@ -221,19 +221,11 @@ int fscrypt_inherit_context(struct inode *parent, struct inode *child,
 		return -ENOKEY;
 
 	ctx.format = FS_ENCRYPTION_CONTEXT_FORMAT_V1;
-	if (fscrypt_dummy_context_enabled(parent)) {
-		ctx.contents_encryption_mode = FS_ENCRYPTION_MODE_AES_256_XTS;
-		ctx.filenames_encryption_mode = FS_ENCRYPTION_MODE_AES_256_CTS;
-		ctx.flags = 0;
-		memset(ctx.master_key_descriptor, 0x42, FS_KEY_DESCRIPTOR_SIZE);
-		res = 0;
-	} else {
-		ctx.contents_encryption_mode = ci->ci_data_mode;
-		ctx.filenames_encryption_mode = ci->ci_filename_mode;
-		ctx.flags = ci->ci_flags;
-		memcpy(ctx.master_key_descriptor, ci->ci_master_key,
-				FS_KEY_DESCRIPTOR_SIZE);
-	}
+	ctx.contents_encryption_mode = ci->ci_data_mode;
+	ctx.filenames_encryption_mode = ci->ci_filename_mode;
+	ctx.flags = ci->ci_flags;
+	memcpy(ctx.master_key_descriptor, ci->ci_master_key,
+	       FS_KEY_DESCRIPTOR_SIZE);
 	get_random_bytes(ctx.nonce, FS_KEY_DERIVATION_NONCE_SIZE);
 	res = parent->i_sb->s_cop->set_context(child, &ctx,
 						sizeof(ctx), fs_data);