fscrypto: improved validation when loading inode encryption metadata

- Validate fscrypt_context.format and fscrypt_context.flags.  If
  unrecognized values are set, then the kernel may not know how to
  interpret the encrypted file, so it should fail the operation.

- Validate that AES_256_XTS is used for contents and that AES_256_CTS is
  used for filenames.  It was previously possible for the kernel to
  accept these reversed, though it would have taken manual editing of
  the block device.  This was not intended.

- Fail cleanly rather than BUG()-ing if a file has an unexpected type.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/crypto/keyinfo.c

@@ -139,6 +139,38 @@ static int validate_user_key(struct fscrypt_info *crypt_info,
 	return res;
 }
 
+static int determine_cipher_type(struct fscrypt_info *ci, struct inode *inode,
+				 const char **cipher_str_ret, int *keysize_ret)
+{
+	if (S_ISREG(inode->i_mode)) {
+		if (ci->ci_data_mode == FS_ENCRYPTION_MODE_AES_256_XTS) {
+			*cipher_str_ret = "xts(aes)";
+			*keysize_ret = FS_AES_256_XTS_KEY_SIZE;
+			return 0;
+		}
+		pr_warn_once("fscrypto: unsupported contents encryption mode "
+			     "%d for inode %lu\n",
+			     ci->ci_data_mode, inode->i_ino);
+		return -ENOKEY;
+	}
+
+	if (S_ISDIR(inode->i_mode) || S_ISLNK(inode->i_mode)) {
+		if (ci->ci_filename_mode == FS_ENCRYPTION_MODE_AES_256_CTS) {
+			*cipher_str_ret = "cts(cbc(aes))";
+			*keysize_ret = FS_AES_256_CTS_KEY_SIZE;
+			return 0;
+		}
+		pr_warn_once("fscrypto: unsupported filenames encryption mode "
+			     "%d for inode %lu\n",
+			     ci->ci_filename_mode, inode->i_ino);
+		return -ENOKEY;
+	}
+
+	pr_warn_once("fscrypto: unsupported file type %d for inode %lu\n",
+		     (inode->i_mode & S_IFMT), inode->i_ino);
+	return -ENOKEY;
+}
+
 static void put_crypt_info(struct fscrypt_info *ci)
 {
 	if (!ci)
@@ -155,8 +187,8 @@ int get_crypt_info(struct inode *inode)
 	struct fscrypt_context ctx;
 	struct crypto_skcipher *ctfm;
 	const char *cipher_str;
+	int keysize;
 	u8 raw_key[FS_MAX_KEY_SIZE];
-	u8 mode;
 	int res;
 
 	res = fscrypt_initialize();
@@ -179,13 +211,19 @@ int get_crypt_info(struct inode *inode)
 	if (res < 0) {
 		if (!fscrypt_dummy_context_enabled(inode))
 			return res;
+		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;
 	} else if (res != sizeof(ctx)) {
 		return -EINVAL;
 	}
-	res = 0;
+
+	if (ctx.format != FS_ENCRYPTION_CONTEXT_FORMAT_V1)
+		return -EINVAL;
+
+	if (ctx.flags & ~FS_POLICY_FLAGS_VALID)
+		return -EINVAL;
 
 	crypt_info = kmem_cache_alloc(fscrypt_info_cachep, GFP_NOFS);
 	if (!crypt_info)
@@ -198,27 +236,11 @@ int get_crypt_info(struct inode *inode)
 	crypt_info->ci_keyring_key = NULL;
 	memcpy(crypt_info->ci_master_key, ctx.master_key_descriptor,
 				sizeof(crypt_info->ci_master_key));
-	if (S_ISREG(inode->i_mode))
-		mode = crypt_info->ci_data_mode;
-	else if (S_ISDIR(inode->i_mode) || S_ISLNK(inode->i_mode))
-		mode = crypt_info->ci_filename_mode;
-	else
-		BUG();
-
-	switch (mode) {
-	case FS_ENCRYPTION_MODE_AES_256_XTS:
-		cipher_str = "xts(aes)";
-		break;
-	case FS_ENCRYPTION_MODE_AES_256_CTS:
-		cipher_str = "cts(cbc(aes))";
-		break;
-	default:
-		printk_once(KERN_WARNING
-			    "%s: unsupported key mode %d (ino %u)\n",
-			    __func__, mode, (unsigned) inode->i_ino);
-		res = -ENOKEY;
+
+	res = determine_cipher_type(crypt_info, inode, &cipher_str, &keysize);
+	if (res)
 		goto out;
-	}
+
 	if (fscrypt_dummy_context_enabled(inode)) {
 		memset(raw_key, 0x42, FS_AES_256_XTS_KEY_SIZE);
 		goto got_key;
@@ -253,7 +275,7 @@ int get_crypt_info(struct inode *inode)
 	crypt_info->ci_ctfm = ctfm;
 	crypto_skcipher_clear_flags(ctfm, ~0);
 	crypto_skcipher_set_flags(ctfm, CRYPTO_TFM_REQ_WEAK_KEY);
-	res = crypto_skcipher_setkey(ctfm, raw_key, fscrypt_key_size(mode));
+	res = crypto_skcipher_setkey(ctfm, raw_key, keysize);
 	if (res)
 		goto out;
 

include/linux/fscrypto.h

@@ -111,23 +111,6 @@ struct fscrypt_completion_result {
 	struct fscrypt_completion_result ecr = { \
 		COMPLETION_INITIALIZER((ecr).completion), 0 }
 
-static inline int fscrypt_key_size(int mode)
-{
-	switch (mode) {
-	case FS_ENCRYPTION_MODE_AES_256_XTS:
-		return FS_AES_256_XTS_KEY_SIZE;
-	case FS_ENCRYPTION_MODE_AES_256_GCM:
-		return FS_AES_256_GCM_KEY_SIZE;
-	case FS_ENCRYPTION_MODE_AES_256_CBC:
-		return FS_AES_256_CBC_KEY_SIZE;
-	case FS_ENCRYPTION_MODE_AES_256_CTS:
-		return FS_AES_256_CTS_KEY_SIZE;
-	default:
-		BUG();
-	}
-	return 0;
-}
-
 #define FS_FNAME_NUM_SCATTER_ENTRIES	4
 #define FS_CRYPTO_BLOCK_SIZE		16
 #define FS_FNAME_CRYPTO_DIGEST_SIZE	32
@@ -202,13 +185,6 @@ static inline bool fscrypt_valid_filenames_enc_mode(u32 mode)
 	return (mode == FS_ENCRYPTION_MODE_AES_256_CTS);
 }
 
-static inline u32 fscrypt_validate_encryption_key_size(u32 mode, u32 size)
-{
-	if (size == fscrypt_key_size(mode))
-		return size;
-	return 0;
-}
-
 static inline bool fscrypt_is_dot_dotdot(const struct qstr *str)
 {
 	if (str->len == 1 && str->name[0] == '.')