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ext4 crypto: add padding to filenames before encrypting

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This obscures the length of the filenames, to decrease the amount of
information leakage.  By default, we pad the filenames to the next 4
byte boundaries.  This costs nothing, since the directory entries are
aligned to 4 byte boundaries anyway.  Filenames can also be padded to
8, 16, or 32 bytes, which will consume more directory space.

Change-Id: Ibb7a0fb76d2c48e2061240a709358ff40b14f322
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
This commit is contained in:
Theodore Ts'o
2015-05-01 16:56:50 -04:00
parent 5de0b4d0cd
commit a44cd7a054
5 changed files with 31 additions and 8 deletions
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11
fs/ext4/ext4_crypto.h
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@@ -20,12 +20,20 @@ struct ext4_encryption_policy {
char version;
char contents_encryption_mode;
char filenames_encryption_mode;
char flags;
char master_key_descriptor[EXT4_KEY_DESCRIPTOR_SIZE];
} __attribute__((__packed__));
#define EXT4_ENCRYPTION_CONTEXT_FORMAT_V1 1
#define EXT4_KEY_DERIVATION_NONCE_SIZE 16
#define EXT4_POLICY_FLAGS_PAD_4 0x00
#define EXT4_POLICY_FLAGS_PAD_8 0x01
#define EXT4_POLICY_FLAGS_PAD_16 0x02
#define EXT4_POLICY_FLAGS_PAD_32 0x03
#define EXT4_POLICY_FLAGS_PAD_MASK 0x03
#define EXT4_POLICY_FLAGS_VALID 0x03
/**
* Encryption context for inode
*
@@ -41,7 +49,7 @@ struct ext4_encryption_context {
char format;
char contents_encryption_mode;
char filenames_encryption_mode;
char reserved;
char flags;
char master_key_descriptor[EXT4_KEY_DESCRIPTOR_SIZE];
char nonce[EXT4_KEY_DERIVATION_NONCE_SIZE];
} __attribute__((__packed__));
@@ -120,6 +128,7 @@ struct ext4_fname_crypto_ctx {
struct crypto_hash *htfm;
struct page *workpage;
struct ext4_encryption_key key;
unsigned flags : 8;
unsigned has_valid_key : 1;
unsigned ctfm_key_is_ready : 1;
};