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ext4 crypto: reorganize how we store keys in the inode

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This is a pretty massive patch which does a number of different things:

1) The per-inode encryption information is now stored in an allocated
   data structure, ext4_crypt_info, instead of directly in the node.
   This reduces the size usage of an in-memory inode when it is not
   using encryption.

2) We drop the ext4_fname_crypto_ctx entirely, and use the per-inode
   encryption structure instead.  This remove an unnecessary memory
   allocation and free for the fname_crypto_ctx as well as allowing us
   to reuse the ctfm in a directory for multiple lookups and file
   creations.

3) We also cache the inode's policy information in the ext4_crypt_info
   structure so we don't have to continually read it out of the
   extended attributes.

4) We now keep the keyring key in the inode's encryption structure
   instead of releasing it after we are done using it to derive the
   per-inode key.  This allows us to test to see if the key has been
   revoked; if it has, we prevent the use of the derived key and free
   it.

5) When an inode is released (or when the derived key is freed), we
   will use memset_explicit() to zero out the derived key, so it's not
   left hanging around in memory.  This implies that when a user logs
   out, it is important to first revoke the key, and then unlink it,
   and then finally, to use "echo 3 > /proc/sys/vm/drop_caches" to
   release any decrypted pages and dcache entries from the system
   caches.

6) All this, and we also shrink the number of lines of code by around
   100.  :-)

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
This commit is contained in:
Theodore Ts'o
2015-05-18 13:17:47 -04:00
parent e2881b1b51
commit b7236e21d5
11 changed files with 245 additions and 345 deletions
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16
fs/ext4/ext4_crypto.h
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@@ -76,7 +76,13 @@ struct ext4_encryption_key {
struct ext4_crypt_info {
unsigned char ci_mode;
unsigned char ci_size;
char ci_data_mode;
char ci_filename_mode;
char ci_flags;
struct crypto_ablkcipher *ci_ctfm;
struct key *ci_keyring_key;
char ci_raw[EXT4_MAX_KEY_SIZE];
char ci_master_key[EXT4_KEY_DESCRIPTOR_SIZE];
};
#define EXT4_CTX_REQUIRES_FREE_ENCRYPT_FL 0x00000001
@@ -128,16 +134,6 @@ struct ext4_str {
u32 len;
};
struct ext4_fname_crypto_ctx {
u32 lim;
struct crypto_ablkcipher *ctfm;
struct crypto_hash *htfm;
struct ext4_crypt_info ci;
unsigned flags : 8;
unsigned has_valid_key : 1;
unsigned ctfm_key_is_ready : 1;
};
/**
* For encrypted symlinks, the ciphertext length is stored at the beginning
* of the string in little-endian format.