ext4 crypto: reorganize how we store keys in the inode

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  This allows us to test to see if the key has been
   revoked; if it has, we prevent the use of the derived key and free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>

fs/ext4/ext4.h

@@ -955,7 +955,7 @@ struct ext4_inode_info {
 
 #ifdef CONFIG_EXT4_FS_ENCRYPTION
 	/* Encryption params */
-	struct ext4_crypt_info i_crypt_info;
+	struct ext4_crypt_info *i_crypt_info;
 #endif
 };
 
@@ -2096,37 +2096,30 @@ static inline int ext4_sb_has_crypto(struct super_block *sb)
 /* crypto_fname.c */
 bool ext4_valid_filenames_enc_mode(uint32_t mode);
 u32 ext4_fname_crypto_round_up(u32 size, u32 blksize);
-int ext4_fname_crypto_alloc_buffer(struct ext4_fname_crypto_ctx *ctx,
+int ext4_fname_crypto_alloc_buffer(struct inode *inode,
 				   u32 ilen, struct ext4_str *crypto_str);
-int _ext4_fname_disk_to_usr(struct ext4_fname_crypto_ctx *ctx,
+int _ext4_fname_disk_to_usr(struct inode *inode,
 			    struct dx_hash_info *hinfo,
 			    const struct ext4_str *iname,
 			    struct ext4_str *oname);
-int ext4_fname_disk_to_usr(struct ext4_fname_crypto_ctx *ctx,
+int ext4_fname_disk_to_usr(struct inode *inode,
 			   struct dx_hash_info *hinfo,
 			   const struct ext4_dir_entry_2 *de,
 			   struct ext4_str *oname);
-int ext4_fname_usr_to_disk(struct ext4_fname_crypto_ctx *ctx,
+int ext4_fname_usr_to_disk(struct inode *inode,
 			   const struct qstr *iname,
 			   struct ext4_str *oname);
-int ext4_fname_crypto_namelen_on_disk(struct ext4_fname_crypto_ctx *ctx,
-				      u32 namelen);
 #ifdef CONFIG_EXT4_FS_ENCRYPTION
-void ext4_put_fname_crypto_ctx(struct ext4_fname_crypto_ctx **ctx);
-struct ext4_fname_crypto_ctx *ext4_get_fname_crypto_ctx(struct inode *inode,
-							u32 max_len);
+int ext4_setup_fname_crypto(struct inode *inode);
 void ext4_fname_crypto_free_buffer(struct ext4_str *crypto_str);
 int ext4_fname_setup_filename(struct inode *dir, const struct qstr *iname,
 			      int lookup, struct ext4_filename *fname);
 void ext4_fname_free_filename(struct ext4_filename *fname);
 #else
 static inline
-void ext4_put_fname_crypto_ctx(struct ext4_fname_crypto_ctx **ctx) { }
-static inline
-struct ext4_fname_crypto_ctx *ext4_get_fname_crypto_ctx(struct inode *inode,
-							u32 max_len)
+int ext4_setup_fname_crypto(struct inode *inode)
 {
-	return NULL;
+	return 0;
 }
 static inline void ext4_fname_crypto_free_buffer(struct ext4_str *p) { }
 static inline int ext4_fname_setup_filename(struct inode *dir,
@@ -2143,15 +2136,34 @@ static inline void ext4_fname_free_filename(struct ext4_filename *fname) { }
 
 
 /* crypto_key.c */
-int ext4_generate_encryption_key(struct inode *inode);
+void ext4_free_encryption_info(struct inode *inode);
+int _ext4_get_encryption_info(struct inode *inode);
 
 #ifdef CONFIG_EXT4_FS_ENCRYPTION
 int ext4_has_encryption_key(struct inode *inode);
+
+static inline int ext4_get_encryption_info(struct inode *inode)
+{
+	struct ext4_crypt_info *ci = EXT4_I(inode)->i_crypt_info;
+
+	if (!ci ||
+	    (ci->ci_keyring_key &&
+	     (ci->ci_keyring_key->flags & ((1 << KEY_FLAG_INVALIDATED) |
+					   (1 << KEY_FLAG_REVOKED) |
+					   (1 << KEY_FLAG_DEAD)))))
+		return _ext4_get_encryption_info(inode);
+	return 0;
+}
+
 #else
 static inline int ext4_has_encryption_key(struct inode *inode)
 {
 	return 0;
 }
+static inline int ext4_get_encryption_info(struct inode *inode)
+{
+	return 0;
+}
 #endif