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  This allows us to test to see if the key has been
   revoked; if it has, we prevent the use of the derived key and free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

1 files changed, 57 insertions, 17 deletions

diff --git a/fs/ext4/crypto_key.c b/fs/ext4/crypto_key.c
index ec6635dc50f9..0075e43ffea6 100644
--- a/fs/ext4/crypto_key.c
+++ b/fs/ext4/crypto_key.c
@@ -84,14 +84,26 @@ out:
         return res;
 }
 
-/**
- * ext4_generate_encryption_key() - generates an encryption key
- * @inode: The inode to generate the encryption key for.
- */
-int ext4_generate_encryption_key(struct inode *inode)
+void ext4_free_encryption_info(struct inode *inode)
+{
+        struct ext4_inode_info *ei = EXT4_I(inode);
+        struct ext4_crypt_info *ci = ei->i_crypt_info;
+
+        if (!ci)
+                return;
+
+        if (ci->ci_keyring_key)
+                key_put(ci->ci_keyring_key);
+        crypto_free_ablkcipher(ci->ci_ctfm);
+        memzero_explicit(&ci->ci_raw, sizeof(ci->ci_raw));
+        kfree(ci);
+        ei->i_crypt_info = NULL;
+}
+
+int _ext4_get_encryption_info(struct inode *inode)
 {
         struct ext4_inode_info *ei = EXT4_I(inode);
-        struct ext4_crypt_info *crypt_info = &ei->i_crypt_info;
+        struct ext4_crypt_info *crypt_info;
         char full_key_descriptor[EXT4_KEY_DESC_PREFIX_SIZE +
                                  (EXT4_KEY_DESCRIPTOR_SIZE * 2) + 1];
         struct key *keyring_key = NULL;
@@ -99,18 +111,40 @@ int ext4_generate_encryption_key(struct inode *inode)
         struct ext4_encryption_context ctx;
         struct user_key_payload *ukp;
         struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
-        int res = ext4_xattr_get(inode, EXT4_XATTR_INDEX_ENCRYPTION,
-                                 EXT4_XATTR_NAME_ENCRYPTION_CONTEXT,
-                                 &ctx, sizeof(ctx));
+        int res;
 
-        if (res != sizeof(ctx)) {
-                if (res > 0)
-                        res = -EINVAL;
-                goto out;
+        if (ei->i_crypt_info) {
+                if (!ei->i_crypt_info->ci_keyring_key ||
+                    key_validate(ei->i_crypt_info->ci_keyring_key) == 0)
+                        return 0;
+                ext4_free_encryption_info(inode);
         }
+
+        res = ext4_xattr_get(inode, EXT4_XATTR_INDEX_ENCRYPTION,
+                                 EXT4_XATTR_NAME_ENCRYPTION_CONTEXT,
+                                 &ctx, sizeof(ctx));
+        if (res < 0) {
+                if (!DUMMY_ENCRYPTION_ENABLED(sbi))
+                        return res;
+                ctx.contents_encryption_mode = EXT4_ENCRYPTION_MODE_AES_256_XTS;
+                ctx.filenames_encryption_mode =
+                        EXT4_ENCRYPTION_MODE_AES_256_CTS;
+                ctx.flags = 0;
+        } else if (res != sizeof(ctx))
+                return -EINVAL;
         res = 0;
 
+        crypt_info = kmalloc(sizeof(struct ext4_crypt_info), GFP_KERNEL);
+        if (!crypt_info)
+                return -ENOMEM;
+
         ei->i_crypt_policy_flags = ctx.flags;
+        crypt_info->ci_flags = ctx.flags;
+        crypt_info->ci_data_mode = ctx.contents_encryption_mode;
+        crypt_info->ci_filename_mode = ctx.filenames_encryption_mode;
+        crypt_info->ci_ctfm = NULL;
+        memcpy(crypt_info->ci_master_key, ctx.master_key_descriptor,
+               sizeof(crypt_info->ci_master_key));
         if (S_ISREG(inode->i_mode))
                 crypt_info->ci_mode = ctx.contents_encryption_mode;
         else if (S_ISDIR(inode->i_mode) || S_ISLNK(inode->i_mode))
@@ -151,17 +185,23 @@ int ext4_generate_encryption_key(struct inode *inode)
         res = ext4_derive_key_aes(ctx.nonce, master_key->raw,
                                   crypt_info->ci_raw);
 out:
+        if (res < 0) {
+                if (res == -ENOKEY)
+                        res = 0;
+                kfree(crypt_info);
+        } else {
+                ei->i_crypt_info = crypt_info;
+                crypt_info->ci_keyring_key = keyring_key;
+                keyring_key = NULL;
+        }
         if (keyring_key)
                 key_put(keyring_key);
-        if (res < 0)
-                crypt_info->ci_mode = EXT4_ENCRYPTION_MODE_INVALID;
         return res;
 }
 
 int ext4_has_encryption_key(struct inode *inode)
 {
         struct ext4_inode_info *ei = EXT4_I(inode);
-        struct ext4_crypt_info *crypt_info = &ei->i_crypt_info;
 
-        return (crypt_info->ci_mode != EXT4_ENCRYPTION_MODE_INVALID);
+        return (ei->i_crypt_info != NULL);
 }