fscrypto/f2fs: allow fs-specific key prefix for fs encryption

This patch allows fscrypto to handle a second key prefix given by filesystem.
The main reason is to provide backward compatibility, since previously f2fs
used "f2fs:" as a crypto prefix instead of "fscrypt:".
Later, ext4 should also provide key_prefix() to give "ext4:".

One concern decribed by Ted would be kinda double check overhead of prefixes.
In x86, for example, validate_user_key consumes 8 ms after boot-up, which turns
out derive_key_aes() consumed most of the time to load specific crypto module.
After such the cold miss, it shows almost zero latencies, which treats as a
negligible overhead.
Note that request_key() detects wrong prefix in prior to derive_key_aes() even.

Cc: Ted Tso <tytso@mit.edu>
Cc: stable@vger.kernel.org # v4.6
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

1 files changed, 13 insertions, 0 deletions

diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
index 8a28f79..28c8992 100644
--- a/fs/f2fs/super.c
+++ b/fs/f2fs/super.c
@@ -964,6 +964,12 @@ static int f2fs_get_context(struct inode *inode, void *ctx, size_t len)
 				ctx, len, NULL);
 }
 
+static int f2fs_key_prefix(struct inode *inode, u8 **key)
+{
+	*key = F2FS_I_SB(inode)->key_prefix;
+	return F2FS_I_SB(inode)->key_prefix_size;
+}
+
 static int f2fs_set_context(struct inode *inode, const void *ctx, size_t len,
 							void *fs_data)
 {
@@ -980,6 +986,7 @@ static unsigned f2fs_max_namelen(struct inode *inode)
 
 static struct fscrypt_operations f2fs_cryptops = {
 	.get_context	= f2fs_get_context,
+	.key_prefix	= f2fs_key_prefix,
 	.set_context	= f2fs_set_context,
 	.is_encrypted	= f2fs_encrypted_inode,
 	.empty_dir	= f2fs_empty_dir,
@@ -1305,6 +1312,12 @@ static void init_sb_info(struct f2fs_sb_info *sbi)
 
 	INIT_LIST_HEAD(&sbi->s_list);
 	mutex_init(&sbi->umount_mutex);
+
+#ifdef CONFIG_F2FS_FS_ENCRYPTION
+	memcpy(sbi->key_prefix, F2FS_KEY_DESC_PREFIX,
+				F2FS_KEY_DESC_PREFIX_SIZE);
+	sbi->key_prefix_size = F2FS_KEY_DESC_PREFIX_SIZE;
+#endif
 }
 
 /*