This patches allow backing up encrypted files without having access to the key. Unfortunately, the key *is* necessary to restore the files, because establishing a link means that we have to manipulate the both the encrypted directory and the encrypted file, and doing this through the VFS interface is non-trivial. So we have an ioctl which extracts the encrypted file name, and that in combination with the encryption metadata for the directory should be sufficient to restore the file name assuming the restore is done with access to the user's master key. The other tricky bit is that if the file's i_size is not a multiple of the AES block size, we need to be able to copy a handful of bytes before i_size --- and O_DIRECT reads don't allow that. There are two ways of solving this. One would be an new DIO_FLAG that rounds i_size up to the file system blocksize, which we would pass when reading encrypted files using O_DIRECT. This would require changes to the core direct I/O, and may be controversial. It also may make it more difficult to back port these patches to ancient BSP kernels. So what we're doing for now is admittedly a hack. Since encrypted files are read-only without access to the key, it is safe to create a shadow copy of the inode structure, and round up i_size in the shadow structure. We only do this when reading the last block in the file, so the overhead shouldn't be too bad. The process doing the store will need to truncate the file back down to the original file when it has access to the key. We can't do this without the key because the kernel zero fills the block between i_size and the end of the block. (This is also why it's not a security issue to round i_size up to the end of the block; there is no chance we will be revealing stale data.) So this means that for the purposes of doing the encrypted backup, the backup will need to store the encrypted file name, the directory's encrypted metadata, and the original i_size in some convenient ouf-of-band backup metadata store. Theodore Ts'o (3): ext4 crypto: add ciphertext_access mount option ext4 crypto: add ioctls to allow backup of encryption metadata ext4 crypto: add missing locking for keyring_key access fs/ext4/crypto_key.c | 61 +++++++++++++++++++++++++++++++++++++++++++++++- fs/ext4/ext4.h | 12 ++++++++++ fs/ext4/ext4_crypto.h | 8 +++++++ fs/ext4/file.c | 5 +++- fs/ext4/indirect.c | 24 +++++++++++++++---- fs/ext4/inode.c | 17 ++++++++------ fs/ext4/ioctl.c | 64 +++++++++++++++++++++++++++++++++++++++++++++++++++ fs/ext4/namei.c | 24 +++++++++++++++++++ fs/ext4/super.c | 48 ++++++++++++++++++++++++++++++++++++++ 9 files changed, 249 insertions(+), 14 deletions(-) -- 2.5.0 -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
