Re: [PATCH 1/3] ext4: require key for truncate(2) of encrypted file

[Eric Biggers <ebiggers3@gmail.com>]

Hi Christoph,

On Tue, Jun 13, 2017 at 11:52:10PM -0700, Christoph Hellwig wrote:
> On Tue, Jun 13, 2017 at 04:47:53PM -0700, Eric Biggers wrote:
> > From: Eric Biggers <ebiggers@google.com>
> > 
> > Currently, filesystems allow truncate(2) on an encrypted file without
> > the encryption key.  However, it's impossible to correctly handle the
> > case where the size being truncated to is not a multiple of the
> > filesystem block size, because that would require decrypting the final
> > block, zeroing the part beyond i_size, then encrypting the block.
> > 
> > As other modifications to encrypted file contents are prohibited without
> > the key, just prohibit truncate(2) as well, making it fail with ENOKEY.
> 
> What about hole punches?  What about fallocate which just adds zeroes
> but still changes the content.  What about insert or collapse range?

None of those are allowed because fallocate() requires a file descriptor, and
open() fails with ENOKEY if the encryption key is not available.  truncate() is
different because it takes a path, not a file descriptor.

Eric