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

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Message-ID: <20170614070357.GB605@zzz>
Date:   Wed, 14 Jun 2017 00:03:57 -0700
From:   Eric Biggers <ebiggers3@...il.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     linux-fscrypt@...r.kernel.org, Theodore Ts'o <tytso@....edu>,
        linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-mtd@...ts.infradead.org, Eric Biggers <ebiggers@...gle.com>
Subject: Re: [PATCH 1/3] ext4: require key for truncate(2) of encrypted file

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@...gle.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