Re: [PATCH 1/4] fscrypt: add FS_IOC_GET_ENCRYPTION_NONCE ioctl

From: "Theodore Y. Ts'o" <tytso@mit.edu>
To: Eric Biggers <ebiggers@kernel.org>
Cc: linux-fscrypt@vger.kernel.org, linux-ext4@vger.kernel.org,
	linux-f2fs-devel@lists.sourceforge.net,
	linux-mtd@lists.infradead.org, linux-fsdevel@vger.kernel.org,
	linux-api@vger.kernel.org
Subject: Re: [PATCH 1/4] fscrypt: add FS_IOC_GET_ENCRYPTION_NONCE ioctl
Date: Thu, 19 Mar 2020 16:19:14 -0400	[thread overview]
Message-ID: <20200319201914.GD1067245@mit.edu> (raw)
In-Reply-To: <20200314205052.93294-2-ebiggers@kernel.org>

On Sat, Mar 14, 2020 at 01:50:49PM -0700, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
> 
> Add an ioctl FS_IOC_GET_ENCRYPTION_NONCE which retrieves the nonce from
> an encrypted file or directory.  The nonce is the 16-byte random value
> stored in the inode's encryption xattr.  It is normally used together
> with the master key to derive the inode's actual encryption key.
> 
> The nonces are needed by automated tests that verify the correctness of
> the ciphertext on-disk.  Except for the IV_INO_LBLK_64 case, there's no
> way to replicate a file's ciphertext without knowing that file's nonce.
> 
> The nonces aren't secret, and the existing ciphertext verification tests
> in xfstests retrieve them from disk using debugfs or dump.f2fs.  But in
> environments that lack these debugging tools, getting the nonces by
> manually parsing the filesystem structure would be very hard.
> 
> To make this important type of testing much easier, let's just add an
> ioctl that retrieves the nonce.
> 
> Signed-off-by: Eric Biggers <ebiggers@google.com>

Reviewed-by: Theodore Ts'o <tytso@mit.edu>