Re: [PATCH] fscrypt: restrict IV_INO_LBLK_* to AES-256-XTS

[Eric Biggers <ebiggers@kernel.org>]

On Tue, Jul 21, 2020 at 11:10:12AM -0700, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
> 
> IV_INO_LBLK_* exist only because of hardware limitations, and currently
> the only known use case for them involves AES-256-XTS.  Therefore, for
> now only allow them in combination with AES-256-XTS.  This way we don't
> have to worry about them being combined with other encryption modes.
> 
> (To be clear, combining IV_INO_LBLK_* with other encryption modes
> *should* work just fine.  It's just not being tested, so we can't be
> 100% sure it works.  So with no known use case, it's best to disallow it
> for now, just like we don't allow other weird combinations like
> AES-256-XTS contents encryption with Adiantum filenames encryption.)
> 
> This can be relaxed later if a use case for other combinations arises.
> 
> Fixes: b103fb7653ff ("fscrypt: add support for IV_INO_LBLK_64 policies")
> Fixes: e3b1078bedd3 ("fscrypt: add support for IV_INO_LBLK_32 policies")
> Signed-off-by: Eric Biggers <ebiggers@google.com>
> ---
>  fs/crypto/policy.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/fs/crypto/policy.c b/fs/crypto/policy.c
> index 8a8ad0e44bb8..8e667aadf271 100644
> --- a/fs/crypto/policy.c
> +++ b/fs/crypto/policy.c
> @@ -77,6 +77,20 @@ static bool supported_iv_ino_lblk_policy(const struct fscrypt_policy_v2 *policy,
>  	struct super_block *sb = inode->i_sb;
>  	int ino_bits = 64, lblk_bits = 64;
>  
> +	/*
> +	 * IV_INO_LBLK_* exist only because of hardware limitations, and
> +	 * currently the only known use case for them involves AES-256-XTS.
> +	 * That's also all we test currently.  For these reasons, for now only
> +	 * allow AES-256-XTS here.  This can be relaxed later if a use case for
> +	 * IV_INO_LBLK_* with other encryption modes arises.
> +	 */
> +	if (policy->contents_encryption_mode != FSCRYPT_MODE_AES_256_XTS) {
> +		fscrypt_warn(inode,
> +			     "Can't use %s policy with contents mode other than AES-256-XTS",
> +			     type);
> +		return false;
> +	}
> +
>  	/*
>  	 * It's unsafe to include inode numbers in the IVs if the filesystem can
>  	 * potentially renumber inodes, e.g. via filesystem shrinking.
> -- 

Applied to fscrypt.git#master for 5.9.

- Eric