Re: [PATCH AUTOSEL 5.10 021/101] fscrypt: allow 256-bit master keys with AES-256-XTS

[Sasha Levin <sashal@kernel.org>]

On Mon, Nov 08, 2021 at 05:48:40PM -0800, Eric Biggers wrote:
>On Mon, Nov 08, 2021 at 12:47:11PM -0500, Sasha Levin wrote:
>> From: Eric Biggers <ebiggers@google.com>
>>
>> [ Upstream commit 7f595d6a6cdc336834552069a2e0a4f6d4756ddf ]
>>
>> fscrypt currently requires a 512-bit master key when AES-256-XTS is
>> used, since AES-256-XTS keys are 512-bit and fscrypt requires that the
>> master key be at least as long any key that will be derived from it.
>>
>> However, this is overly strict because AES-256-XTS doesn't actually have
>> a 512-bit security strength, but rather 256-bit.  The fact that XTS
>> takes twice the expected key size is a quirk of the XTS mode.  It is
>> sufficient to use 256 bits of entropy for AES-256-XTS, provided that it
>> is first properly expanded into a 512-bit key, which HKDF-SHA512 does.
>>
>> Therefore, relax the check of the master key size to use the security
>> strength of the derived key rather than the size of the derived key
>> (except for v1 encryption policies, which don't use HKDF).
>>
>> Besides making things more flexible for userspace, this is needed in
>> order for the use of a KDF which only takes a 256-bit key to be
>> introduced into the fscrypt key hierarchy.  This will happen with
>> hardware-wrapped keys support, as all known hardware which supports that
>> feature uses an SP800-108 KDF using AES-256-CMAC, so the wrapped keys
>> are wrapped 256-bit AES keys.  Moreover, there is interest in fscrypt
>> supporting the same type of AES-256-CMAC based KDF in software as an
>> alternative to HKDF-SHA512.  There is no security problem with such
>> features, so fix the key length check to work properly with them.
>>
>> Reviewed-by: Paul Crowley <paulcrowley@google.com>
>> Link: https://lore.kernel.org/r/20210921030303.5598-1-ebiggers@kernel.org
>> Signed-off-by: Eric Biggers <ebiggers@google.com>
>> Signed-off-by: Sasha Levin <sashal@kernel.org>
>
>I don't expect any problem with backporting this, but I don't see how this
>follows the stable kernel rules (Documentation/process/stable-kernel-rules.rst).
>I don't see what distinguishes this patch from ones that don't get picked up by
>AUTOSEL; it seems pretty arbitrary to me.

It is, to some extent. My understanding was that this is a minor fix to
make something that should have worked, work.

-- 
Thanks,
Sasha