On Mon, Nov 08, 2021 at 12:47:11PM -0500, Sasha Levin wrote: > From: Eric Biggers <ebiggers@xxxxxxxxxx> > > [ 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@xxxxxxxxxx> > Link: https://lore.kernel.org/r/20210921030303.5598-1-ebiggers@xxxxxxxxxx > Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx> > Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> 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. - Eric
