Messages in this thread | | | Date | Sun, 14 Nov 2021 09:16:13 -0500 | From | Sasha Levin <> | Subject | Re: [PATCH AUTOSEL 5.10 021/101] fscrypt: allow 256-bit master keys with AES-256-XTS |
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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
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