From patchwork Fri Jul 26 22:41:34 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Eric Biggers X-Patchwork-Id: 11061885 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id D9353746 for ; Fri, 26 Jul 2019 22:46:03 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id CC16A28B4C for ; Fri, 26 Jul 2019 22:46:03 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id C07F728B5D; Fri, 26 Jul 2019 22:46:03 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.0 required=2.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,RCVD_IN_DNSWL_HI autolearn=ham version=3.3.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 0A27228B4C for ; Fri, 26 Jul 2019 22:46:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2387571AbfGZWqC (ORCPT ); Fri, 26 Jul 2019 18:46:02 -0400 Received: from mail.kernel.org ([198.145.29.99]:52488 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2387535AbfGZWqB (ORCPT ); Fri, 26 Jul 2019 18:46:01 -0400 Received: from sol.localdomain (c-24-5-143-220.hsd1.ca.comcast.net [24.5.143.220]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id D805722CD1; Fri, 26 Jul 2019 22:45:59 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1564181160; bh=mErALqCTk6E120NHiNPGCl67c6DS3WtUjVeqDW8HkQQ=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=aP8xwJngaLfWwxnNi43kE2r6HE30luj1Xir4YUV41o2RfNBHukBMA3lBA42ErawIX Bb4VzINmo5VgMgyKm34WCQ/h4y73MFTO94loqsDjz+XHzJkfaJv734Db3L9bXUq8rh 8hSJk/rJPMqXtr0XbHGyP3aNSH1sFQtsPg2LasWg= From: Eric Biggers To: linux-fscrypt@vger.kernel.org Cc: linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, linux-mtd@lists.infradead.org, linux-api@vger.kernel.org, linux-crypto@vger.kernel.org, keyrings@vger.kernel.org, Paul Crowley , Satya Tangirala Subject: [PATCH v7 09/16] fscrypt: add an HKDF-SHA512 implementation Date: Fri, 26 Jul 2019 15:41:34 -0700 Message-Id: <20190726224141.14044-10-ebiggers@kernel.org> X-Mailer: git-send-email 2.22.0 In-Reply-To: <20190726224141.14044-1-ebiggers@kernel.org> References: <20190726224141.14044-1-ebiggers@kernel.org> MIME-Version: 1.0 Sender: linux-fscrypt-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fscrypt@vger.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP From: Eric Biggers Add an implementation of HKDF (RFC 5869) to fscrypt, for the purpose of deriving additional key material from the fscrypt master keys for v2 encryption policies. HKDF is a key derivation function built on top of HMAC. We choose SHA-512 for the underlying unkeyed hash, and use an "hmac(sha512)" transform allocated from the crypto API. We'll be using this to replace the AES-ECB based KDF currently used to derive the per-file encryption keys. While the AES-ECB based KDF is believed to meet the original security requirements, it is nonstandard and has problems that don't exist in modern KDFs such as HKDF: 1. It's reversible. Given a derived key and nonce, an attacker can easily compute the master key. This is okay if the master key and derived keys are equally hard to compromise, but now we'd like to be more robust against threats such as a derived key being compromised through a timing attack, or a derived key for an in-use file being compromised after the master key has already been removed. 2. It doesn't evenly distribute the entropy from the master key; each 16 input bytes only affects the corresponding 16 output bytes. 3. It isn't easily extensible to deriving other values or keys, such as a public hash for securely identifying the key, or per-mode keys. Per-mode keys will be immediately useful for Adiantum encryption, for which fscrypt currently uses the master key directly, introducing unnecessary usage constraints. Per-mode keys will also be useful for hardware inline encryption, which is currently being worked on. HKDF solves all the above problems. Signed-off-by: Eric Biggers Reviewed-by: Theodore Ts'o --- fs/crypto/Kconfig | 2 + fs/crypto/Makefile | 1 + fs/crypto/fscrypt_private.h | 15 +++ fs/crypto/hkdf.c | 181 ++++++++++++++++++++++++++++++++++++ 4 files changed, 199 insertions(+) create mode 100644 fs/crypto/hkdf.c diff --git a/fs/crypto/Kconfig b/fs/crypto/Kconfig index 5fdf24877c178..ff5a1746cbae4 100644 --- a/fs/crypto/Kconfig +++ b/fs/crypto/Kconfig @@ -7,6 +7,8 @@ config FS_ENCRYPTION select CRYPTO_ECB select CRYPTO_XTS select CRYPTO_CTS + select CRYPTO_SHA512 + select CRYPTO_HMAC select KEYS help Enable encryption of files and directories. This diff --git a/fs/crypto/Makefile b/fs/crypto/Makefile index a640d486800da..3333e2ac859fa 100644 --- a/fs/crypto/Makefile +++ b/fs/crypto/Makefile @@ -3,6 +3,7 @@ obj-$(CONFIG_FS_ENCRYPTION) += fscrypto.o fscrypto-y := crypto.o \ fname.o \ + hkdf.o \ hooks.o \ keyring.o \ keysetup.o \ diff --git a/fs/crypto/fscrypt_private.h b/fs/crypto/fscrypt_private.h index 3616232b4798e..92567efec2cd5 100644 --- a/fs/crypto/fscrypt_private.h +++ b/fs/crypto/fscrypt_private.h @@ -172,6 +172,21 @@ extern bool fscrypt_fname_encrypted_size(const struct inode *inode, u32 orig_len, u32 max_len, u32 *encrypted_len_ret); +/* hkdf.c */ + +struct fscrypt_hkdf { + struct crypto_shash *hmac_tfm; +}; + +extern int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key, + unsigned int master_key_size); + +extern int fscrypt_hkdf_expand(struct fscrypt_hkdf *hkdf, u8 context, + const u8 *info, unsigned int infolen, + u8 *okm, unsigned int okmlen); + +extern void fscrypt_destroy_hkdf(struct fscrypt_hkdf *hkdf); + /* keyring.c */ /* diff --git a/fs/crypto/hkdf.c b/fs/crypto/hkdf.c new file mode 100644 index 0000000000000..f21873e1b4674 --- /dev/null +++ b/fs/crypto/hkdf.c @@ -0,0 +1,181 @@ +// SPDX-License-Identifier: GPL-2.0 +/* + * Implementation of HKDF ("HMAC-based Extract-and-Expand Key Derivation + * Function"), aka RFC 5869. See also the original paper (Krawczyk 2010): + * "Cryptographic Extraction and Key Derivation: The HKDF Scheme". + * + * This is used to derive keys from the fscrypt master keys. + * + * Copyright 2019 Google LLC + */ + +#include +#include + +#include "fscrypt_private.h" + +/* + * HKDF supports any unkeyed cryptographic hash algorithm, but fscrypt uses + * SHA-512 because it is reasonably secure and efficient; and since it produces + * a 64-byte digest, deriving an AES-256-XTS key preserves all 64 bytes of + * entropy from the master key and requires only one iteration of HKDF-Expand. + */ +#define HKDF_HMAC_ALG "hmac(sha512)" +#define HKDF_HASHLEN SHA512_DIGEST_SIZE + +/* + * HKDF consists of two steps: + * + * 1. HKDF-Extract: extract a pseudorandom key of length HKDF_HASHLEN bytes from + * the input keying material and optional salt. + * 2. HKDF-Expand: expand the pseudorandom key into output keying material of + * any length, parameterized by an application-specific info string. + * + * HKDF-Extract can be skipped if the input is already a pseudorandom key of + * length HKDF_HASHLEN bytes. However, cipher modes other than AES-256-XTS take + * shorter keys, and we don't want to force users of those modes to provide + * unnecessarily long master keys. Thus fscrypt still does HKDF-Extract. No + * salt is used, since fscrypt master keys should already be pseudorandom and + * there's no way to persist a random salt per master key from kernel mode. + */ + +/* HKDF-Extract (RFC 5869 section 2.2), unsalted */ +static int hkdf_extract(struct crypto_shash *hmac_tfm, const u8 *ikm, + unsigned int ikmlen, u8 prk[HKDF_HASHLEN]) +{ + static const u8 default_salt[HKDF_HASHLEN]; + SHASH_DESC_ON_STACK(desc, hmac_tfm); + int err; + + err = crypto_shash_setkey(hmac_tfm, default_salt, HKDF_HASHLEN); + if (err) + return err; + + desc->tfm = hmac_tfm; + err = crypto_shash_digest(desc, ikm, ikmlen, prk); + shash_desc_zero(desc); + return err; +} + +/* + * Compute HKDF-Extract using the given master key as the input keying material, + * and prepare an HMAC transform object keyed by the resulting pseudorandom key. + * + * Afterwards, the keyed HMAC transform object can be used for HKDF-Expand many + * times without having to recompute HKDF-Extract each time. + */ +int fscrypt_init_hkdf(struct fscrypt_hkdf *hkdf, const u8 *master_key, + unsigned int master_key_size) +{ + struct crypto_shash *hmac_tfm; + u8 prk[HKDF_HASHLEN]; + int err; + + hmac_tfm = crypto_alloc_shash(HKDF_HMAC_ALG, 0, 0); + if (IS_ERR(hmac_tfm)) { + fscrypt_err(NULL, "Error allocating " HKDF_HMAC_ALG ": %ld", + PTR_ERR(hmac_tfm)); + return PTR_ERR(hmac_tfm); + } + + if (WARN_ON(crypto_shash_digestsize(hmac_tfm) != sizeof(prk))) { + err = -EINVAL; + goto err_free_tfm; + } + + err = hkdf_extract(hmac_tfm, master_key, master_key_size, prk); + if (err) + goto err_free_tfm; + + err = crypto_shash_setkey(hmac_tfm, prk, sizeof(prk)); + if (err) + goto err_free_tfm; + + hkdf->hmac_tfm = hmac_tfm; + goto out; + +err_free_tfm: + crypto_free_shash(hmac_tfm); +out: + memzero_explicit(prk, sizeof(prk)); + return err; +} + +/* + * HKDF-Expand (RFC 5869 section 2.3). This expands the pseudorandom key, which + * was already keyed into 'hkdf->hmac_tfm' by fscrypt_init_hkdf(), into 'okmlen' + * bytes of output keying material parameterized by the application-specific + * 'info' of length 'infolen' bytes, prefixed by "fscrypt\0" and the 'context' + * byte. This is thread-safe and may be called by multiple threads in parallel. + * + * ('context' isn't part of the HKDF specification; it's just a prefix fscrypt + * adds to its application-specific info strings to guarantee that it doesn't + * accidentally repeat an info string when using HKDF for different purposes.) + */ +int fscrypt_hkdf_expand(struct fscrypt_hkdf *hkdf, u8 context, + const u8 *info, unsigned int infolen, + u8 *okm, unsigned int okmlen) +{ + SHASH_DESC_ON_STACK(desc, hkdf->hmac_tfm); + u8 prefix[9]; + unsigned int i; + int err; + const u8 *prev = NULL; + u8 counter = 1; + u8 tmp[HKDF_HASHLEN]; + + if (WARN_ON(okmlen > 255 * HKDF_HASHLEN)) + return -EINVAL; + + desc->tfm = hkdf->hmac_tfm; + + memcpy(prefix, "fscrypt\0", 8); + prefix[8] = context; + + for (i = 0; i < okmlen; i += HKDF_HASHLEN) { + + err = crypto_shash_init(desc); + if (err) + goto out; + + if (prev) { + err = crypto_shash_update(desc, prev, HKDF_HASHLEN); + if (err) + goto out; + } + + err = crypto_shash_update(desc, prefix, sizeof(prefix)); + if (err) + goto out; + + err = crypto_shash_update(desc, info, infolen); + if (err) + goto out; + + BUILD_BUG_ON(sizeof(counter) != 1); + if (okmlen - i < HKDF_HASHLEN) { + err = crypto_shash_finup(desc, &counter, 1, tmp); + if (err) + goto out; + memcpy(&okm[i], tmp, okmlen - i); + memzero_explicit(tmp, sizeof(tmp)); + } else { + err = crypto_shash_finup(desc, &counter, 1, &okm[i]); + if (err) + goto out; + } + counter++; + prev = &okm[i]; + } + err = 0; +out: + if (unlikely(err)) + memzero_explicit(okm, okmlen); /* so caller doesn't need to */ + shash_desc_zero(desc); + return err; +} + +void fscrypt_destroy_hkdf(struct fscrypt_hkdf *hkdf) +{ + crypto_free_shash(hkdf->hmac_tfm); +}