From patchwork Mon Aug 5 16:25:13 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Eric Biggers X-Patchwork-Id: 11077389 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 4E2911398 for ; Mon, 5 Aug 2019 16:28:48 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 3FAEB2887E for ; Mon, 5 Aug 2019 16:28:48 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id 32FEC28893; Mon, 5 Aug 2019 16:28:48 +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=unavailable 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 80A3C288D0 for ; Mon, 5 Aug 2019 16:28:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730017AbfHEQ2k (ORCPT ); Mon, 5 Aug 2019 12:28:40 -0400 Received: from mail.kernel.org ([198.145.29.99]:60448 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729840AbfHEQ2h (ORCPT ); Mon, 5 Aug 2019 12:28:37 -0400 Received: from ebiggers-linuxstation.mtv.corp.google.com (unknown [104.132.1.77]) (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 000342187F; Mon, 5 Aug 2019 16:28:35 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1565022516; bh=0ls8jW8dCCf95aDr2AU1QCbKDs6ncwu2A9zTiU4BtRw=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=NUaFFyANM3Vh1ldyGJ0R9vG2Ljo8Aovu5CyjpxfCT+l34sX75Yt1fdd7tUjk5Y4u4 85CdVan5nakgL7o/9Xqs7cTvnlYOQ4641Yq5g6cpX44y8M1wB32Ij/zh1W3uiA3fXs TtZg0OBoUG8Hy0cj7h/OwQE5sZTtXawWvunQFjKw= From: Eric Biggers To: linux-fscrypt@vger.kernel.org Cc: linux-ext4@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, linux-mtd@lists.infradead.org, linux-fsdevel@vger.kernel.org, linux-crypto@vger.kernel.org, keyrings@vger.kernel.org, linux-api@vger.kernel.org, Satya Tangirala , Paul Crowley , Theodore Ts'o , Jaegeuk Kim Subject: [PATCH v8 12/20] fscrypt: add an HKDF-SHA512 implementation Date: Mon, 5 Aug 2019 09:25:13 -0700 Message-Id: <20190805162521.90882-13-ebiggers@kernel.org> X-Mailer: git-send-email 2.22.0.770.g0f2c4a37fd-goog In-Reply-To: <20190805162521.90882-1-ebiggers@kernel.org> References: <20190805162521.90882-1-ebiggers@kernel.org> MIME-Version: 1.0 Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@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. Reviewed-by: Theodore Ts'o Signed-off-by: Eric Biggers Reviewed-by: Paul Crowley --- 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 5fdf24877c1785..ff5a1746cbae4c 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 6b2485b4139335..232e2bb5a337b2 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 fc804f4a03fc92..9556e9499dc59c 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 00000000000000..f21873e1b46749 --- /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); +}