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![Daniel Rosenberg](/assets/img/avatar_default.png)
When an encrypted directory is listed without the key, the filesystem must show "no-key names" that uniquely identify directory entries, are at most 255 (NAME_MAX) bytes long, and don't contain '/' or '\0'. Currently, for short names the no-key name is the base64 encoding of the ciphertext filename, while for long names it's the base64 encoding of the ciphertext filename's dirhash and second-to-last 16-byte block. This format has the following problems: - Since it doesn't always include the dirhash, it's incompatible with directories that will use a secret-keyed dirhash over the plaintext filenames. In this case, the dirhash won't be computable from the ciphertext name without the key, so it instead must be retrieved from the directory entry and always included in the no-key name. Casefolded encrypted directories will use this type of dirhash. - It's ambiguous: it's possible to craft two filenames that map to the same no-key name, since the method used to abbreviate long filenames doesn't use a proper cryptographic hash function. Solve both these problems by switching to a new no-key name format that is the base64 encoding of a variable-length structure that contains the dirhash, up to 149 bytes of the ciphertext filename, and (if any bytes remain) the SHA-256 of the remaining bytes of the ciphertext filename. This ensures that each no-key name contains everything needed to find the directory entry again, contains only legal characters, doesn't exceed NAME_MAX, is unambiguous unless there's a SHA-256 collision, and that we only take the performance hit of SHA-256 on very long filenames. Note: this change does *not* address the existing issue where users can modify the 'dirhash' part of a no-key name and the filesystem may still accept the name. Signed-off-by: Daniel Rosenberg <drosen@google.com> [EB: improved comments and commit message, fixed checking return value of base64_decode(), check for SHA-256 error, continue to set disk_name for short names to keep matching simpler, and many other cleanups] Link: https://lore.kernel.org/r/20200120223201.241390-7-ebiggers@kernel.org Signed-off-by: Eric Biggers <ebiggers@google.com>
588 lines
18 KiB
C
588 lines
18 KiB
C
// SPDX-License-Identifier: GPL-2.0
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/*
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* This contains functions for filename crypto management
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*
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* Copyright (C) 2015, Google, Inc.
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* Copyright (C) 2015, Motorola Mobility
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*
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* Written by Uday Savagaonkar, 2014.
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* Modified by Jaegeuk Kim, 2015.
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*
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* This has not yet undergone a rigorous security audit.
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*/
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#include <linux/namei.h>
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#include <linux/scatterlist.h>
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#include <crypto/hash.h>
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#include <crypto/sha.h>
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#include <crypto/skcipher.h>
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#include "fscrypt_private.h"
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/**
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* struct fscrypt_nokey_name - identifier for directory entry when key is absent
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*
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* When userspace lists an encrypted directory without access to the key, the
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* filesystem must present a unique "no-key name" for each filename that allows
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* it to find the directory entry again if requested. Naively, that would just
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* mean using the ciphertext filenames. However, since the ciphertext filenames
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* can contain illegal characters ('\0' and '/'), they must be encoded in some
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* way. We use base64. But that can cause names to exceed NAME_MAX (255
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* bytes), so we also need to use a strong hash to abbreviate long names.
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*
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* The filesystem may also need another kind of hash, the "dirhash", to quickly
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* find the directory entry. Since filesystems normally compute the dirhash
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* over the on-disk filename (i.e. the ciphertext), it's not computable from
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* no-key names that abbreviate the ciphertext using the strong hash to fit in
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* NAME_MAX. It's also not computable if it's a keyed hash taken over the
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* plaintext (but it may still be available in the on-disk directory entry);
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* casefolded directories use this type of dirhash. At least in these cases,
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* each no-key name must include the name's dirhash too.
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*
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* To meet all these requirements, we base64-encode the following
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* variable-length structure. It contains the dirhash, or 0's if the filesystem
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* didn't provide one; up to 149 bytes of the ciphertext name; and for
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* ciphertexts longer than 149 bytes, also the SHA-256 of the remaining bytes.
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*
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* This ensures that each no-key name contains everything needed to find the
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* directory entry again, contains only legal characters, doesn't exceed
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* NAME_MAX, is unambiguous unless there's a SHA-256 collision, and that we only
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* take the performance hit of SHA-256 on very long filenames (which are rare).
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*/
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struct fscrypt_nokey_name {
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u32 dirhash[2];
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u8 bytes[149];
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u8 sha256[SHA256_DIGEST_SIZE];
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}; /* 189 bytes => 252 bytes base64-encoded, which is <= NAME_MAX (255) */
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/*
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* Decoded size of max-size nokey name, i.e. a name that was abbreviated using
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* the strong hash and thus includes the 'sha256' field. This isn't simply
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* sizeof(struct fscrypt_nokey_name), as the padding at the end isn't included.
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*/
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#define FSCRYPT_NOKEY_NAME_MAX offsetofend(struct fscrypt_nokey_name, sha256)
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static struct crypto_shash *sha256_hash_tfm;
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static int fscrypt_do_sha256(const u8 *data, unsigned int data_len, u8 *result)
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{
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struct crypto_shash *tfm = READ_ONCE(sha256_hash_tfm);
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if (unlikely(!tfm)) {
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struct crypto_shash *prev_tfm;
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tfm = crypto_alloc_shash("sha256", 0, 0);
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if (IS_ERR(tfm)) {
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fscrypt_err(NULL,
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"Error allocating SHA-256 transform: %ld",
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PTR_ERR(tfm));
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return PTR_ERR(tfm);
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}
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prev_tfm = cmpxchg(&sha256_hash_tfm, NULL, tfm);
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if (prev_tfm) {
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crypto_free_shash(tfm);
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tfm = prev_tfm;
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}
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}
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{
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SHASH_DESC_ON_STACK(desc, tfm);
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desc->tfm = tfm;
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return crypto_shash_digest(desc, data, data_len, result);
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}
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}
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static inline bool fscrypt_is_dot_dotdot(const struct qstr *str)
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{
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if (str->len == 1 && str->name[0] == '.')
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return true;
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if (str->len == 2 && str->name[0] == '.' && str->name[1] == '.')
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return true;
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return false;
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}
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/**
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* fscrypt_fname_encrypt() - encrypt a filename
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*
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* The output buffer must be at least as large as the input buffer.
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* Any extra space is filled with NUL padding before encryption.
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*
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* Return: 0 on success, -errno on failure
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*/
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int fscrypt_fname_encrypt(const struct inode *inode, const struct qstr *iname,
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u8 *out, unsigned int olen)
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{
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struct skcipher_request *req = NULL;
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DECLARE_CRYPTO_WAIT(wait);
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const struct fscrypt_info *ci = inode->i_crypt_info;
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struct crypto_skcipher *tfm = ci->ci_ctfm;
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union fscrypt_iv iv;
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struct scatterlist sg;
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int res;
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/*
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* Copy the filename to the output buffer for encrypting in-place and
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* pad it with the needed number of NUL bytes.
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*/
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if (WARN_ON(olen < iname->len))
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return -ENOBUFS;
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memcpy(out, iname->name, iname->len);
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memset(out + iname->len, 0, olen - iname->len);
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/* Initialize the IV */
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fscrypt_generate_iv(&iv, 0, ci);
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/* Set up the encryption request */
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req = skcipher_request_alloc(tfm, GFP_NOFS);
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if (!req)
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return -ENOMEM;
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skcipher_request_set_callback(req,
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CRYPTO_TFM_REQ_MAY_BACKLOG | CRYPTO_TFM_REQ_MAY_SLEEP,
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crypto_req_done, &wait);
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sg_init_one(&sg, out, olen);
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skcipher_request_set_crypt(req, &sg, &sg, olen, &iv);
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/* Do the encryption */
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res = crypto_wait_req(crypto_skcipher_encrypt(req), &wait);
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skcipher_request_free(req);
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if (res < 0) {
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fscrypt_err(inode, "Filename encryption failed: %d", res);
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return res;
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}
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return 0;
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}
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/**
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* fname_decrypt() - decrypt a filename
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*
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* The caller must have allocated sufficient memory for the @oname string.
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*
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* Return: 0 on success, -errno on failure
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*/
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static int fname_decrypt(const struct inode *inode,
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const struct fscrypt_str *iname,
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struct fscrypt_str *oname)
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{
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struct skcipher_request *req = NULL;
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DECLARE_CRYPTO_WAIT(wait);
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struct scatterlist src_sg, dst_sg;
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const struct fscrypt_info *ci = inode->i_crypt_info;
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struct crypto_skcipher *tfm = ci->ci_ctfm;
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union fscrypt_iv iv;
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int res;
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/* Allocate request */
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req = skcipher_request_alloc(tfm, GFP_NOFS);
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if (!req)
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return -ENOMEM;
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skcipher_request_set_callback(req,
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CRYPTO_TFM_REQ_MAY_BACKLOG | CRYPTO_TFM_REQ_MAY_SLEEP,
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crypto_req_done, &wait);
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/* Initialize IV */
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fscrypt_generate_iv(&iv, 0, ci);
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/* Create decryption request */
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sg_init_one(&src_sg, iname->name, iname->len);
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sg_init_one(&dst_sg, oname->name, oname->len);
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skcipher_request_set_crypt(req, &src_sg, &dst_sg, iname->len, &iv);
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res = crypto_wait_req(crypto_skcipher_decrypt(req), &wait);
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skcipher_request_free(req);
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if (res < 0) {
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fscrypt_err(inode, "Filename decryption failed: %d", res);
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return res;
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}
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oname->len = strnlen(oname->name, iname->len);
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return 0;
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}
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static const char lookup_table[65] =
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"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+,";
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#define BASE64_CHARS(nbytes) DIV_ROUND_UP((nbytes) * 4, 3)
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/**
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* base64_encode() -
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*
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* Encodes the input string using characters from the set [A-Za-z0-9+,].
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* The encoded string is roughly 4/3 times the size of the input string.
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*
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* Return: length of the encoded string
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*/
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static int base64_encode(const u8 *src, int len, char *dst)
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{
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int i, bits = 0, ac = 0;
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char *cp = dst;
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for (i = 0; i < len; i++) {
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ac += src[i] << bits;
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bits += 8;
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do {
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*cp++ = lookup_table[ac & 0x3f];
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ac >>= 6;
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bits -= 6;
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} while (bits >= 6);
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}
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if (bits)
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*cp++ = lookup_table[ac & 0x3f];
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return cp - dst;
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}
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static int base64_decode(const char *src, int len, u8 *dst)
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{
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int i, bits = 0, ac = 0;
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const char *p;
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u8 *cp = dst;
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for (i = 0; i < len; i++) {
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p = strchr(lookup_table, src[i]);
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if (p == NULL || src[i] == 0)
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return -2;
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ac += (p - lookup_table) << bits;
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bits += 6;
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if (bits >= 8) {
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*cp++ = ac & 0xff;
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ac >>= 8;
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bits -= 8;
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}
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}
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if (ac)
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return -1;
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return cp - dst;
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}
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bool fscrypt_fname_encrypted_size(const struct inode *inode, u32 orig_len,
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u32 max_len, u32 *encrypted_len_ret)
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{
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const struct fscrypt_info *ci = inode->i_crypt_info;
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int padding = 4 << (fscrypt_policy_flags(&ci->ci_policy) &
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FSCRYPT_POLICY_FLAGS_PAD_MASK);
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u32 encrypted_len;
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if (orig_len > max_len)
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return false;
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encrypted_len = max(orig_len, (u32)FS_CRYPTO_BLOCK_SIZE);
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encrypted_len = round_up(encrypted_len, padding);
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*encrypted_len_ret = min(encrypted_len, max_len);
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return true;
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}
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/**
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* fscrypt_fname_alloc_buffer - allocate a buffer for presented filenames
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*
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* Allocate a buffer that is large enough to hold any decrypted or encoded
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* filename (null-terminated), for the given maximum encrypted filename length.
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*
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* Return: 0 on success, -errno on failure
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*/
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int fscrypt_fname_alloc_buffer(const struct inode *inode,
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u32 max_encrypted_len,
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struct fscrypt_str *crypto_str)
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{
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const u32 max_encoded_len = BASE64_CHARS(FSCRYPT_NOKEY_NAME_MAX);
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u32 max_presented_len;
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max_presented_len = max(max_encoded_len, max_encrypted_len);
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crypto_str->name = kmalloc(max_presented_len + 1, GFP_NOFS);
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if (!crypto_str->name)
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return -ENOMEM;
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crypto_str->len = max_presented_len;
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return 0;
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}
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EXPORT_SYMBOL(fscrypt_fname_alloc_buffer);
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/**
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* fscrypt_fname_free_buffer - free the buffer for presented filenames
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*
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* Free the buffer allocated by fscrypt_fname_alloc_buffer().
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*/
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void fscrypt_fname_free_buffer(struct fscrypt_str *crypto_str)
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{
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if (!crypto_str)
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return;
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kfree(crypto_str->name);
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crypto_str->name = NULL;
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}
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EXPORT_SYMBOL(fscrypt_fname_free_buffer);
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/**
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* fscrypt_fname_disk_to_usr() - converts a filename from disk space to user
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* space
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*
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* The caller must have allocated sufficient memory for the @oname string.
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*
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* If the key is available, we'll decrypt the disk name. Otherwise, we'll
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* encode it for presentation in fscrypt_nokey_name format.
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* See struct fscrypt_nokey_name for details.
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*
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* Return: 0 on success, -errno on failure
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*/
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int fscrypt_fname_disk_to_usr(const struct inode *inode,
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u32 hash, u32 minor_hash,
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const struct fscrypt_str *iname,
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struct fscrypt_str *oname)
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{
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const struct qstr qname = FSTR_TO_QSTR(iname);
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struct fscrypt_nokey_name nokey_name;
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u32 size; /* size of the unencoded no-key name */
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int err;
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if (fscrypt_is_dot_dotdot(&qname)) {
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oname->name[0] = '.';
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oname->name[iname->len - 1] = '.';
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oname->len = iname->len;
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return 0;
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}
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if (iname->len < FS_CRYPTO_BLOCK_SIZE)
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return -EUCLEAN;
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if (fscrypt_has_encryption_key(inode))
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return fname_decrypt(inode, iname, oname);
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/*
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* Sanity check that struct fscrypt_nokey_name doesn't have padding
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* between fields and that its encoded size never exceeds NAME_MAX.
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*/
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BUILD_BUG_ON(offsetofend(struct fscrypt_nokey_name, dirhash) !=
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offsetof(struct fscrypt_nokey_name, bytes));
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BUILD_BUG_ON(offsetofend(struct fscrypt_nokey_name, bytes) !=
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offsetof(struct fscrypt_nokey_name, sha256));
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BUILD_BUG_ON(BASE64_CHARS(FSCRYPT_NOKEY_NAME_MAX) > NAME_MAX);
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if (hash) {
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nokey_name.dirhash[0] = hash;
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nokey_name.dirhash[1] = minor_hash;
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} else {
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nokey_name.dirhash[0] = 0;
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nokey_name.dirhash[1] = 0;
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}
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if (iname->len <= sizeof(nokey_name.bytes)) {
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memcpy(nokey_name.bytes, iname->name, iname->len);
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size = offsetof(struct fscrypt_nokey_name, bytes[iname->len]);
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} else {
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memcpy(nokey_name.bytes, iname->name, sizeof(nokey_name.bytes));
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/* Compute strong hash of remaining part of name. */
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err = fscrypt_do_sha256(&iname->name[sizeof(nokey_name.bytes)],
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iname->len - sizeof(nokey_name.bytes),
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nokey_name.sha256);
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if (err)
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return err;
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size = FSCRYPT_NOKEY_NAME_MAX;
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}
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oname->len = base64_encode((const u8 *)&nokey_name, size, oname->name);
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return 0;
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}
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EXPORT_SYMBOL(fscrypt_fname_disk_to_usr);
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/**
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* fscrypt_setup_filename() - prepare to search a possibly encrypted directory
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* @dir: the directory that will be searched
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* @iname: the user-provided filename being searched for
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* @lookup: 1 if we're allowed to proceed without the key because it's
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* ->lookup() or we're finding the dir_entry for deletion; 0 if we cannot
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* proceed without the key because we're going to create the dir_entry.
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* @fname: the filename information to be filled in
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*
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* Given a user-provided filename @iname, this function sets @fname->disk_name
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* to the name that would be stored in the on-disk directory entry, if possible.
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* If the directory is unencrypted this is simply @iname. Else, if we have the
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* directory's encryption key, then @iname is the plaintext, so we encrypt it to
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* get the disk_name.
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*
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* Else, for keyless @lookup operations, @iname is the presented ciphertext, so
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* we decode it to get the fscrypt_nokey_name. Non-@lookup operations will be
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* impossible in this case, so we fail them with ENOKEY.
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*
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* If successful, fscrypt_free_filename() must be called later to clean up.
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*
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* Return: 0 on success, -errno on failure
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*/
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int fscrypt_setup_filename(struct inode *dir, const struct qstr *iname,
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int lookup, struct fscrypt_name *fname)
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{
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struct fscrypt_nokey_name *nokey_name;
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int ret;
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memset(fname, 0, sizeof(struct fscrypt_name));
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fname->usr_fname = iname;
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if (!IS_ENCRYPTED(dir) || fscrypt_is_dot_dotdot(iname)) {
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fname->disk_name.name = (unsigned char *)iname->name;
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fname->disk_name.len = iname->len;
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return 0;
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}
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ret = fscrypt_get_encryption_info(dir);
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if (ret)
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return ret;
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if (fscrypt_has_encryption_key(dir)) {
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if (!fscrypt_fname_encrypted_size(dir, iname->len,
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dir->i_sb->s_cop->max_namelen,
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&fname->crypto_buf.len))
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return -ENAMETOOLONG;
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fname->crypto_buf.name = kmalloc(fname->crypto_buf.len,
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GFP_NOFS);
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if (!fname->crypto_buf.name)
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return -ENOMEM;
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ret = fscrypt_fname_encrypt(dir, iname, fname->crypto_buf.name,
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fname->crypto_buf.len);
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if (ret)
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goto errout;
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fname->disk_name.name = fname->crypto_buf.name;
|
|
fname->disk_name.len = fname->crypto_buf.len;
|
|
return 0;
|
|
}
|
|
if (!lookup)
|
|
return -ENOKEY;
|
|
fname->is_ciphertext_name = true;
|
|
|
|
/*
|
|
* We don't have the key and we are doing a lookup; decode the
|
|
* user-supplied name
|
|
*/
|
|
|
|
if (iname->len > BASE64_CHARS(FSCRYPT_NOKEY_NAME_MAX))
|
|
return -ENOENT;
|
|
|
|
fname->crypto_buf.name = kmalloc(FSCRYPT_NOKEY_NAME_MAX, GFP_KERNEL);
|
|
if (fname->crypto_buf.name == NULL)
|
|
return -ENOMEM;
|
|
|
|
ret = base64_decode(iname->name, iname->len, fname->crypto_buf.name);
|
|
if (ret < (int)offsetof(struct fscrypt_nokey_name, bytes[1]) ||
|
|
(ret > offsetof(struct fscrypt_nokey_name, sha256) &&
|
|
ret != FSCRYPT_NOKEY_NAME_MAX)) {
|
|
ret = -ENOENT;
|
|
goto errout;
|
|
}
|
|
fname->crypto_buf.len = ret;
|
|
|
|
nokey_name = (void *)fname->crypto_buf.name;
|
|
fname->hash = nokey_name->dirhash[0];
|
|
fname->minor_hash = nokey_name->dirhash[1];
|
|
if (ret != FSCRYPT_NOKEY_NAME_MAX) {
|
|
/* The full ciphertext filename is available. */
|
|
fname->disk_name.name = nokey_name->bytes;
|
|
fname->disk_name.len =
|
|
ret - offsetof(struct fscrypt_nokey_name, bytes);
|
|
}
|
|
return 0;
|
|
|
|
errout:
|
|
kfree(fname->crypto_buf.name);
|
|
return ret;
|
|
}
|
|
EXPORT_SYMBOL(fscrypt_setup_filename);
|
|
|
|
/**
|
|
* fscrypt_match_name() - test whether the given name matches a directory entry
|
|
* @fname: the name being searched for
|
|
* @de_name: the name from the directory entry
|
|
* @de_name_len: the length of @de_name in bytes
|
|
*
|
|
* Normally @fname->disk_name will be set, and in that case we simply compare
|
|
* that to the name stored in the directory entry. The only exception is that
|
|
* if we don't have the key for an encrypted directory and the name we're
|
|
* looking for is very long, then we won't have the full disk_name and instead
|
|
* we'll need to match against a fscrypt_nokey_name that includes a strong hash.
|
|
*
|
|
* Return: %true if the name matches, otherwise %false.
|
|
*/
|
|
bool fscrypt_match_name(const struct fscrypt_name *fname,
|
|
const u8 *de_name, u32 de_name_len)
|
|
{
|
|
const struct fscrypt_nokey_name *nokey_name =
|
|
(const void *)fname->crypto_buf.name;
|
|
u8 sha256[SHA256_DIGEST_SIZE];
|
|
|
|
if (likely(fname->disk_name.name)) {
|
|
if (de_name_len != fname->disk_name.len)
|
|
return false;
|
|
return !memcmp(de_name, fname->disk_name.name, de_name_len);
|
|
}
|
|
if (de_name_len <= sizeof(nokey_name->bytes))
|
|
return false;
|
|
if (memcmp(de_name, nokey_name->bytes, sizeof(nokey_name->bytes)))
|
|
return false;
|
|
if (fscrypt_do_sha256(&de_name[sizeof(nokey_name->bytes)],
|
|
de_name_len - sizeof(nokey_name->bytes), sha256))
|
|
return false;
|
|
return !memcmp(sha256, nokey_name->sha256, sizeof(sha256));
|
|
}
|
|
EXPORT_SYMBOL_GPL(fscrypt_match_name);
|
|
|
|
/**
|
|
* fscrypt_fname_siphash() - calculate the SipHash of a filename
|
|
* @dir: the parent directory
|
|
* @name: the filename to calculate the SipHash of
|
|
*
|
|
* Given a plaintext filename @name and a directory @dir which uses SipHash as
|
|
* its dirhash method and has had its fscrypt key set up, this function
|
|
* calculates the SipHash of that name using the directory's secret dirhash key.
|
|
*
|
|
* Return: the SipHash of @name using the hash key of @dir
|
|
*/
|
|
u64 fscrypt_fname_siphash(const struct inode *dir, const struct qstr *name)
|
|
{
|
|
const struct fscrypt_info *ci = dir->i_crypt_info;
|
|
|
|
WARN_ON(!ci->ci_dirhash_key_initialized);
|
|
|
|
return siphash(name->name, name->len, &ci->ci_dirhash_key);
|
|
}
|
|
EXPORT_SYMBOL_GPL(fscrypt_fname_siphash);
|
|
|
|
/*
|
|
* Validate dentries in encrypted directories to make sure we aren't potentially
|
|
* caching stale dentries after a key has been added.
|
|
*/
|
|
static int fscrypt_d_revalidate(struct dentry *dentry, unsigned int flags)
|
|
{
|
|
struct dentry *dir;
|
|
int err;
|
|
int valid;
|
|
|
|
/*
|
|
* Plaintext names are always valid, since fscrypt doesn't support
|
|
* reverting to ciphertext names without evicting the directory's inode
|
|
* -- which implies eviction of the dentries in the directory.
|
|
*/
|
|
if (!(dentry->d_flags & DCACHE_ENCRYPTED_NAME))
|
|
return 1;
|
|
|
|
/*
|
|
* Ciphertext name; valid if the directory's key is still unavailable.
|
|
*
|
|
* Although fscrypt forbids rename() on ciphertext names, we still must
|
|
* use dget_parent() here rather than use ->d_parent directly. That's
|
|
* because a corrupted fs image may contain directory hard links, which
|
|
* the VFS handles by moving the directory's dentry tree in the dcache
|
|
* each time ->lookup() finds the directory and it already has a dentry
|
|
* elsewhere. Thus ->d_parent can be changing, and we must safely grab
|
|
* a reference to some ->d_parent to prevent it from being freed.
|
|
*/
|
|
|
|
if (flags & LOOKUP_RCU)
|
|
return -ECHILD;
|
|
|
|
dir = dget_parent(dentry);
|
|
err = fscrypt_get_encryption_info(d_inode(dir));
|
|
valid = !fscrypt_has_encryption_key(d_inode(dir));
|
|
dput(dir);
|
|
|
|
if (err < 0)
|
|
return err;
|
|
|
|
return valid;
|
|
}
|
|
|
|
const struct dentry_operations fscrypt_d_ops = {
|
|
.d_revalidate = fscrypt_d_revalidate,
|
|
};
|