linux/kernel/module_signing.c
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   1/* Module signature checker
   2 *
   3 * Copyright (C) 2012 Red Hat, Inc. All Rights Reserved.
   4 * Written by David Howells (dhowells@redhat.com)
   5 *
   6 * This program is free software; you can redistribute it and/or
   7 * modify it under the terms of the GNU General Public Licence
   8 * as published by the Free Software Foundation; either version
   9 * 2 of the Licence, or (at your option) any later version.
  10 */
  11
  12#include <linux/kernel.h>
  13#include <linux/err.h>
  14#include <crypto/public_key.h>
  15#include <crypto/hash.h>
  16#include <keys/asymmetric-type.h>
  17#include "module-internal.h"
  18
  19/*
  20 * Module signature information block.
  21 *
  22 * The constituents of the signature section are, in order:
  23 *
  24 *      - Signer's name
  25 *      - Key identifier
  26 *      - Signature data
  27 *      - Information block
  28 */
  29struct module_signature {
  30        u8      algo;           /* Public-key crypto algorithm [enum pkey_algo] */
  31        u8      hash;           /* Digest algorithm [enum pkey_hash_algo] */
  32        u8      id_type;        /* Key identifier type [enum pkey_id_type] */
  33        u8      signer_len;     /* Length of signer's name */
  34        u8      key_id_len;     /* Length of key identifier */
  35        u8      __pad[3];
  36        __be32  sig_len;        /* Length of signature data */
  37};
  38
  39/*
  40 * Digest the module contents.
  41 */
  42static struct public_key_signature *mod_make_digest(enum pkey_hash_algo hash,
  43                                                    const void *mod,
  44                                                    unsigned long modlen)
  45{
  46        struct public_key_signature *pks;
  47        struct crypto_shash *tfm;
  48        struct shash_desc *desc;
  49        size_t digest_size, desc_size;
  50        int ret;
  51
  52        pr_devel("==>%s()\n", __func__);
  53        
  54        /* Allocate the hashing algorithm we're going to need and find out how
  55         * big the hash operational data will be.
  56         */
  57        tfm = crypto_alloc_shash(pkey_hash_algo[hash], 0, 0);
  58        if (IS_ERR(tfm))
  59                return (PTR_ERR(tfm) == -ENOENT) ? ERR_PTR(-ENOPKG) : ERR_CAST(tfm);
  60
  61        desc_size = crypto_shash_descsize(tfm) + sizeof(*desc);
  62        digest_size = crypto_shash_digestsize(tfm);
  63
  64        /* We allocate the hash operational data storage on the end of our
  65         * context data and the digest output buffer on the end of that.
  66         */
  67        ret = -ENOMEM;
  68        pks = kzalloc(digest_size + sizeof(*pks) + desc_size, GFP_KERNEL);
  69        if (!pks)
  70                goto error_no_pks;
  71
  72        pks->pkey_hash_algo     = hash;
  73        pks->digest             = (u8 *)pks + sizeof(*pks) + desc_size;
  74        pks->digest_size        = digest_size;
  75
  76        desc = (void *)pks + sizeof(*pks);
  77        desc->tfm   = tfm;
  78        desc->flags = CRYPTO_TFM_REQ_MAY_SLEEP;
  79
  80        ret = crypto_shash_init(desc);
  81        if (ret < 0)
  82                goto error;
  83
  84        ret = crypto_shash_finup(desc, mod, modlen, pks->digest);
  85        if (ret < 0)
  86                goto error;
  87
  88        crypto_free_shash(tfm);
  89        pr_devel("<==%s() = ok\n", __func__);
  90        return pks;
  91
  92error:
  93        kfree(pks);
  94error_no_pks:
  95        crypto_free_shash(tfm);
  96        pr_devel("<==%s() = %d\n", __func__, ret);
  97        return ERR_PTR(ret);
  98}
  99
 100/*
 101 * Extract an MPI array from the signature data.  This represents the actual
 102 * signature.  Each raw MPI is prefaced by a BE 2-byte value indicating the
 103 * size of the MPI in bytes.
 104 *
 105 * RSA signatures only have one MPI, so currently we only read one.
 106 */
 107static int mod_extract_mpi_array(struct public_key_signature *pks,
 108                                 const void *data, size_t len)
 109{
 110        size_t nbytes;
 111        MPI mpi;
 112
 113        if (len < 3)
 114                return -EBADMSG;
 115        nbytes = ((const u8 *)data)[0] << 8 | ((const u8 *)data)[1];
 116        data += 2;
 117        len -= 2;
 118        if (len != nbytes)
 119                return -EBADMSG;
 120
 121        mpi = mpi_read_raw_data(data, nbytes);
 122        if (!mpi)
 123                return -ENOMEM;
 124        pks->mpi[0] = mpi;
 125        pks->nr_mpi = 1;
 126        return 0;
 127}
 128
 129/*
 130 * Request an asymmetric key.
 131 */
 132static struct key *request_asymmetric_key(const char *signer, size_t signer_len,
 133                                          const u8 *key_id, size_t key_id_len)
 134{
 135        key_ref_t key;
 136        size_t i;
 137        char *id, *q;
 138
 139        pr_devel("==>%s(,%zu,,%zu)\n", __func__, signer_len, key_id_len);
 140
 141        /* Construct an identifier. */
 142        id = kmalloc(signer_len + 2 + key_id_len * 2 + 1, GFP_KERNEL);
 143        if (!id)
 144                return ERR_PTR(-ENOKEY);
 145
 146        memcpy(id, signer, signer_len);
 147
 148        q = id + signer_len;
 149        *q++ = ':';
 150        *q++ = ' ';
 151        for (i = 0; i < key_id_len; i++) {
 152                *q++ = hex_asc[*key_id >> 4];
 153                *q++ = hex_asc[*key_id++ & 0x0f];
 154        }
 155
 156        *q = 0;
 157
 158        pr_debug("Look up: \"%s\"\n", id);
 159
 160        key = keyring_search(make_key_ref(modsign_keyring, 1),
 161                             &key_type_asymmetric, id);
 162        if (IS_ERR(key))
 163                pr_warn("Request for unknown module key '%s' err %ld\n",
 164                        id, PTR_ERR(key));
 165        kfree(id);
 166
 167        if (IS_ERR(key)) {
 168                switch (PTR_ERR(key)) {
 169                        /* Hide some search errors */
 170                case -EACCES:
 171                case -ENOTDIR:
 172                case -EAGAIN:
 173                        return ERR_PTR(-ENOKEY);
 174                default:
 175                        return ERR_CAST(key);
 176                }
 177        }
 178
 179        pr_devel("<==%s() = 0 [%x]\n", __func__, key_serial(key_ref_to_ptr(key)));
 180        return key_ref_to_ptr(key);
 181}
 182
 183/*
 184 * Verify the signature on a module.
 185 */
 186int mod_verify_sig(const void *mod, unsigned long *_modlen)
 187{
 188        struct public_key_signature *pks;
 189        struct module_signature ms;
 190        struct key *key;
 191        const void *sig;
 192        size_t modlen = *_modlen, sig_len;
 193        int ret;
 194
 195        pr_devel("==>%s(,%zu)\n", __func__, modlen);
 196
 197        if (modlen <= sizeof(ms))
 198                return -EBADMSG;
 199
 200        memcpy(&ms, mod + (modlen - sizeof(ms)), sizeof(ms));
 201        modlen -= sizeof(ms);
 202
 203        sig_len = be32_to_cpu(ms.sig_len);
 204        if (sig_len >= modlen)
 205                return -EBADMSG;
 206        modlen -= sig_len;
 207        if ((size_t)ms.signer_len + ms.key_id_len >= modlen)
 208                return -EBADMSG;
 209        modlen -= (size_t)ms.signer_len + ms.key_id_len;
 210
 211        *_modlen = modlen;
 212        sig = mod + modlen;
 213
 214        /* For the moment, only support RSA and X.509 identifiers */
 215        if (ms.algo != PKEY_ALGO_RSA ||
 216            ms.id_type != PKEY_ID_X509)
 217                return -ENOPKG;
 218
 219        if (ms.hash >= PKEY_HASH__LAST ||
 220            !pkey_hash_algo[ms.hash])
 221                return -ENOPKG;
 222
 223        key = request_asymmetric_key(sig, ms.signer_len,
 224                                     sig + ms.signer_len, ms.key_id_len);
 225        if (IS_ERR(key))
 226                return PTR_ERR(key);
 227
 228        pks = mod_make_digest(ms.hash, mod, modlen);
 229        if (IS_ERR(pks)) {
 230                ret = PTR_ERR(pks);
 231                goto error_put_key;
 232        }
 233
 234        ret = mod_extract_mpi_array(pks, sig + ms.signer_len + ms.key_id_len,
 235                                    sig_len);
 236        if (ret < 0)
 237                goto error_free_pks;
 238
 239        ret = verify_signature(key, pks);
 240        pr_devel("verify_signature() = %d\n", ret);
 241
 242error_free_pks:
 243        mpi_free(pks->rsa.s);
 244        kfree(pks);
 245error_put_key:
 246        key_put(key);
 247        pr_devel("<==%s() = %d\n", __func__, ret);
 248        return ret;     
 249}
 250
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