Root/kernel/module_signing.c

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 <keys/system_keyring.h>
18#include "module-internal.h"
19
20/*
21 * Module signature information block.
22 *
23 * The constituents of the signature section are, in order:
24 *
25 * - Signer's name
26 * - Key identifier
27 * - Signature data
28 * - Information block
29 */
30struct module_signature {
31    u8 algo; /* Public-key crypto algorithm [enum pkey_algo] */
32    u8 hash; /* Digest algorithm [enum hash_algo] */
33    u8 id_type; /* Key identifier type [enum pkey_id_type] */
34    u8 signer_len; /* Length of signer's name */
35    u8 key_id_len; /* Length of key identifier */
36    u8 __pad[3];
37    __be32 sig_len; /* Length of signature data */
38};
39
40/*
41 * Digest the module contents.
42 */
43static struct public_key_signature *mod_make_digest(enum hash_algo hash,
44                            const void *mod,
45                            unsigned long modlen)
46{
47    struct public_key_signature *pks;
48    struct crypto_shash *tfm;
49    struct shash_desc *desc;
50    size_t digest_size, desc_size;
51    int ret;
52
53    pr_devel("==>%s()\n", __func__);
54    
55    /* Allocate the hashing algorithm we're going to need and find out how
56     * big the hash operational data will be.
57     */
58    tfm = crypto_alloc_shash(hash_algo_name[hash], 0, 0);
59    if (IS_ERR(tfm))
60        return (PTR_ERR(tfm) == -ENOENT) ? ERR_PTR(-ENOPKG) : ERR_CAST(tfm);
61
62    desc_size = crypto_shash_descsize(tfm) + sizeof(*desc);
63    digest_size = crypto_shash_digestsize(tfm);
64
65    /* We allocate the hash operational data storage on the end of our
66     * context data and the digest output buffer on the end of that.
67     */
68    ret = -ENOMEM;
69    pks = kzalloc(digest_size + sizeof(*pks) + desc_size, GFP_KERNEL);
70    if (!pks)
71        goto error_no_pks;
72
73    pks->pkey_hash_algo = hash;
74    pks->digest = (u8 *)pks + sizeof(*pks) + desc_size;
75    pks->digest_size = digest_size;
76
77    desc = (void *)pks + sizeof(*pks);
78    desc->tfm = tfm;
79    desc->flags = CRYPTO_TFM_REQ_MAY_SLEEP;
80
81    ret = crypto_shash_init(desc);
82    if (ret < 0)
83        goto error;
84
85    ret = crypto_shash_finup(desc, mod, modlen, pks->digest);
86    if (ret < 0)
87        goto error;
88
89    crypto_free_shash(tfm);
90    pr_devel("<==%s() = ok\n", __func__);
91    return pks;
92
93error:
94    kfree(pks);
95error_no_pks:
96    crypto_free_shash(tfm);
97    pr_devel("<==%s() = %d\n", __func__, ret);
98    return ERR_PTR(ret);
99}
100
101/*
102 * Extract an MPI array from the signature data. This represents the actual
103 * signature. Each raw MPI is prefaced by a BE 2-byte value indicating the
104 * size of the MPI in bytes.
105 *
106 * RSA signatures only have one MPI, so currently we only read one.
107 */
108static int mod_extract_mpi_array(struct public_key_signature *pks,
109                 const void *data, size_t len)
110{
111    size_t nbytes;
112    MPI mpi;
113
114    if (len < 3)
115        return -EBADMSG;
116    nbytes = ((const u8 *)data)[0] << 8 | ((const u8 *)data)[1];
117    data += 2;
118    len -= 2;
119    if (len != nbytes)
120        return -EBADMSG;
121
122    mpi = mpi_read_raw_data(data, nbytes);
123    if (!mpi)
124        return -ENOMEM;
125    pks->mpi[0] = mpi;
126    pks->nr_mpi = 1;
127    return 0;
128}
129
130/*
131 * Request an asymmetric key.
132 */
133static struct key *request_asymmetric_key(const char *signer, size_t signer_len,
134                      const u8 *key_id, size_t key_id_len)
135{
136    key_ref_t key;
137    size_t i;
138    char *id, *q;
139
140    pr_devel("==>%s(,%zu,,%zu)\n", __func__, signer_len, key_id_len);
141
142    /* Construct an identifier. */
143    id = kmalloc(signer_len + 2 + key_id_len * 2 + 1, GFP_KERNEL);
144    if (!id)
145        return ERR_PTR(-ENOKEY);
146
147    memcpy(id, signer, signer_len);
148
149    q = id + signer_len;
150    *q++ = ':';
151    *q++ = ' ';
152    for (i = 0; i < key_id_len; i++) {
153        *q++ = hex_asc[*key_id >> 4];
154        *q++ = hex_asc[*key_id++ & 0x0f];
155    }
156
157    *q = 0;
158
159    pr_debug("Look up: \"%s\"\n", id);
160
161    key = keyring_search(make_key_ref(system_trusted_keyring, 1),
162                 &key_type_asymmetric, id);
163    if (IS_ERR(key))
164        pr_warn("Request for unknown module key '%s' err %ld\n",
165            id, PTR_ERR(key));
166    kfree(id);
167
168    if (IS_ERR(key)) {
169        switch (PTR_ERR(key)) {
170            /* Hide some search errors */
171        case -EACCES:
172        case -ENOTDIR:
173        case -EAGAIN:
174            return ERR_PTR(-ENOKEY);
175        default:
176            return ERR_CAST(key);
177        }
178    }
179
180    pr_devel("<==%s() = 0 [%x]\n", __func__, key_serial(key_ref_to_ptr(key)));
181    return key_ref_to_ptr(key);
182}
183
184/*
185 * Verify the signature on a module.
186 */
187int mod_verify_sig(const void *mod, unsigned long *_modlen)
188{
189    struct public_key_signature *pks;
190    struct module_signature ms;
191    struct key *key;
192    const void *sig;
193    size_t modlen = *_modlen, sig_len;
194    int ret;
195
196    pr_devel("==>%s(,%zu)\n", __func__, modlen);
197
198    if (modlen <= sizeof(ms))
199        return -EBADMSG;
200
201    memcpy(&ms, mod + (modlen - sizeof(ms)), sizeof(ms));
202    modlen -= sizeof(ms);
203
204    sig_len = be32_to_cpu(ms.sig_len);
205    if (sig_len >= modlen)
206        return -EBADMSG;
207    modlen -= sig_len;
208    if ((size_t)ms.signer_len + ms.key_id_len >= modlen)
209        return -EBADMSG;
210    modlen -= (size_t)ms.signer_len + ms.key_id_len;
211
212    *_modlen = modlen;
213    sig = mod + modlen;
214
215    /* For the moment, only support RSA and X.509 identifiers */
216    if (ms.algo != PKEY_ALGO_RSA ||
217        ms.id_type != PKEY_ID_X509)
218        return -ENOPKG;
219
220    if (ms.hash >= PKEY_HASH__LAST ||
221        !hash_algo_name[ms.hash])
222        return -ENOPKG;
223
224    key = request_asymmetric_key(sig, ms.signer_len,
225                     sig + ms.signer_len, ms.key_id_len);
226    if (IS_ERR(key))
227        return PTR_ERR(key);
228
229    pks = mod_make_digest(ms.hash, mod, modlen);
230    if (IS_ERR(pks)) {
231        ret = PTR_ERR(pks);
232        goto error_put_key;
233    }
234
235    ret = mod_extract_mpi_array(pks, sig + ms.signer_len + ms.key_id_len,
236                    sig_len);
237    if (ret < 0)
238        goto error_free_pks;
239
240    ret = verify_signature(key, pks);
241    pr_devel("verify_signature() = %d\n", ret);
242
243error_free_pks:
244    mpi_free(pks->rsa.s);
245    kfree(pks);
246error_put_key:
247    key_put(key);
248    pr_devel("<==%s() = %d\n", __func__, ret);
249    return ret;
250}
251

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