module_signing.c

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/* Module signature checker
 *
 * Copyright (C) 2012 Red Hat, Inc. All Rights Reserved.
 * Written by David Howells ([email protected])
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public Licence
 * as published by the Free Software Foundation; either version
 * 2 of the Licence, or (at your option) any later version.
 */

#include <linux/kernel.h>
#include <linux/err.h>
#include <crypto/public_key.h>
#include <crypto/hash.h>
#include <keys/asymmetric-type.h>
#include "module-internal.h"

/*
 * Module signature information block.
 *
 * The constituents of the signature section are, in order:
 *
 *  - Signer's name
 *  - Key identifier
 *  - Signature data
 *  - Information block
 */
struct module_signature {
    u8  algo;       /* Public-key crypto algorithm [enum pkey_algo] */
    u8  hash;       /* Digest algorithm [enum pkey_hash_algo] */
    u8  id_type;    /* Key identifier type [enum pkey_id_type] */
    u8  signer_len; /* Length of signer's name */
    u8  key_id_len; /* Length of key identifier */
    u8  __pad[3];
    __be32  sig_len;    /* Length of signature data */
};

/*
 * Digest the module contents.
 */
static struct public_key_signature *mod_make_digest(enum pkey_hash_algo hash,
                            const void *mod,
                            unsigned long modlen)
{
    struct public_key_signature *pks;
    struct crypto_shash *tfm;
    struct shash_desc *desc;
    size_t digest_size, desc_size;
    int ret;

    pr_devel("==>%s()\n", __func__);
    
    /* Allocate the hashing algorithm we're going to need and find out how
     * big the hash operational data will be.
     */
    tfm = crypto_alloc_shash(pkey_hash_algo[hash], 0, 0);
    if (IS_ERR(tfm))
        return (PTR_ERR(tfm) == -ENOENT) ? ERR_PTR(-ENOPKG) : ERR_CAST(tfm);

    desc_size = crypto_shash_descsize(tfm) + sizeof(*desc);
    digest_size = crypto_shash_digestsize(tfm);

    /* We allocate the hash operational data storage on the end of our
     * context data and the digest output buffer on the end of that.
     */
    ret = -ENOMEM;
    pks = kzalloc(digest_size + sizeof(*pks) + desc_size, GFP_KERNEL);
    if (!pks)
        goto error_no_pks;

    pks->pkey_hash_algo = hash;
    pks->digest     = (u8 *)pks + sizeof(*pks) + desc_size;
    pks->digest_size    = digest_size;

    desc = (void *)pks + sizeof(*pks);
    desc->tfm   = tfm;
    desc->flags = CRYPTO_TFM_REQ_MAY_SLEEP;

    ret = crypto_shash_init(desc);
    if (ret < 0)
        goto error;

    ret = crypto_shash_finup(desc, mod, modlen, pks->digest);
    if (ret < 0)
        goto error;

    crypto_free_shash(tfm);
    pr_devel("<==%s() = ok\n", __func__);
    return pks;

error:
    kfree(pks);
error_no_pks:
    crypto_free_shash(tfm);
    pr_devel("<==%s() = %d\n", __func__, ret);
    return ERR_PTR(ret);
}

/*
 * Extract an MPI array from the signature data.  This represents the actual
 * signature.  Each raw MPI is prefaced by a BE 2-byte value indicating the
 * size of the MPI in bytes.
 *
 * RSA signatures only have one MPI, so currently we only read one.
 */
static int mod_extract_mpi_array(struct public_key_signature *pks,
                 const void *data, size_t len)
{
    size_t nbytes;
    MPI mpi;

    if (len < 3)
        return -EBADMSG;
    nbytes = ((const u8 *)data)[0] << 8 | ((const u8 *)data)[1];
    data += 2;
    len -= 2;
    if (len != nbytes)
        return -EBADMSG;

    mpi = mpi_read_raw_data(data, nbytes);
    if (!mpi)
        return -ENOMEM;
    pks->mpi[0] = mpi;
    pks->nr_mpi = 1;
    return 0;
}

/*
 * Request an asymmetric key.
 */
static struct key *request_asymmetric_key(const char *signer, size_t signer_len,
                      const u8 *key_id, size_t key_id_len)
{
    key_ref_t key;
    size_t i;
    char *id, *q;

    pr_devel("==>%s(,%zu,,%zu)\n", __func__, signer_len, key_id_len);

    /* Construct an identifier. */
    id = kmalloc(signer_len + 2 + key_id_len * 2 + 1, GFP_KERNEL);
    if (!id)
        return ERR_PTR(-ENOKEY);

    memcpy(id, signer, signer_len);

    q = id + signer_len;
    *q++ = ':';
    *q++ = ' ';
    for (i = 0; i < key_id_len; i++) {
        *q++ = hex_asc[*key_id >> 4];
        *q++ = hex_asc[*key_id++ & 0x0f];
    }

    *q = 0;

    pr_debug("Look up: \"%s\"\n", id);

    key = keyring_search(make_key_ref(modsign_keyring, 1),
                 &key_type_asymmetric, id);
    if (IS_ERR(key))
        pr_warn("Request for unknown module key '%s' err %ld\n",
            id, PTR_ERR(key));
    kfree(id);

    if (IS_ERR(key)) {
        switch (PTR_ERR(key)) {
            /* Hide some search errors */
        case -EACCES:
        case -ENOTDIR:
        case -EAGAIN:
            return ERR_PTR(-ENOKEY);
        default:
            return ERR_CAST(key);
        }
    }

    pr_devel("<==%s() = 0 [%x]\n", __func__, key_serial(key_ref_to_ptr(key)));
    return key_ref_to_ptr(key);
}

/*
 * Verify the signature on a module.
 */
int mod_verify_sig(const void *mod, unsigned long *_modlen)
{
    struct public_key_signature *pks;
    struct module_signature ms;
    struct key *key;
    const void *sig;
    size_t modlen = *_modlen, sig_len;
    int ret;

    pr_devel("==>%s(,%zu)\n", __func__, modlen);

    if (modlen <= sizeof(ms))
        return -EBADMSG;

    memcpy(&ms, mod + (modlen - sizeof(ms)), sizeof(ms));
    modlen -= sizeof(ms);

    sig_len = be32_to_cpu(ms.sig_len);
    if (sig_len >= modlen)
        return -EBADMSG;
    modlen -= sig_len;
    if ((size_t)ms.signer_len + ms.key_id_len >= modlen)
        return -EBADMSG;
    modlen -= (size_t)ms.signer_len + ms.key_id_len;

    *_modlen = modlen;
    sig = mod + modlen;

    /* For the moment, only support RSA and X.509 identifiers */
    if (ms.algo != PKEY_ALGO_RSA ||
        ms.id_type != PKEY_ID_X509)
        return -ENOPKG;

    if (ms.hash >= PKEY_HASH__LAST ||
        !pkey_hash_algo[ms.hash])
        return -ENOPKG;

    key = request_asymmetric_key(sig, ms.signer_len,
                     sig + ms.signer_len, ms.key_id_len);
    if (IS_ERR(key))
        return PTR_ERR(key);

    pks = mod_make_digest(ms.hash, mod, modlen);
    if (IS_ERR(pks)) {
        ret = PTR_ERR(pks);
        goto error_put_key;
    }

    ret = mod_extract_mpi_array(pks, sig + ms.signer_len + ms.key_id_len,
                    sig_len);
    if (ret < 0)
        goto error_free_pks;

    ret = verify_signature(key, pks);
    pr_devel("verify_signature() = %d\n", ret);

error_free_pks:
    mpi_free(pks->rsa.s);
    kfree(pks);
error_put_key:
    key_put(key);
    pr_devel("<==%s() = %d\n", __func__, ret);
    return ret; 
}