digsig.c

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/*
 * Copyright (C) 2011 Nokia Corporation
 * Copyright (C) 2011 Intel Corporation
 *
 * Author:
 * Dmitry Kasatkin <[email protected]>
 *                 <[email protected]>
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, version 2 of the License.
 *
 * File: sign.c
 *  implements signature (RSA) verification
 *  pkcs decoding is based on LibTomCrypt code
 */

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <linux/err.h>
#include <linux/module.h>
#include <linux/slab.h>
#include <linux/key.h>
#include <linux/crypto.h>
#include <crypto/hash.h>
#include <crypto/sha.h>
#include <keys/user-type.h>
#include <linux/mpi.h>
#include <linux/digsig.h>

static struct crypto_shash *shash;

static int pkcs_1_v1_5_decode_emsa(const unsigned char *msg,
            unsigned long  msglen,
            unsigned long  modulus_bitlen,
            unsigned char *out,
            unsigned long *outlen)
{
    unsigned long modulus_len, ps_len, i;

    modulus_len = (modulus_bitlen >> 3) + (modulus_bitlen & 7 ? 1 : 0);

    /* test message size */
    if ((msglen > modulus_len) || (modulus_len < 11))
        return -EINVAL;

    /* separate encoded message */
    if ((msg[0] != 0x00) || (msg[1] != (unsigned char)1))
        return -EINVAL;

    for (i = 2; i < modulus_len - 1; i++)
        if (msg[i] != 0xFF)
            break;

    /* separator check */
    if (msg[i] != 0)
        /* There was no octet with hexadecimal value 0x00
        to separate ps from m. */
        return -EINVAL;

    ps_len = i - 2;

    if (*outlen < (msglen - (2 + ps_len + 1))) {
        *outlen = msglen - (2 + ps_len + 1);
        return -EOVERFLOW;
    }

    *outlen = (msglen - (2 + ps_len + 1));
    memcpy(out, &msg[2 + ps_len + 1], *outlen);

    return 0;
}

/*
 * RSA Signature verification with public key
 */
static int digsig_verify_rsa(struct key *key,
            const char *sig, int siglen,
               const char *h, int hlen)
{
    int err = -EINVAL;
    unsigned long len;
    unsigned long mlen, mblen;
    unsigned nret, l;
    int head, i;
    unsigned char *out1 = NULL, *out2 = NULL;
    MPI in = NULL, res = NULL, pkey[2];
    uint8_t *p, *datap, *endp;
    struct user_key_payload *ukp;
    struct pubkey_hdr *pkh;

    down_read(&key->sem);
    ukp = key->payload.data;

    if (ukp->datalen < sizeof(*pkh))
        goto err1;

    pkh = (struct pubkey_hdr *)ukp->data;

    if (pkh->version != 1)
        goto err1;

    if (pkh->algo != PUBKEY_ALGO_RSA)
        goto err1;

    if (pkh->nmpi != 2)
        goto err1;

    datap = pkh->mpi;
    endp = ukp->data + ukp->datalen;

    err = -ENOMEM;

    for (i = 0; i < pkh->nmpi; i++) {
        unsigned int remaining = endp - datap;
        pkey[i] = mpi_read_from_buffer(datap, &remaining);
        if (!pkey[i])
            goto err;
        datap += remaining;
    }

    mblen = mpi_get_nbits(pkey[0]);
    mlen = (mblen + 7)/8;

    if (mlen == 0)
        goto err;

    out1 = kzalloc(mlen, GFP_KERNEL);
    if (!out1)
        goto err;

    out2 = kzalloc(mlen, GFP_KERNEL);
    if (!out2)
        goto err;

    nret = siglen;
    in = mpi_read_from_buffer(sig, &nret);
    if (!in)
        goto err;

    res = mpi_alloc(mpi_get_nlimbs(in) * 2);
    if (!res)
        goto err;

    err = mpi_powm(res, in, pkey[1], pkey[0]);
    if (err)
        goto err;

    if (mpi_get_nlimbs(res) * BYTES_PER_MPI_LIMB > mlen) {
        err = -EINVAL;
        goto err;
    }

    p = mpi_get_buffer(res, &l, NULL);
    if (!p) {
        err = -EINVAL;
        goto err;
    }

    len = mlen;
    head = len - l;
    memset(out1, 0, head);
    memcpy(out1 + head, p, l);

    err = pkcs_1_v1_5_decode_emsa(out1, len, mblen, out2, &len);
    if (err)
        goto err;

    if (len != hlen || memcmp(out2, h, hlen))
        err = -EINVAL;

err:
    mpi_free(in);
    mpi_free(res);
    kfree(out1);
    kfree(out2);
    while (--i >= 0)
        mpi_free(pkey[i]);
err1:
    up_read(&key->sem);

    return err;
}

int digsig_verify(struct key *keyring, const char *sig, int siglen,
                        const char *data, int datalen)
{
    int err = -ENOMEM;
    struct signature_hdr *sh = (struct signature_hdr *)sig;
    struct shash_desc *desc = NULL;
    unsigned char hash[SHA1_DIGEST_SIZE];
    struct key *key;
    char name[20];

    if (siglen < sizeof(*sh) + 2)
        return -EINVAL;

    if (sh->algo != PUBKEY_ALGO_RSA)
        return -ENOTSUPP;

    sprintf(name, "%llX", __be64_to_cpup((uint64_t *)sh->keyid));

    if (keyring) {
        /* search in specific keyring */
        key_ref_t kref;
        kref = keyring_search(make_key_ref(keyring, 1UL),
                        &key_type_user, name);
        if (IS_ERR(kref))
            key = ERR_PTR(PTR_ERR(kref));
        else
            key = key_ref_to_ptr(kref);
    } else {
        key = request_key(&key_type_user, name, NULL);
    }
    if (IS_ERR(key)) {
        pr_err("key not found, id: %s\n", name);
        return PTR_ERR(key);
    }

    desc = kzalloc(sizeof(*desc) + crypto_shash_descsize(shash),
               GFP_KERNEL);
    if (!desc)
        goto err;

    desc->tfm = shash;
    desc->flags = CRYPTO_TFM_REQ_MAY_SLEEP;

    crypto_shash_init(desc);
    crypto_shash_update(desc, data, datalen);
    crypto_shash_update(desc, sig, sizeof(*sh));
    crypto_shash_final(desc, hash);

    kfree(desc);

    /* pass signature mpis address */
    err = digsig_verify_rsa(key, sig + sizeof(*sh), siglen - sizeof(*sh),
                 hash, sizeof(hash));

err:
    key_put(key);

    return err ? -EINVAL : 0;
}
EXPORT_SYMBOL_GPL(digsig_verify);

static int __init digsig_init(void)
{
    shash = crypto_alloc_shash("sha1", 0, 0);
    if (IS_ERR(shash)) {
        pr_err("shash allocation failed\n");
        return  PTR_ERR(shash);
    }

    return 0;

}

static void __exit digsig_cleanup(void)
{
    crypto_free_shash(shash);
}

module_init(digsig_init);
module_exit(digsig_cleanup);

MODULE_LICENSE("GPL");