/*
 * Copyright (C) 2011 Nokia Corporation
 * Copyright (C) 2011 Intel Corporation
 *
 * Author:
 * Dmitry Kasatkin <dmitry.kasatkin@nokia.com>
 *                 <dmitry.kasatkin@intel.com>
 *
 * 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 const char *pkcs_1_v1_5_decode_emsa(const unsigned char *msg,
                                                unsigned long  msglen,
                                                unsigned long  modulus_bitlen,
                                                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 NULL;

        /* separate encoded message */
        if (msg[0] != 0x00 || msg[1] != 0x01)
                return NULL;

        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 NULL;

        ps_len = i - 2;

        *outlen = (msglen - (2 + ps_len + 1));

        return msg + 2 + ps_len + 1;
}

/*
 * 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;
        const char *m;
        MPI in = NULL, res = NULL, pkey[2];
        uint8_t *p, *datap;
        const uint8_t *endp;
        const struct user_key_payload *ukp;
        struct pubkey_hdr *pkh;

        down_read(&key->sem);
        ukp = user_key_payload_locked(key);

        if (!ukp) {
                /* key was revoked before we acquired its semaphore */
                err = -EKEYREVOKED;
                goto err1;
        }

        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;

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

        mblen = mpi_get_nbits(pkey[0]);
        mlen = DIV_ROUND_UP(mblen, 8);

        if (mlen == 0) {
                err = -EINVAL;
                goto err;
        }

        err = -ENOMEM;

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

        nret = siglen;
        in = mpi_read_from_buffer(sig, &nret);
        if (IS_ERR(in)) {
                err = PTR_ERR(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);

        kfree(p);

        m = pkcs_1_v1_5_decode_emsa(out1, len, mblen, &len);

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

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

        return err;
}

/**
 * digsig_verify() - digital signature verification with public key
 * @keyring:    keyring to search key in
 * @sig:        digital signature
 * @siglen:     length of the signature
 * @data:       data
 * @datalen:    length of the data
 *
 * Returns 0 on success, -EINVAL otherwise
 *
 * Verifies data integrity against digital signature.
 * Currently only RSA is supported.
 * Normally hash of the content is used as a data for this function.
 *
 */
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_CAST(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");