/* Large capacity key type
 *
 * Copyright (C) 2017 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
 * Copyright (C) 2013 Red Hat, Inc. All Rights Reserved.
 * Written by David Howells (dhowells@redhat.com)
 *
 * 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.
 */

#define pr_fmt(fmt) "big_key: "fmt
#include <linux/init.h>
#include <linux/seq_file.h>
#include <linux/file.h>
#include <linux/shmem_fs.h>
#include <linux/err.h>
#include <linux/scatterlist.h>
#include <linux/random.h>
#include <keys/user-type.h>
#include <keys/big_key-type.h>
#include <crypto/aead.h>

/*
 * Layout of key payload words.
 */
enum {
        big_key_data,
        big_key_path,
        big_key_path_2nd_part,
        big_key_len,
};

/*
 * Crypto operation with big_key data
 */
enum big_key_op {
        BIG_KEY_ENC,
        BIG_KEY_DEC,
};

/*
 * If the data is under this limit, there's no point creating a shm file to
 * hold it as the permanently resident metadata for the shmem fs will be at
 * least as large as the data.
 */
#define BIG_KEY_FILE_THRESHOLD (sizeof(struct inode) + sizeof(struct dentry))

/*
 * Key size for big_key data encryption
 */
#define ENC_KEY_SIZE 32

/*
 * Authentication tag length
 */
#define ENC_AUTHTAG_SIZE 16

/*
 * big_key defined keys take an arbitrary string as the description and an
 * arbitrary blob of data as the payload
 */
struct key_type key_type_big_key = {
        .name                   = "big_key",
        .preparse               = big_key_preparse,
        .free_preparse          = big_key_free_preparse,
        .instantiate            = generic_key_instantiate,
        .revoke                 = big_key_revoke,
        .destroy                = big_key_destroy,
        .describe               = big_key_describe,
        .read                   = big_key_read,
        /* no ->update(); don't add it without changing big_key_crypt() nonce */
};

/*
 * Crypto names for big_key data authenticated encryption
 */
static const char big_key_alg_name[] = "gcm(aes)";

/*
 * Crypto algorithms for big_key data authenticated encryption
 */
static struct crypto_aead *big_key_aead;

/*
 * Since changing the key affects the entire object, we need a mutex.
 */
static DEFINE_MUTEX(big_key_aead_lock);

/*
 * Encrypt/decrypt big_key data
 */
static int big_key_crypt(enum big_key_op op, u8 *data, size_t datalen, u8 *key)
{
        int ret;
        struct scatterlist sgio;
        struct aead_request *aead_req;
        /* We always use a zero nonce. The reason we can get away with this is
         * because we're using a different randomly generated key for every
         * different encryption. Notably, too, key_type_big_key doesn't define
         * an .update function, so there's no chance we'll wind up reusing the
         * key to encrypt updated data. Simply put: one key, one encryption.
         */
        u8 zero_nonce[crypto_aead_ivsize(big_key_aead)];

        aead_req = aead_request_alloc(big_key_aead, GFP_KERNEL);
        if (!aead_req)
                return -ENOMEM;

        memset(zero_nonce, 0, sizeof(zero_nonce));
        sg_init_one(&sgio, data, datalen + (op == BIG_KEY_ENC ? ENC_AUTHTAG_SIZE : 0));
        aead_request_set_crypt(aead_req, &sgio, &sgio, datalen, zero_nonce);
        aead_request_set_callback(aead_req, CRYPTO_TFM_REQ_MAY_SLEEP, NULL, NULL);
        aead_request_set_ad(aead_req, 0);

        mutex_lock(&big_key_aead_lock);
        if (crypto_aead_setkey(big_key_aead, key, ENC_KEY_SIZE)) {
                ret = -EAGAIN;
                goto error;
        }
        if (op == BIG_KEY_ENC)
                ret = crypto_aead_encrypt(aead_req);
        else
                ret = crypto_aead_decrypt(aead_req);
error:
        mutex_unlock(&big_key_aead_lock);
        aead_request_free(aead_req);
        return ret;
}

/*
 * Preparse a big key
 */
int big_key_preparse(struct key_preparsed_payload *prep)
{
        struct path *path = (struct path *)&prep->payload.data[big_key_path];
        struct file *file;
        u8 *enckey;
        u8 *data = NULL;
        ssize_t written;
        size_t datalen = prep->datalen;
        int ret;

        ret = -EINVAL;
        if (datalen <= 0 || datalen > 1024 * 1024 || !prep->data)
                goto error;

        /* Set an arbitrary quota */
        prep->quotalen = 16;

        prep->payload.data[big_key_len] = (void *)(unsigned long)datalen;

        if (datalen > BIG_KEY_FILE_THRESHOLD) {
                /* Create a shmem file to store the data in.  This will permit the data
                 * to be swapped out if needed.
                 *
                 * File content is stored encrypted with randomly generated key.
                 */
                size_t enclen = datalen + ENC_AUTHTAG_SIZE;
                loff_t pos = 0;

                data = kmalloc(enclen, GFP_KERNEL);
                if (!data)
                        return -ENOMEM;
                memcpy(data, prep->data, datalen);

                /* generate random key */
                enckey = kmalloc(ENC_KEY_SIZE, GFP_KERNEL);
                if (!enckey) {
                        ret = -ENOMEM;
                        goto error;
                }
                ret = get_random_bytes_wait(enckey, ENC_KEY_SIZE);
                if (unlikely(ret))
                        goto err_enckey;

                /* encrypt aligned data */
                ret = big_key_crypt(BIG_KEY_ENC, data, datalen, enckey);
                if (ret)
                        goto err_enckey;

                /* save aligned data to file */
                file = shmem_kernel_file_setup("", enclen, 0);
                if (IS_ERR(file)) {
                        ret = PTR_ERR(file);
                        goto err_enckey;
                }

                written = kernel_write(file, data, enclen, &pos);
                if (written != enclen) {
                        ret = written;
                        if (written >= 0)
                                ret = -ENOMEM;
                        goto err_fput;
                }

                /* Pin the mount and dentry to the key so that we can open it again
                 * later
                 */
                prep->payload.data[big_key_data] = enckey;
                *path = file->f_path;
                path_get(path);
                fput(file);
                kzfree(data);
        } else {
                /* Just store the data in a buffer */
                void *data = kmalloc(datalen, GFP_KERNEL);

                if (!data)
                        return -ENOMEM;

                prep->payload.data[big_key_data] = data;
                memcpy(data, prep->data, prep->datalen);
        }
        return 0;

err_fput:
        fput(file);
err_enckey:
        kzfree(enckey);
error:
        kzfree(data);
        return ret;
}

/*
 * Clear preparsement.
 */
void big_key_free_preparse(struct key_preparsed_payload *prep)
{
        if (prep->datalen > BIG_KEY_FILE_THRESHOLD) {
                struct path *path = (struct path *)&prep->payload.data[big_key_path];

                path_put(path);
        }
        kzfree(prep->payload.data[big_key_data]);
}

/*
 * dispose of the links from a revoked keyring
 * - called with the key sem write-locked
 */
void big_key_revoke(struct key *key)
{
        struct path *path = (struct path *)&key->payload.data[big_key_path];

        /* clear the quota */
        key_payload_reserve(key, 0);
        if (key_is_positive(key) &&
            (size_t)key->payload.data[big_key_len] > BIG_KEY_FILE_THRESHOLD)
                vfs_truncate(path, 0);
}

/*
 * dispose of the data dangling from the corpse of a big_key key
 */
void big_key_destroy(struct key *key)
{
        size_t datalen = (size_t)key->payload.data[big_key_len];

        if (datalen > BIG_KEY_FILE_THRESHOLD) {
                struct path *path = (struct path *)&key->payload.data[big_key_path];

                path_put(path);
                path->mnt = NULL;
                path->dentry = NULL;
        }
        kzfree(key->payload.data[big_key_data]);
        key->payload.data[big_key_data] = NULL;
}

/*
 * describe the big_key key
 */
void big_key_describe(const struct key *key, struct seq_file *m)
{
        size_t datalen = (size_t)key->payload.data[big_key_len];

        seq_puts(m, key->description);

        if (key_is_positive(key))
                seq_printf(m, ": %zu [%s]",
                           datalen,
                           datalen > BIG_KEY_FILE_THRESHOLD ? "file" : "buff");
}

/*
 * read the key data
 * - the key's semaphore is read-locked
 */
long big_key_read(const struct key *key, char __user *buffer, size_t buflen)
{
        size_t datalen = (size_t)key->payload.data[big_key_len];
        long ret;

        if (!buffer || buflen < datalen)
                return datalen;

        if (datalen > BIG_KEY_FILE_THRESHOLD) {
                struct path *path = (struct path *)&key->payload.data[big_key_path];
                struct file *file;
                u8 *data;
                u8 *enckey = (u8 *)key->payload.data[big_key_data];
                size_t enclen = datalen + ENC_AUTHTAG_SIZE;
                loff_t pos = 0;

                data = kmalloc(enclen, GFP_KERNEL);
                if (!data)
                        return -ENOMEM;

                file = dentry_open(path, O_RDONLY, current_cred());
                if (IS_ERR(file)) {
                        ret = PTR_ERR(file);
                        goto error;
                }

                /* read file to kernel and decrypt */
                ret = kernel_read(file, data, enclen, &pos);
                if (ret >= 0 && ret != enclen) {
                        ret = -EIO;
                        goto err_fput;
                }

                ret = big_key_crypt(BIG_KEY_DEC, data, enclen, enckey);
                if (ret)
                        goto err_fput;

                ret = datalen;

                /* copy decrypted data to user */
                if (copy_to_user(buffer, data, datalen) != 0)
                        ret = -EFAULT;

err_fput:
                fput(file);
error:
                kzfree(data);
        } else {
                ret = datalen;
                if (copy_to_user(buffer, key->payload.data[big_key_data],
                                 datalen) != 0)
                        ret = -EFAULT;
        }

        return ret;
}

/*
 * Register key type
 */
static int __init big_key_init(void)
{
        int ret;

        /* init block cipher */
        big_key_aead = crypto_alloc_aead(big_key_alg_name, 0, CRYPTO_ALG_ASYNC);
        if (IS_ERR(big_key_aead)) {
                ret = PTR_ERR(big_key_aead);
                pr_err("Can't alloc crypto: %d\n", ret);
                return ret;
        }
        ret = crypto_aead_setauthsize(big_key_aead, ENC_AUTHTAG_SIZE);
        if (ret < 0) {
                pr_err("Can't set crypto auth tag len: %d\n", ret);
                goto free_aead;
        }

        ret = register_key_type(&key_type_big_key);
        if (ret < 0) {
                pr_err("Can't register type: %d\n", ret);
                goto free_aead;
        }

        return 0;

free_aead:
        crypto_free_aead(big_key_aead);
        return ret;
}

late_initcall(big_key_init);