/* Key type used to cache DNS lookups made by the kernel
 *
 * See Documentation/networking/dns_resolver.rst
 *
 *   Copyright (c) 2007 Igor Mammedov
 *   Author(s): Igor Mammedov (niallain@gmail.com)
 *              Steve French (sfrench@us.ibm.com)
 *              Wang Lei (wang840925@gmail.com)
 *              David Howells (dhowells@redhat.com)
 *
 *   This library is free software; you can redistribute it and/or modify
 *   it under the terms of the GNU Lesser General Public License as published
 *   by the Free Software Foundation; either version 2.1 of the License, or
 *   (at your option) any later version.
 *
 *   This library is distributed in the hope that it will be useful,
 *   but WITHOUT ANY WARRANTY; without even the implied warranty of
 *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See
 *   the GNU Lesser General Public License for more details.
 *
 *   You should have received a copy of the GNU Lesser General Public License
 *   along with this library; if not, see <http://www.gnu.org/licenses/>.
 */
#include <linux/module.h>
#include <linux/moduleparam.h>
#include <linux/slab.h>
#include <linux/string.h>
#include <linux/kernel.h>
#include <linux/keyctl.h>
#include <linux/err.h>
#include <linux/seq_file.h>
#include <linux/dns_resolver.h>
#include <keys/dns_resolver-type.h>
#include <keys/user-type.h>
#include "internal.h"

MODULE_DESCRIPTION("DNS Resolver");
MODULE_AUTHOR("Wang Lei");
MODULE_LICENSE("GPL");

unsigned int dns_resolver_debug;
module_param_named(debug, dns_resolver_debug, uint, 0644);
MODULE_PARM_DESC(debug, "DNS Resolver debugging mask");

const struct cred *dns_resolver_cache;

#define DNS_ERRORNO_OPTION      "dnserror"

/*
 * Preparse instantiation data for a dns_resolver key.
 *
 * For normal hostname lookups, the data must be a NUL-terminated string, with
 * the NUL char accounted in datalen.
 *
 * If the data contains a '#' characters, then we take the clause after each
 * one to be an option of the form 'key=value'.  The actual data of interest is
 * the string leading up to the first '#'.  For instance:
 *
 *        "ip1,ip2,...#foo=bar"
 *
 * For server list requests, the data must begin with a NUL char and be
 * followed by a byte indicating the version of the data format.  Version 1
 * looks something like (note this is packed):
 *
 *      u8      Non-string marker (ie. 0)
 *      u8      Content (DNS_PAYLOAD_IS_*)
 *      u8      Version (e.g. 1)
 *      u8      Source of server list
 *      u8      Lookup status of server list
 *      u8      Number of servers
 *      foreach-server {
 *              __le16  Name length
 *              __le16  Priority (as per SRV record, low first)
 *              __le16  Weight (as per SRV record, higher first)
 *              __le16  Port
 *              u8      Source of address list
 *              u8      Lookup status of address list
 *              u8      Protocol (DNS_SERVER_PROTOCOL_*)
 *              u8      Number of addresses
 *              char[]  Name (not NUL-terminated)
 *              foreach-address {
 *                      u8              Family (DNS_ADDRESS_IS_*)
 *                      union {
 *                              u8[4]   ipv4_addr
 *                              u8[16]  ipv6_addr
 *                      }
 *              }
 *      }
 *
 */
static int
dns_resolver_preparse(struct key_preparsed_payload *prep)
{
        struct user_key_payload *upayload;
        unsigned long derrno;
        int ret;
        int datalen = prep->datalen, result_len = 0;
        const char *data = prep->data, *end, *opt;

        if (datalen <= 1 || !data)
                return -EINVAL;

        if (data[0] == 0) {
                const struct dns_server_list_v1_header *v1;

                /* It may be a server list. */
                if (datalen < sizeof(*v1))
                        return -EINVAL;

                v1 = (const struct dns_server_list_v1_header *)data;
                kenter("[%u,%u],%u", v1->hdr.content, v1->hdr.version, datalen);
                if (v1->hdr.content != DNS_PAYLOAD_IS_SERVER_LIST) {
                        pr_warn_ratelimited(
                                "dns_resolver: Unsupported content type (%u)\n",
                                v1->hdr.content);
                        return -EINVAL;
                }

                if (v1->hdr.version != 1) {
                        pr_warn_ratelimited(
                                "dns_resolver: Unsupported server list version (%u)\n",
                                v1->hdr.version);
                        return -EINVAL;
                }

                if ((v1->status != DNS_LOOKUP_GOOD &&
                     v1->status != DNS_LOOKUP_GOOD_WITH_BAD)) {
                        if (prep->expiry == TIME64_MAX)
                                prep->expiry = ktime_get_real_seconds() + 1;
                }

                result_len = datalen;
                goto store_result;
        }

        kenter("'%*.*s',%u", datalen, datalen, data, datalen);

        if (!data || data[datalen - 1] != '\0')
                return -EINVAL;
        datalen--;

        /* deal with any options embedded in the data */
        end = data + datalen;
        opt = memchr(data, '#', datalen);
        if (!opt) {
                /* no options: the entire data is the result */
                kdebug("no options");
                result_len = datalen;
        } else {
                const char *next_opt;

                result_len = opt - data;
                opt++;
                kdebug("options: '%s'", opt);
                do {
                        int opt_len, opt_nlen;
                        const char *eq;
                        char optval[128];

                        next_opt = memchr(opt, '#', end - opt) ?: end;
                        opt_len = next_opt - opt;
                        if (opt_len <= 0 || opt_len > sizeof(optval)) {
                                pr_warn_ratelimited("Invalid option length (%d) for dns_resolver key\n",
                                                    opt_len);
                                return -EINVAL;
                        }

                        eq = memchr(opt, '=', opt_len);
                        if (eq) {
                                opt_nlen = eq - opt;
                                eq++;
                                memcpy(optval, eq, next_opt - eq);
                                optval[next_opt - eq] = '\0';
                        } else {
                                opt_nlen = opt_len;
                                optval[0] = '\0';
                        }

                        kdebug("option '%*.*s' val '%s'",
                               opt_nlen, opt_nlen, opt, optval);

                        /* see if it's an error number representing a DNS error
                         * that's to be recorded as the result in this key */
                        if (opt_nlen == sizeof(DNS_ERRORNO_OPTION) - 1 &&
                            memcmp(opt, DNS_ERRORNO_OPTION, opt_nlen) == 0) {
                                kdebug("dns error number option");

                                ret = kstrtoul(optval, 10, &derrno);
                                if (ret < 0)
                                        goto bad_option_value;

                                if (derrno < 1 || derrno > 511)
                                        goto bad_option_value;

                                kdebug("dns error no. = %lu", derrno);
                                prep->payload.data[dns_key_error] = ERR_PTR(-derrno);
                                continue;
                        }

                bad_option_value:
                        pr_warn_ratelimited("Option '%*.*s' to dns_resolver key: bad/missing value\n",
                                            opt_nlen, opt_nlen, opt);
                        return -EINVAL;
                } while (opt = next_opt + 1, opt < end);
        }

        /* don't cache the result if we're caching an error saying there's no
         * result */
        if (prep->payload.data[dns_key_error]) {
                kleave(" = 0 [h_error %ld]", PTR_ERR(prep->payload.data[dns_key_error]));
                return 0;
        }

store_result:
        kdebug("store result");
        prep->quotalen = result_len;

        upayload = kmalloc(sizeof(*upayload) + result_len + 1, GFP_KERNEL);
        if (!upayload) {
                kleave(" = -ENOMEM");
                return -ENOMEM;
        }

        upayload->datalen = result_len;
        memcpy(upayload->data, data, result_len);
        upayload->data[result_len] = '\0';

        prep->payload.data[dns_key_data] = upayload;
        kleave(" = 0");
        return 0;
}

/*
 * Clean up the preparse data
 */
static void dns_resolver_free_preparse(struct key_preparsed_payload *prep)
{
        pr_devel("==>%s()\n", __func__);

        kfree(prep->payload.data[dns_key_data]);
}

/*
 * The description is of the form "[<type>:]<domain_name>"
 *
 * The domain name may be a simple name or an absolute domain name (which
 * should end with a period).  The domain name is case-independent.
 */
static bool dns_resolver_cmp(const struct key *key,
                             const struct key_match_data *match_data)
{
        int slen, dlen, ret = 0;
        const char *src = key->description, *dsp = match_data->raw_data;

        kenter("%s,%s", src, dsp);

        if (!src || !dsp)
                goto no_match;

        if (strcasecmp(src, dsp) == 0)
                goto matched;

        slen = strlen(src);
        dlen = strlen(dsp);
        if (slen <= 0 || dlen <= 0)
                goto no_match;
        if (src[slen - 1] == '.')
                slen--;
        if (dsp[dlen - 1] == '.')
                dlen--;
        if (slen != dlen || strncasecmp(src, dsp, slen) != 0)
                goto no_match;

matched:
        ret = 1;
no_match:
        kleave(" = %d", ret);
        return ret;
}

/*
 * Preparse the match criterion.
 */
static int dns_resolver_match_preparse(struct key_match_data *match_data)
{
        match_data->lookup_type = KEYRING_SEARCH_LOOKUP_ITERATE;
        match_data->cmp = dns_resolver_cmp;
        return 0;
}

/*
 * Describe a DNS key
 */
static void dns_resolver_describe(const struct key *key, struct seq_file *m)
{
        seq_puts(m, key->description);
        if (key_is_positive(key)) {
                int err = PTR_ERR(key->payload.data[dns_key_error]);

                if (err)
                        seq_printf(m, ": %d", err);
                else
                        seq_printf(m, ": %u", key->datalen);
        }
}

/*
 * read the DNS data
 * - the key's semaphore is read-locked
 */
static long dns_resolver_read(const struct key *key,
                              char *buffer, size_t buflen)
{
        int err = PTR_ERR(key->payload.data[dns_key_error]);

        if (err)
                return err;

        return user_read(key, buffer, buflen);
}

struct key_type key_type_dns_resolver = {
        .name           = "dns_resolver",
        .flags          = KEY_TYPE_NET_DOMAIN | KEY_TYPE_INSTANT_REAP,
        .preparse       = dns_resolver_preparse,
        .free_preparse  = dns_resolver_free_preparse,
        .instantiate    = generic_key_instantiate,
        .match_preparse = dns_resolver_match_preparse,
        .revoke         = user_revoke,
        .destroy        = user_destroy,
        .describe       = dns_resolver_describe,
        .read           = dns_resolver_read,
};

static int __init init_dns_resolver(void)
{
        struct cred *cred;
        struct key *keyring;
        int ret;

        /* create an override credential set with a special thread keyring in
         * which DNS requests are cached
         *
         * this is used to prevent malicious redirections from being installed
         * with add_key().
         */
        cred = prepare_kernel_cred(&init_task);
        if (!cred)
                return -ENOMEM;

        keyring = keyring_alloc(".dns_resolver",
                                GLOBAL_ROOT_UID, GLOBAL_ROOT_GID, cred,
                                (KEY_POS_ALL & ~KEY_POS_SETATTR) |
                                KEY_USR_VIEW | KEY_USR_READ,
                                KEY_ALLOC_NOT_IN_QUOTA, NULL, NULL);
        if (IS_ERR(keyring)) {
                ret = PTR_ERR(keyring);
                goto failed_put_cred;
        }

        ret = register_key_type(&key_type_dns_resolver);
        if (ret < 0)
                goto failed_put_key;

        /* instruct request_key() to use this special keyring as a cache for
         * the results it looks up */
        set_bit(KEY_FLAG_ROOT_CAN_CLEAR, &keyring->flags);
        cred->thread_keyring = keyring;
        cred->jit_keyring = KEY_REQKEY_DEFL_THREAD_KEYRING;
        dns_resolver_cache = cred;

        kdebug("DNS resolver keyring: %d\n", key_serial(keyring));
        return 0;

failed_put_key:
        key_put(keyring);
failed_put_cred:
        put_cred(cred);
        return ret;
}

static void __exit exit_dns_resolver(void)
{
        key_revoke(dns_resolver_cache->thread_keyring);
        unregister_key_type(&key_type_dns_resolver);
        put_cred(dns_resolver_cache);
}

module_init(init_dns_resolver)
module_exit(exit_dns_resolver)
MODULE_LICENSE("GPL");