dns_key.c

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/* Key type used to cache DNS lookups made by the kernel
 *
 * See Documentation/networking/dns_resolver.txt
 *
 *   Copyright (c) 2007 Igor Mammedov
 *   Author(s): Igor Mammedov ([email protected])
 *              Steve French ([email protected])
 *              Wang Lei ([email protected])
 *      David Howells ([email protected])
 *
 *   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, write to the Free Software
 *   Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 */
#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 <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, S_IWUSR | S_IRUGO);
MODULE_PARM_DESC(debug, "DNS Resolver debugging mask");

const struct cred *dns_resolver_cache;

#define DNS_ERRORNO_OPTION  "dnserror"

/*
 * Instantiate a user defined key for dns_resolver.
 *
 * 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"
 */
static int
dns_resolver_instantiate(struct key *key, struct key_preparsed_payload *prep)
{
    struct user_key_payload *upayload;
    unsigned long derrno;
    int ret;
    size_t datalen = prep->datalen, result_len = 0;
    const char *data = prep->data, *end, *opt;

    kenter("%%%d,%s,'%*.*s',%zu",
           key->serial, key->description,
           (int)datalen, (int)datalen, data, datalen);

    if (datalen <= 1 || !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 {
            const char *eq;
            int opt_len, opt_nlen, opt_vlen, tmp;

            next_opt = memchr(opt, '#', end - opt) ?: end;
            opt_len = next_opt - opt;
            if (!opt_len) {
                printk(KERN_WARNING
                       "Empty option to dns_resolver key %d\n",
                       key->serial);
                return -EINVAL;
            }

            eq = memchr(opt, '=', opt_len) ?: end;
            opt_nlen = eq - opt;
            eq++;
            opt_vlen = next_opt - eq; /* will be -1 if no value */

            tmp = opt_vlen >= 0 ? opt_vlen : 0;
            kdebug("option '%*.*s' val '%*.*s'",
                   opt_nlen, opt_nlen, opt, tmp, tmp, eq);

            /* 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");
                if (opt_vlen <= 0)
                    goto bad_option_value;

                ret = strict_strtoul(eq, 10, &derrno);
                if (ret < 0)
                    goto bad_option_value;

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

                kdebug("dns error no. = %lu", derrno);
                key->type_data.x[0] = -derrno;
                continue;
            }

        bad_option_value:
            printk(KERN_WARNING
                   "Option '%*.*s' to dns_resolver key %d:"
                   " bad/missing value\n",
                   opt_nlen, opt_nlen, opt, key->serial);
            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 (key->type_data.x[0]) {
        kleave(" = 0 [h_error %ld]", key->type_data.x[0]);
        return 0;
    }

    kdebug("store result");
    ret = key_payload_reserve(key, result_len);
    if (ret < 0)
        return -EINVAL;

    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';
    rcu_assign_pointer(key->payload.data, upayload);

    kleave(" = 0");
    return 0;
}

/*
 * 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 int
dns_resolver_match(const struct key *key, const void *description)
{
    int slen, dlen, ret = 0;
    const char *src = key->description, *dsp = description;

    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;
}

/*
 * Describe a DNS key
 */
static void dns_resolver_describe(const struct key *key, struct seq_file *m)
{
    int err = key->type_data.x[0];

    seq_puts(m, key->description);
    if (key_is_instantiated(key)) {
        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 __user *buffer, size_t buflen)
{
    if (key->type_data.x[0])
        return key->type_data.x[0];

    return user_read(key, buffer, buflen);
}

struct key_type key_type_dns_resolver = {
    .name       = "dns_resolver",
    .instantiate    = dns_resolver_instantiate,
    .match      = dns_resolver_match,
    .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(NULL);
    if (!cred)
        return -ENOMEM;

    keyring = key_alloc(&key_type_keyring, ".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);
    if (IS_ERR(keyring)) {
        ret = PTR_ERR(keyring);
        goto failed_put_cred;
    }

    ret = key_instantiate_and_link(keyring, NULL, 0, NULL, NULL);
    if (ret < 0)
        goto failed_put_key;

    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");