xt_limit.c

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/* (C) 1999 Jérôme de Vivie <[email protected]>
 * (C) 1999 Hervé Eychenne <[email protected]>
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 as
 * published by the Free Software Foundation.
 */
#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <linux/slab.h>
#include <linux/module.h>
#include <linux/skbuff.h>
#include <linux/spinlock.h>
#include <linux/interrupt.h>

#include <linux/netfilter/x_tables.h>
#include <linux/netfilter/xt_limit.h>

struct xt_limit_priv {
    unsigned long prev;
    uint32_t credit;
};

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Herve Eychenne <[email protected]>");
MODULE_DESCRIPTION("Xtables: rate-limit match");
MODULE_ALIAS("ipt_limit");
MODULE_ALIAS("ip6t_limit");

/* The algorithm used is the Simple Token Bucket Filter (TBF)
 * see net/sched/sch_tbf.c in the linux source tree
 */

static DEFINE_SPINLOCK(limit_lock);

/* Rusty: This is my (non-mathematically-inclined) understanding of
   this algorithm.  The `average rate' in jiffies becomes your initial
   amount of credit `credit' and the most credit you can ever have
   `credit_cap'.  The `peak rate' becomes the cost of passing the
   test, `cost'.

   `prev' tracks the last packet hit: you gain one credit per jiffy.
   If you get credit balance more than this, the extra credit is
   discarded.  Every time the match passes, you lose `cost' credits;
   if you don't have that many, the test fails.

   See Alexey's formal explanation in net/sched/sch_tbf.c.

   To get the maxmum range, we multiply by this factor (ie. you get N
   credits per jiffy).  We want to allow a rate as low as 1 per day
   (slowest userspace tool allows), which means
   CREDITS_PER_JIFFY*HZ*60*60*24 < 2^32. ie. */
#define MAX_CPJ (0xFFFFFFFF / (HZ*60*60*24))

/* Repeated shift and or gives us all 1s, final shift and add 1 gives
 * us the power of 2 below the theoretical max, so GCC simply does a
 * shift. */
#define _POW2_BELOW2(x) ((x)|((x)>>1))
#define _POW2_BELOW4(x) (_POW2_BELOW2(x)|_POW2_BELOW2((x)>>2))
#define _POW2_BELOW8(x) (_POW2_BELOW4(x)|_POW2_BELOW4((x)>>4))
#define _POW2_BELOW16(x) (_POW2_BELOW8(x)|_POW2_BELOW8((x)>>8))
#define _POW2_BELOW32(x) (_POW2_BELOW16(x)|_POW2_BELOW16((x)>>16))
#define POW2_BELOW32(x) ((_POW2_BELOW32(x)>>1) + 1)

#define CREDITS_PER_JIFFY POW2_BELOW32(MAX_CPJ)

static bool
limit_mt(const struct sk_buff *skb, struct xt_action_param *par)
{
    const struct xt_rateinfo *r = par->matchinfo;
    struct xt_limit_priv *priv = r->master;
    unsigned long now = jiffies;

    spin_lock_bh(&limit_lock);
    priv->credit += (now - xchg(&priv->prev, now)) * CREDITS_PER_JIFFY;
    if (priv->credit > r->credit_cap)
        priv->credit = r->credit_cap;

    if (priv->credit >= r->cost) {
        /* We're not limited. */
        priv->credit -= r->cost;
        spin_unlock_bh(&limit_lock);
        return true;
    }

    spin_unlock_bh(&limit_lock);
    return false;
}

/* Precision saver. */
static u32 user2credits(u32 user)
{
    /* If multiplying would overflow... */
    if (user > 0xFFFFFFFF / (HZ*CREDITS_PER_JIFFY))
        /* Divide first. */
        return (user / XT_LIMIT_SCALE) * HZ * CREDITS_PER_JIFFY;

    return (user * HZ * CREDITS_PER_JIFFY) / XT_LIMIT_SCALE;
}

static int limit_mt_check(const struct xt_mtchk_param *par)
{
    struct xt_rateinfo *r = par->matchinfo;
    struct xt_limit_priv *priv;

    /* Check for overflow. */
    if (r->burst == 0
        || user2credits(r->avg * r->burst) < user2credits(r->avg)) {
        pr_info("Overflow, try lower: %u/%u\n",
            r->avg, r->burst);
        return -ERANGE;
    }

    priv = kmalloc(sizeof(*priv), GFP_KERNEL);
    if (priv == NULL)
        return -ENOMEM;

    /* For SMP, we only want to use one set of state. */
    r->master = priv;
    /* User avg in seconds * XT_LIMIT_SCALE: convert to jiffies *
       128. */
    priv->prev = jiffies;
    priv->credit = user2credits(r->avg * r->burst); /* Credits full. */
    if (r->cost == 0) {
        r->credit_cap = priv->credit; /* Credits full. */
        r->cost = user2credits(r->avg);
    }
    return 0;
}

static void limit_mt_destroy(const struct xt_mtdtor_param *par)
{
    const struct xt_rateinfo *info = par->matchinfo;

    kfree(info->master);
}

#ifdef CONFIG_COMPAT
struct compat_xt_rateinfo {
    u_int32_t avg;
    u_int32_t burst;

    compat_ulong_t prev;
    u_int32_t credit;
    u_int32_t credit_cap, cost;

    u_int32_t master;
};

/* To keep the full "prev" timestamp, the upper 32 bits are stored in the
 * master pointer, which does not need to be preserved. */
static void limit_mt_compat_from_user(void *dst, const void *src)
{
    const struct compat_xt_rateinfo *cm = src;
    struct xt_rateinfo m = {
        .avg        = cm->avg,
        .burst      = cm->burst,
        .prev       = cm->prev | (unsigned long)cm->master << 32,
        .credit     = cm->credit,
        .credit_cap = cm->credit_cap,
        .cost       = cm->cost,
    };
    memcpy(dst, &m, sizeof(m));
}

static int limit_mt_compat_to_user(void __user *dst, const void *src)
{
    const struct xt_rateinfo *m = src;
    struct compat_xt_rateinfo cm = {
        .avg        = m->avg,
        .burst      = m->burst,
        .prev       = m->prev,
        .credit     = m->credit,
        .credit_cap = m->credit_cap,
        .cost       = m->cost,
        .master     = m->prev >> 32,
    };
    return copy_to_user(dst, &cm, sizeof(cm)) ? -EFAULT : 0;
}
#endif /* CONFIG_COMPAT */

static struct xt_match limit_mt_reg __read_mostly = {
    .name             = "limit",
    .revision         = 0,
    .family           = NFPROTO_UNSPEC,
    .match            = limit_mt,
    .checkentry       = limit_mt_check,
    .destroy          = limit_mt_destroy,
    .matchsize        = sizeof(struct xt_rateinfo),
#ifdef CONFIG_COMPAT
    .compatsize       = sizeof(struct compat_xt_rateinfo),
    .compat_from_user = limit_mt_compat_from_user,
    .compat_to_user   = limit_mt_compat_to_user,
#endif
    .me               = THIS_MODULE,
};

static int __init limit_mt_init(void)
{
    return xt_register_match(&limit_mt_reg);
}

static void __exit limit_mt_exit(void)
{
    xt_unregister_match(&limit_mt_reg);
}

module_init(limit_mt_init);
module_exit(limit_mt_exit);