linux/net/netfilter/xt_limit.c
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   1/* (C) 1999 Jérôme de Vivie <devivie@info.enserb.u-bordeaux.fr>
   2 * (C) 1999 Hervé Eychenne <eychenne@info.enserb.u-bordeaux.fr>
   3 *
   4 * This program is free software; you can redistribute it and/or modify
   5 * it under the terms of the GNU General Public License version 2 as
   6 * published by the Free Software Foundation.
   7 */
   8#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
   9
  10#include <linux/slab.h>
  11#include <linux/module.h>
  12#include <linux/skbuff.h>
  13#include <linux/spinlock.h>
  14#include <linux/interrupt.h>
  15
  16#include <linux/netfilter/x_tables.h>
  17#include <linux/netfilter/xt_limit.h>
  18
  19struct xt_limit_priv {
  20        unsigned long prev;
  21        uint32_t credit;
  22};
  23
  24MODULE_LICENSE("GPL");
  25MODULE_AUTHOR("Herve Eychenne <rv@wallfire.org>");
  26MODULE_DESCRIPTION("Xtables: rate-limit match");
  27MODULE_ALIAS("ipt_limit");
  28MODULE_ALIAS("ip6t_limit");
  29
  30/* The algorithm used is the Simple Token Bucket Filter (TBF)
  31 * see net/sched/sch_tbf.c in the linux source tree
  32 */
  33
  34static DEFINE_SPINLOCK(limit_lock);
  35
  36/* Rusty: This is my (non-mathematically-inclined) understanding of
  37   this algorithm.  The `average rate' in jiffies becomes your initial
  38   amount of credit `credit' and the most credit you can ever have
  39   `credit_cap'.  The `peak rate' becomes the cost of passing the
  40   test, `cost'.
  41
  42   `prev' tracks the last packet hit: you gain one credit per jiffy.
  43   If you get credit balance more than this, the extra credit is
  44   discarded.  Every time the match passes, you lose `cost' credits;
  45   if you don't have that many, the test fails.
  46
  47   See Alexey's formal explanation in net/sched/sch_tbf.c.
  48
  49   To get the maxmum range, we multiply by this factor (ie. you get N
  50   credits per jiffy).  We want to allow a rate as low as 1 per day
  51   (slowest userspace tool allows), which means
  52   CREDITS_PER_JIFFY*HZ*60*60*24 < 2^32. ie. */
  53#define MAX_CPJ (0xFFFFFFFF / (HZ*60*60*24))
  54
  55/* Repeated shift and or gives us all 1s, final shift and add 1 gives
  56 * us the power of 2 below the theoretical max, so GCC simply does a
  57 * shift. */
  58#define _POW2_BELOW2(x) ((x)|((x)>>1))
  59#define _POW2_BELOW4(x) (_POW2_BELOW2(x)|_POW2_BELOW2((x)>>2))
  60#define _POW2_BELOW8(x) (_POW2_BELOW4(x)|_POW2_BELOW4((x)>>4))
  61#define _POW2_BELOW16(x) (_POW2_BELOW8(x)|_POW2_BELOW8((x)>>8))
  62#define _POW2_BELOW32(x) (_POW2_BELOW16(x)|_POW2_BELOW16((x)>>16))
  63#define POW2_BELOW32(x) ((_POW2_BELOW32(x)>>1) + 1)
  64
  65#define CREDITS_PER_JIFFY POW2_BELOW32(MAX_CPJ)
  66
  67static bool
  68limit_mt(const struct sk_buff *skb, struct xt_action_param *par)
  69{
  70        const struct xt_rateinfo *r = par->matchinfo;
  71        struct xt_limit_priv *priv = r->master;
  72        unsigned long now = jiffies;
  73
  74        spin_lock_bh(&limit_lock);
  75        priv->credit += (now - xchg(&priv->prev, now)) * CREDITS_PER_JIFFY;
  76        if (priv->credit > r->credit_cap)
  77                priv->credit = r->credit_cap;
  78
  79        if (priv->credit >= r->cost) {
  80                /* We're not limited. */
  81                priv->credit -= r->cost;
  82                spin_unlock_bh(&limit_lock);
  83                return true;
  84        }
  85
  86        spin_unlock_bh(&limit_lock);
  87        return false;
  88}
  89
  90/* Precision saver. */
  91static u32 user2credits(u32 user)
  92{
  93        /* If multiplying would overflow... */
  94        if (user > 0xFFFFFFFF / (HZ*CREDITS_PER_JIFFY))
  95                /* Divide first. */
  96                return (user / XT_LIMIT_SCALE) * HZ * CREDITS_PER_JIFFY;
  97
  98        return (user * HZ * CREDITS_PER_JIFFY) / XT_LIMIT_SCALE;
  99}
 100
 101static int limit_mt_check(const struct xt_mtchk_param *par)
 102{
 103        struct xt_rateinfo *r = par->matchinfo;
 104        struct xt_limit_priv *priv;
 105
 106        /* Check for overflow. */
 107        if (r->burst == 0
 108            || user2credits(r->avg * r->burst) < user2credits(r->avg)) {
 109                pr_info("Overflow, try lower: %u/%u\n",
 110                        r->avg, r->burst);
 111                return -ERANGE;
 112        }
 113
 114        priv = kmalloc(sizeof(*priv), GFP_KERNEL);
 115        if (priv == NULL)
 116                return -ENOMEM;
 117
 118        /* For SMP, we only want to use one set of state. */
 119        r->master = priv;
 120        /* User avg in seconds * XT_LIMIT_SCALE: convert to jiffies *
 121           128. */
 122        priv->prev = jiffies;
 123        priv->credit = user2credits(r->avg * r->burst); /* Credits full. */
 124        if (r->cost == 0) {
 125                r->credit_cap = priv->credit; /* Credits full. */
 126                r->cost = user2credits(r->avg);
 127        }
 128        return 0;
 129}
 130
 131static void limit_mt_destroy(const struct xt_mtdtor_param *par)
 132{
 133        const struct xt_rateinfo *info = par->matchinfo;
 134
 135        kfree(info->master);
 136}
 137
 138#ifdef CONFIG_COMPAT
 139struct compat_xt_rateinfo {
 140        u_int32_t avg;
 141        u_int32_t burst;
 142
 143        compat_ulong_t prev;
 144        u_int32_t credit;
 145        u_int32_t credit_cap, cost;
 146
 147        u_int32_t master;
 148};
 149
 150/* To keep the full "prev" timestamp, the upper 32 bits are stored in the
 151 * master pointer, which does not need to be preserved. */
 152static void limit_mt_compat_from_user(void *dst, const void *src)
 153{
 154        const struct compat_xt_rateinfo *cm = src;
 155        struct xt_rateinfo m = {
 156                .avg            = cm->avg,
 157                .burst          = cm->burst,
 158                .prev           = cm->prev | (unsigned long)cm->master << 32,
 159                .credit         = cm->credit,
 160                .credit_cap     = cm->credit_cap,
 161                .cost           = cm->cost,
 162        };
 163        memcpy(dst, &m, sizeof(m));
 164}
 165
 166static int limit_mt_compat_to_user(void __user *dst, const void *src)
 167{
 168        const struct xt_rateinfo *m = src;
 169        struct compat_xt_rateinfo cm = {
 170                .avg            = m->avg,
 171                .burst          = m->burst,
 172                .prev           = m->prev,
 173                .credit         = m->credit,
 174                .credit_cap     = m->credit_cap,
 175                .cost           = m->cost,
 176                .master         = m->prev >> 32,
 177        };
 178        return copy_to_user(dst, &cm, sizeof(cm)) ? -EFAULT : 0;
 179}
 180#endif /* CONFIG_COMPAT */
 181
 182static struct xt_match limit_mt_reg __read_mostly = {
 183        .name             = "limit",
 184        .revision         = 0,
 185        .family           = NFPROTO_UNSPEC,
 186        .match            = limit_mt,
 187        .checkentry       = limit_mt_check,
 188        .destroy          = limit_mt_destroy,
 189        .matchsize        = sizeof(struct xt_rateinfo),
 190#ifdef CONFIG_COMPAT
 191        .compatsize       = sizeof(struct compat_xt_rateinfo),
 192        .compat_from_user = limit_mt_compat_from_user,
 193        .compat_to_user   = limit_mt_compat_to_user,
 194#endif
 195        .me               = THIS_MODULE,
 196};
 197
 198static int __init limit_mt_init(void)
 199{
 200        return xt_register_match(&limit_mt_reg);
 201}
 202
 203static void __exit limit_mt_exit(void)
 204{
 205        xt_unregister_match(&limit_mt_reg);
 206}
 207
 208module_init(limit_mt_init);
 209module_exit(limit_mt_exit);
 210
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